[RecoilRob]

You guys (and gals) are going to hate me for this.  I chopped the intake stacks down a couple inches because I feel they're too close to the back of the air box.   We've always gone by the idea that if you place a ball of similar displacement on the intake stack....there should be nothing that intrudes into this space.   In the case of the 650 that is slightly larger than a baseball (which is about 65% VE) and obviously no baseball is going to fit in that tiny space they allowed for airflow....so all along this has bothered me and finally set out to do something about it.

With an extra airbox the first problem is getting the damn stacks out of it as they're expoxied in.  Grrr....but the stuff can be pried off and the stacks removed.   Going in, the simple sounding 'shorten the stacks' ended up being a bit of a puzzle.   I thought about a stainless thinwall tube to bond the remaining ends together...but had trouble sourcing the exact right size and then saw that it wouldn't work anyhow because the ribs on the tubes are larger than the hole in the box they go through...meaning they have to be flexible.   This ruled out epoxy soaked cloth as a cast and I ended up using Super Z RC-56 canopy glue from my RC building kit.   It is flexible, dries clear and seems up to the task of holding the ends together.   One reason I cut them where they are is to use both ends worth of the stiffening ribs which when ground flat made a decently large surface area for the glue to work on.   Found a plastic bottle close to the inner diameter, wrapped it with masking tape until the right fit then used it to align the halves while the glue was setting.   Ended up with a nice smooth transition of the internal surfaces..   One 'could' just cut the tube...which would solve most of the problems, but the air really does like having a nice radius to flow around and into the stacks so leaving the end with a straight cut would almost surely compromise the total airflow...but might be good enough.  I didn't want to take the chance and kept the inlet trumpets.

So....how does it run?  BRILLIANT!   I know some people just love the instant torque the 650's have...and this is mostly being supplied by the induction tubes though for this you pay a price on the other end of the tach.   Chopping 2" from the tubes unmasks the inlets and also moves the resonant peaks about 500 rpm up down low on the 4th harmonic to about 1000 higher on the 2nd which is around 7000 rpm.   No more falling torque at 6000!!   It pulls harder and harder until the redline comes up...nice.   And down low it's still VERY tractable with only a slight loss of torque at 2000'ish though I did plunk around a bit and found it perfectly acceptable tradeoff....we're giving up something I never use, to gain things that make me smile.   If you do serious idling over rocks and such...leave the tubes in place, but if you ride motorcycles like most of us do....the shorter tubes really uncork the powerband.

All is not good in this report however.....getting the airbox out of the bike is a PAIN.   Of course...I could have attacked it totally wrong and some might laugh at the stupidity of trying to get the box out of the left side opening...but that's all I could figure...and it wasn't easy.  Looks like they built the frame then first operation is to stuff the airbox in then build the bike around it.   It WILL come out the left side if you're good with puzzles and persistent as it needs to be turned upside down to finally come out.   Perhaps someone would be able to get the intakes out with the box still in the bike...but I think that would be really hard.   Obviously...if you have the jugs off for ANY reason...that is the time to pull the airbox and trim the intakes. 

Anyone running one of these bikes who is searching for more power....here it is.  Anyone with a Big Bore kit...I'd consider this mod a requirement and now am wondering how a completely stock bike would like it?   The stock ECU probably won't like the additional airflow and I had to add some fuel to the PowerTronic accordingly.   From 3000 rpm and up...the throttle feels wonderful....responsive and lots of torque available if you want it...but let the revs come up and it really gets with the program.   Best improvement is out on the open highway where 70-80 mph cruise is just barely cracking the throttle now...  And..funny enough, I swear the exhaust is quieter ....which would make sense if the throttle isn't being opened as far to provide the air needed for that power setting.   Aside from the difficulty of actually performing the mod...it's a VERY good one.   Perhaps others can come up with an easier way of doing it.....Good luck!

[John Mullen]

Thanks for documenting all of that.  I for one, really appreciate getting a sneak peak into the mystery of the illusive air box.  As strong as my machine is now, with cam and pistons, I'm never going to find your energy to go down that road.  But thanks so much for giving all that useful information to the forum.

[Blazingatom]

Good work, be easy to 3d print an new airbox, any sort of PA12 or glass filled media for the airbox, if you want flexible pipes TPU would work well. There are a number of free CAD software stuff online and it's just a case of measuring and drawing onto there then using a slicing software

[NVDucati]

...   Chopping 2" from the tubes unmasks the inlets and also moves the resonant peaks about 500 rpm up down low on the 4th harmonic to about 1000 higher on the 2nd which is around 7000 rpm.   No more falling torque at 6000!!   It pulls harder and harder until the redline comes up...nice.   And down low it's still VERY tractable with only a slight loss of torque at 2000'ish though I did plunk around a bit and found it perfectly acceptable tradeoff....we're giving up something I never use, to gain things that make me smile.   If you do serious idling over rocks and such...leave the tubes in place, but if you ride motorcycles like most of us do....the shorter tubes really uncork the powerband.
....   Aside from the difficulty of actually performing the mod...it's a VERY good one.   Perhaps others can come up with an easier way of doing it.....Good luck!

Good stuff! So essentially you "sectioned" 2 inches off each tube by removing it from the middle.
I looked at the airbox when I first got the bike and you are right that It must have been the first thing to go in.
Now that you have broken ground for everyone and more and more owners are making engine mods, there may be more than one way to get to plug&play intake tubes of various lengths.
Thanks for the write up!

[whippers]

Great post and info.

However I think to support your work you should have a dyno chart before and after otherwise it’s yet another personal anecdote. I’d hate to think someone would follow your advice and not get the outcome you describe because your butt dynamometer isn’t as well calibrated as you believe.

[RecoilRob]

Great post and info.

However I think to support your work you should have a dyno chart before and after otherwise it’s yet another personal anecdote. I’d hate to think someone would follow your advice and not get the outcome you describe because your butt dynamometer isn’t as well calibrated as you believe.

Yea....I know....this has kept me from sharing things before just because I can't conclusively document them.   Shortening the stacks isn't rocket science and for sure unshrouds the intakes as well as shortening them...which I doubt anyone would argue isn't a step in the right direction.   How much of a step?   It's some...and part of this is what can't be measured on a dyno..such as how it feels during cruise.  It used to be more locked to the throttle opening where now it runs free and can run at high speeds with very little throttle opening.  Feels like the cork was removed from the bottle so to speak.

Anyone who would be interested in making power enough to invest in oversize pistons or camshafts should look at the airbox situation and if you really dig into it you see that shortening the tubes can't do anything bad for you.   The stack length could be OK for the 650 if the back of the airbox was another 4" or so back away from them...but there's no room for that.   I just wish it was easier to get at the airbox...but perhaps this was part of the 'Anti-Tamper' movement where they try to discourage 'modification' of important hardware and I'm convinced that shoving the tubes back in the box the way they did was part of how they detuned it to 47 hp.   So all of us have been trying harder and harder to pull air through a restricted system.  Makes more sense to derestrict it...yes?

[Locknut]

So,  to be clear ,  have you addressed the micro-tiny intake at the end-cap that feeds the box ?  That's really really tiny,  and coupled with the trumpet/wall proximity it is quite a restrictor, must admit.  Yikes.  Seems like a couple of areas that really limit flow.  Hmmmm. . .Did you address that first, then do the horns ?  If I juuuust happened to have a couple of velocity stacks lying around that I could fit onto some trimmed RE horns,  hmmmm.  Still leaves the cap though, hmmm.

[fireypete]

Those long intakes, I reached in and tried to pull 'em off.  No go.  Its a crappy day and not much to do so that was the level of my effort!  Could just cut 'em off but no smooth opening then.

[RecoilRob]

So,  to be clear ,  have you addressed the micro-tiny intake at the end-cap that feeds the box ?  That's really really tiny,  and coupled with the trumpet/wall proximity it is quite a restrictor, must admit.  Yikes.  Seems like a couple of areas that really limit flow.  Hmmmm. . .Did you address that first, then do the horns ?  If I juuuust happened to have a couple of velocity stacks lying around that I could fit onto some trimmed RE horns,  hmmmm.  Still leaves the cap though, hmmm.

If you measure the stock filter inlet opening it's right on the same as the filter itself...so little restriction and for that it gives you noise reduction and a primary filtering action where anything large or heavy isn't going to be able to go up it.

I thought the stock stack trumpets were really tiny also...and have some nice 50mm aluminum bells here that were intended to be added..but alas there is absolutely no room in there where the stacks live.   The box tapers at the top to follow the frame so the stock bells are about the best that will fit.   

With the shortened stacks feeding on unsrouded air right above the air filter and some actual box volume to play with...it really does breath a lot more freely.  This even with the stock filter inlet though I've got a Sprint filter in it...that thing has less restriction than the oil wetted cotton versions.

[RecoilRob]

Those long intakes, I reached in and tried to pull 'em off.  No go.  Its a crappy day and not much to do so that was the level of my effort!  Could just cut 'em off but no smooth opening then.

I wish it was that easy to mess with the tubes, but they go to the throttle bodies and more importantly...through the front of the airbox.   The seal is very secure and takes quite a bit of force to flex enough to get them out.  They're also keyed so you can't rotate them and really nice construction..as the rest of the bike.  Cutting them off and leaving them that way would probably work decently well...but would offend my mechanical senses to actually do it if I could figure out a way to do it better...even though the ass pain is much worse.

Hopefully this might get other brains ticking and better suggestions thought of.   I'm now positive that the stack situation was involved in the 47 horse power rating.   I know lots of people argue about the restriction of the filter inlet...yet the stacks within the box are massively more restrictive...though difficult to modify.

You know...just had a thought:  if you reach in and cut the tubes...that's not ideal, but if someone with a printer could make a nice bell mouth that would fit over the tube end...that would be super easy and exactly the same result.   Anyone have a printer...?

[GravyDavy]

Just spitballing here, but I wonder if sound control might also be a factor in the tube design. Given the 47hp limit, maximizing low-end torque makes sense, and that often means fairly low airflow. Tightening the clearances from the bellmouth ends to the airbox wall might have some benefit in reducing intake noise, and if that works with the hp limit, it would allow a bit more legal room for the exhaust to be audible. Given how many of us love the exhaust note, I could see that being a consideration. More intake noise would require the exhaust to be quieter in order to pass dB limits.

[whippers]

Yea....I know....this has kept me from sharing things before just because I can't conclusively document them.   Shortening the stacks isn't rocket science and for sure unshrouds the intakes as well as shortening them...which I doubt anyone would argue isn't a step in the right direction.   How much of a step?   It's some...and part of this is what can't be measured on a dyno..such as how it feels during cruise.  It used to be more locked to the throttle opening where now it runs free and can run at high speeds with very little throttle opening.  Feels like the cork was removed from the bottle so to speak.

Anyone who would be interested in making power enough to invest in oversize pistons or camshafts should look at the airbox situation and if you really dig into it you see that shortening the tubes can't do anything bad for you.   The stack length could be OK for the 650 if the back of the airbox was another 4" or so back away from them...but there's no room for that.   I just wish it was easier to get at the airbox...but perhaps this was part of the 'Anti-Tamper' movement where they try to discourage 'modification' of important hardware and I'm convinced that shoving the tubes back in the box the way they did was part of how they detuned it to 47 hp.   So all of us have been trying harder and harder to pull air through a restricted system.  Makes more sense to derestrict it...yes?

I totally agree with the theory and I too have noted how many built dyno curves show the same massive torque dip at higher revs so peak power is never that impressive. Of course peak power is also pretty unimportant on a road bike.  My bike is stock save for s&s silencers and accompanying airfilter and more open retainer. When I do rev it higher I actually find it hard not to smack into the rev limiter. I think you would want a higher rev limit assuming the motor can cope to really make a difference. Mostly I ride the bike in the midrange and it’s brisk enough because I don’t need to slow down much.

[Locknut]

Well, daaaaamn.  Now another project to keep me away from sitting on the lawn mower.  Oh damn.  Maybe do a ZX12 ram air project for the RE now. Duct it into that itty bitty box intake and pressurize the box a few psi at speed..  . .leak in some nitrous for special moments from this mini bottle I have packed away somewhere. . ..you know, not so much that I need fuel added,  but just enough to freak out some guys we encounter.  This can never end.  I'm supposed to be chilling,  being retired, rock on the porch.   

[Marcsen]

Harris used silicone hoses on their bike

[Starpeve]

I wish it was that easy to mess with the tubes, but they go to the throttle bodies and more importantly...through the front of the airbox.   The seal is very secure and takes quite a bit of force to flex enough to get them out.  They're also keyed so you can't rotate them and really nice construction..as the rest of the bike.  Cutting them off and leaving them that way would probably work decently well...but would offend my mechanical senses to actually do it if I could figure out a way to do it better...even though the ass pain is much worse.

Hopefully this might get other brains ticking and better suggestions thought of.   I'm now positive that the stack situation was involved in the 47 horse power rating.   I know lots of people argue about the restriction of the filter inlet...yet the stacks within the box are massively more restrictive...though difficult to modify.

You know...just had a thought:  if you reach in and cut the tubes...that's not ideal, but if someone with a printer could make a nice bell mouth that would fit over the tube end...that would be super easy and exactly the same result.   Anyone have a printer...?

What about removing the airbox completely and fitting pod filters.

[RecoilRob]

What about removing the airbox completely and fitting pod filters.

Well....pods would get rid of the majority of the intake restrictions but also get rid of all the benefits of a properly sized intake tract.   Bottom end and midrange power would be compromised badly with the pods and the FI might be difficult to remap and get it to run anywhere near correctly.   In the SV world we've had many people go to pods with bad results and many have gone back to the airbox as it's the best system if done properly.   The intake at 11" (-2 on the stacks) gives us three resonant points around 3800, 5000 then again at 7000...which I'm feeling is a good spread and the engine is still tractable down to 2000 or so...makes for an excellent street engine powerband.

[NVDucati]

Harris used silicone hoses on their bike

Yes. Silicone tubing can be found in infinite sizes, flex, chemical resistance and shaping properties.
A quick search of the Thomas Register shows 227 makers.
https://www.thomasnet.com/nsearch.html?cov=NA&heading=88669700&typed_term=silicone+tubing&searchterm=silicone+tubing&what=Silicone+Tubing&WTZO=Find+Suppliers&searchsource=suppliers
I haven't done it so my comments are suspect but:
If I ever go through the trouble to get the airbox out, I will convert it to a two piece box for future access.
As for the intake tube shortening, I would think that a "baloney-cut" section removal rejoined by slipping the pieces into a short piece of silicone tube. I think with a scalpel (literally) the cut could be made in situ.

[Locknut]

++1  I've not ever experimented with a plenum for a 270 crank . . . .  but if we had a place for that filter other than the box  it would open up some options for those stacks.  I'd experiment with at least some separation / direction.  180 cranks / box tuning is a different world,  but at least we are dealing with a 7,000 rpm ceiling. 

[Locknut]

RecoilRob, I turned the light on this time:  Ooooff,  we ARE really limited to that air filter-end entry,  as the box sits now.   As gross simple  as it is your moving the horn stack ends away from the wall is so far the ONLY thing we can do ,  other than that filter flow change you did.  Perhaps a side panel becoming a filter system and the box size, as it is, with horn changes to accommodate that 270 crank pulse.   

[RecoilRob]

  Perhaps a side panel becoming a filter system and the box size, as it is, with horn changes to accommodate that 270 crank pulse.

With the size restrictions placed on the airbox...I'm thinking what we have now with a low restriction filter is about as good as it's going to get.   The box is up against the battery which is up against the rear fender, and under the battery the carbon can solenoid and piping so they were using all the available space to keep the dashing good looks of the bike.   I DO really like how the Interceptor looks...so am willing to put up with some difficulty in maintenance I guess.

Early on with a spare airbox I was looking at figuring out the resonant frequency it might have...and then realized that I don't know how to account for all the internal things like the stacks and there also is a breather/oil separator taking up quite a bit of space.   After test riding it and tinkering with the tune to accommodate the increased airflow there's more that's changed here than just letting it breathe easier with a little higher tuned resonant points. 

I'm now thinking that having the stacks so close to the back wall was giving a reflection boost to the reversion pulses and making them stronger than if they're created by encountering just the air within the box.  To me the low end torque always felt a little artificial in that I've never ridden a bike that runs like this.   The term 'agricultural' kept coming to my mind when trying to describe how it felt with stock cam because it was like a tractor.   This was a tractor you could spin to 7000 if you wanted...but it wasn't happy about it and would rather plod along much more slowly.   With the shorter stacks and S&S cam now it feels much like a hot cammed motor that still has good low end behavior but gets progressively stronger the higher it revs.   The throttle response is immediate yet smooth and gentle at first so finding maintenance throttle whilst cranking through a turn very easy then powering out with just as much power as you ask for...no surging or misbehavior.   

[NVDucati]

With the size restrictions placed on the airbox...I'm thinking what we have now with a low restriction filter is about as good as it's going to get.   The box is up against the battery which is up against the rear fender, and under the battery the carbon can solenoid and piping so they were using all the available space to keep the dashing good looks of the bike.   I DO really like how the Interceptor looks...so am willing to put up with some difficulty in maintenance I guess.
... SNIP...

The battery can be easily relocated. With a LITHO battery and a simple "project box", locate it under the rear fender or under the swingarm pivot. The custom bike crowd do that routinely. You would never see it and you can proceed with a longer airbox that has a "rear door".

[Locknut]

+++1 "You B so smart" . .  for  thinking  "out of the box". . . pun intended:
--->   Nothing says the tool pouch has to stay there either.  .    I have a mega size Slime bottle and three huge CO2 cartridges for my initial flat repair attempt stored at a low back rest/sissy bar, there  to keep my wife from ever sliding off.  That area can easily accommodate the OEM tools.  That opens up space for creative volume / custom box. . . . I've done it for small aircraft (mine) lol.  My BATTERY was in the tail, to manage weight & balance for a bigger engine.   This RE box thinking is micro, and should be EZ.  (Should and "will be  " are fantasies)   

 Who the hell started this , anyway .. .

[Intybe]

Who the hell started this , anyway .. .

I think that may have been Rob, whose honorary Grand Gearhead is surely imminent!

Yea....I know....this has kept me from sharing things before just because I can't conclusively document them.   Shortening the stacks isn't rocket science

So glad you shared Rob.  It amazes me that you seem to be the first to have thought of this performance mod: Kudos for not only havin the insight but also busting your balls to achieve it and report the results. Looking forward to your results being replicated... Hopefully, someone with more skills than me figures out an easier process so the rest of us can crack out the scalpal and silicon sleaves.

By the way, what's the science behind it, ie how did you know to chop out two inches and not 2.5. (It's counterintuitive to imagine that length and performance might be inversely proportional... Now to convince my partner!)  I suppose there's some formula you used?
 

[RecoilRob]

By the way, what's the science behind it, ie how did you know to chop out two inches and not 2.5. (It's counterintuitive to imagine that length and performance might be inversely proportional... Now to convince my partner!)  I suppose there's some formula you used?
 

Yes...there is this handy calculator: http://wallaceracing.com/runnertorquecalc.php   Made up a simple little chart with various lengths to see how things varied.   Then because I decided to save the inlet trumpets they needed to be glued back onto the cut stacks so I chose the stiffening ribs to leave on both pieces to give more wheelbase for the glue.   With the ribs ground flat they stuck together pretty nicely....and the resulting length with well within what would work with three resonances available within the stock rev range.   Kind of Karma like. 

[Locknut]

I choose this bike for its as-is simplicity and FI. .  . which I thought would free me from my past: carburated gear-head stupidity.  Now all of a sudden, in one thread,   I'm back into the nightmare my shrink told me to avoid.  I think he was a shrink,  but now I'm not sure.  He WAS drinking a beer,  so I'm not sure.
I just wanted to putt around on a show room bike for once, non-tinkered and virgin.   I'm tired.

But now .  .   BTW,  what the hell is the basis for "Recoil" . . . the back story ?  I've got my theories.

[NVDucati]

We talk, read and write about intake harmonics and laminar flow, etc. Very interesting stuff and it matters for that everything adds up approach to a sweet running engine.
Yet there is still another arena for moving increased volumes of air. Entrainment.
_ For wholly non-motorcycle reasons, I've come across the science and design of entrainment systems. One of the more practical applications is available as a retail product by a Canadian company, Exair. They deal with factories and other industrial applications. They sell adjustable air amplifiers.
https://www.exair.com/products/air-amplifiers.html
Small, light and affordable and transferable to our needs. Video:https://www.exair.com/p6230dx.html
_ If you are curious you can also do a search using "internal combustion engine air intake "entrainment" .
Here is one patent for fuel injection systems; https://patents.google.com/patent/CN107448331B/en
Best yet, it is far less complicated than it appears to be. Suitable for the DIY crowd.

[RecoilRob]

But now .  .   BTW,  what the hell is the basis for "Recoil" . . . the back story ?  I've got my theories.

That's from years ago on THR gun site and I just figured to stay consistent with the name used online.   I used to be quite the recoil junky just as I'm a horsepower junky....obsessiveness seems to be in my genes.   Oh well......

[John Mullen]

This did have me "almost" take off the left cover and dive into "the airbox".  But the more I read and have you guys remind me of my own nightmares surrounding carburetors and intake shit from many years of "never sure I got it quite right", I resisted the airbox calling to me.  So THANK YOU for reminding me of the horrors.  Can I dare ruin the perfect thing, that my machine is now, with the cam and pistons.  I swear it purrs now at 1200 RPM and again at 7000.  There is nothing I would change.  Just keep saying that over and over.  I'm done.  I'm done  I'm done!

[YellowDuck]

There is nothing I would change.  Just keep saying that over and over.  I'm done.  I'm done  I'm done!

Oh who do you think you are fooling...

[John Mullen]

Is that like from one addict to another?  Cause I know my bike is my drug of choice...

[YellowDuck]

Is that like from one addict to another?  Cause I know my bike is my drug of choice...

I'm in recovery, and doing quite well, thanks.  The key step for me was buying this particular bike *because* it is not very powerful, and knowing that for a few thousand more I could have bought something with twice the power, I have so far convinced myself that it would be silly to do anything in search of a few hp.  Same goes for weight reduction. 

The relative freedom from modification compulsion has been very liberating.  Updates so far have included brake pads (necessary and justified), bar end mirrors (totally unnecessary, the OEM mirrors are great), rear suspension (unnecessary, just fucking around for entertainment) and, soon, front suspension (also unnecessary, just messing about).

About the only thing I see left in the future is a very expensive but more comfortable seat, such as a Russell - the RE touring seat is not doing it for me.  Probably seat comfort is the #1 defect for me in the bike as designed.

This restraint is in stark contrast to what I got into with my previous two bikes.  I am proud of myself.

That is my story and I am sticking to it.

[ideola]

On Project MLG, I purchased an extra airbox from a salvaged bike on Fleabay.
I used a hacksaw blade to cut off about 4" of the factory runners (a little tricky but doable through the air filter opening).
I inserted an aluminum trumpet-style velocity stack (35mm x 52mm) into each runner (very tight interference fit, no need for clamp or any other retention)
I used a flap wheel bit on a dremel to port the throttle body interface on the exterior portion of the runner to match the larger bore of the Revelry Racing big bore throttle bodies.
I wrapped the entire airbox in self-adhesive Thermo-tec heat barrier.
I have not yet done any further testing of the CAI setup since the new build was completed.
The bike is currently at the Dyno shop, and I hope to get results any day now, and will be picking it up next Saturday.

[ideola]

I should add, one could very easily just cut off the runners the way I did and insert bellmouths without removing the airbox at all. The runners are accessible through the air filter opening, and while it is a bit fiddly to maneuver the saw blade in that opening, it's better than removing the airbox.

In my case, the bike was already apart for the big bore big valve cnc head big bore throttle body yada yada, so I took the opportunity to wrap the replacement/modified box since it was going to be swapped out anyway. The mechanic did bitch about how much fun it was to get it out and back in. But I think my modification approach could achieve similar results without having to remove the airbox.

[fireypete]

Sooo, cut the intake stacks 4 inches, buy some aluminum stacks, not that expensive and stick/stuff 'em on....thats it?  Jeez thats within the reals of my mechanical prowess...got big hands tho  that airbox is tight!
Perhaps buy a fuel x and use that to fix the mixture if it runs lean due to free flowing air filter and open pipes...Sounds like a plan...

[RecoilRob]

Sooo, cut the intake stacks 4 inches, buy some aluminum stacks, not that expensive and stick/stuff 'em on....thats it?  Jeez thats within the reals of my mechanical prowess...got big hands tho  that airbox is tight!
Perhaps buy a fuel x and use that to fix the mixture if it runs lean due to free flowing air filter and open pipes...Sounds like a plan...

I don't mean to get in the way of your plan...but it might be needing a little rethinking.  The intake stack diameter is fine for a 650, but really starting to get small fast on the Big Bore engines or one intended to make big power so you absolutely don't want to do anything to make the intakes smaller.   Someone was saying about sticking stacks into the cut off tubes...but that would decrease the diameter and while convenient is less than ideal.   A stack that fits over the tube reinforcing ribs while making a smooth and seamless transition to the stack interior would be the better way to go...but I'm not aware of any being offered at the moment.   Almost ready to pull the trigger on a 3D printer so I could make my own...but have been hesitating because I just don't need another obsession at the moment.

The addition of pipes mostly doesn't show much of an improvement and that's because of the stock ECU tuning can't take advantage of them.   I had to add quite a bit of fuel to make it run properly when going to the larger diameter pipes.   Then had to add even more fuel after putting in the S&S cam...which makes sense as it's pumping more air.

The change to 2" shorter stacks needed more tuning than even the cam did as they really change how much air the motor can inhale and where it's most effective.   Sorry but to really make the motor run properly you're going to need to be able to tune it with modified airbox stacks.   I'm liking the PowerTronic as it allows ignition timing changes and came in real handy with the shorter stacks to get it dialed in.   Went for a light throttle 'rideability' ride yesterday where top speed was only 65 mph and for a few miles with the rest in the 50'ish range and it got a little over 75 mpg's....not too shabby for a modified motor and has near perfect behavior.   It gets about 60 when ridden very aggressively and normally just hits 70 on my normal rides....so it's amazingly efficient and if this consideration is taken into account...the power production is pretty impressive for such an economical engine.

[ideola]

https://en.wikipedia.org/wiki/Venturi_effect

[RecoilRob]

https://en.wikipedia.org/wiki/Venturi_effect

I'm well aware of the works of Mr Venturi....how does that relate to the airbox?  I'm guessing you are saying that making the stacks narrower won't cause problems?  But of course it will!  Only a 'perfect venturi' can return all of the energy inputted while still allowing a section of reduced pressure to be used.  We don't have 'perfect venturi's' in the airbox and ANY reduction of area will hurt performance as well as having no need to create a low pressure feed...so why bother with venturi's?

[ideola]

We'll see 

[NVDucati]

I don't mean to get in the way of your plan...but it might be needing a little rethinking.  The intake stack diameter is fine for a 650, but really starting to get small fast on the Big Bore engines or one intended to make big power so you absolutely don't want to do anything to make the intakes smaller.   Someone was saying about sticking stacks into the cut off tubes...but that would decrease the diameter and while convenient is less than ideal.   A stack that fits over the tube reinforcing ribs while making a smooth and seamless transition to the stack interior would be the better way to go...but I'm not aware of any being offered at the moment.   Almost ready to pull the trigger on a 3D printer so I could make my own...but have been hesitating because I just don't need another obsession at the moment.
SNIP...

Yeah, 3D printing is very cool but a bit of a rabbit hole at the same time. The best path is to get your neighbor interested in 3D printing.
But an alternative to attaching the velocity bell to the tube is to use heat and press the shape into the tube itself. Think; brake line flange tool. The shape can be pretty quickly made by using the appropriate metal velocity stack as a mold buck. You can fill it with just about any epoxy based filler including common "liquid nail" construction adhesive. This will not be a long term production run. Don't forget the mold release and a well centered threaded "lolly pop" handle for using your new flange tool.

[Blazingatom]

For 3d printing search for bureaus, in the UK I can recommend 3DPRINTUK in the US shapeways, both are very well priced an for intake stacks probably cost I  the region of £20 to £30

[fireypete]

My bike is the stock 650.  It seems a simple way to get the bike to run a bit better.  I just don't have the knowledge of air flow to really nut it out more than "cut 'em off, stick bell mouths on 'em"  and looking at what I just wrote...perhaps I should stay out of the shed! The fuel x is mentioned on a tec bike youtube vid. Seems like a reasonable idea.
I must also consider, whilst I would like a bit more power the bike is running nicely, a little breathless up top is about it.  And I could manage to duck that up ;

[fireypete]

Hi Rob,
Your engine is quite modified isn't it?  So it would really benefit from more air.  But a stock 650...

[RecoilRob]

Hi Rob,
Your engine is quite modified isn't it?  So it would really benefit from more air.  But a stock 650...

Well...the engine has never had the head off...but does have S&S cam and TEC pipes so we'd call the modification 'minor'.

I AM curious as to what shorter snorkel tubes would do to a totally stock bike?   There is a large chance that doing that would throw the ECU into a tizzy and aftermarket fueling might be needed to retain good ride-ability....but it's too late to experiment on my bike.

[John Mullen]

My Interceptor has pistons and cam and has wicked low-end torque.  It's a torque monster.  So I guess I wouldn't think of modifying the airbox.  It's too sweet the way it is.  I couldn't dream of any better throttle response.  My tuner sure did a great job of dialing in the Power Commander for my bike. Now I have to just leave it and enjoy it.    If it ain't broke........

[ideola]

My tuner sure did a great job of dialing in the Power Commander

None of these mods make much sense if you aren't going to take it to a tuner to be dialed in. The maps provided by Revelry, Hitchcocks, and S&S will all get the bike running decently, but there are so many variables from one build to the next, there's no way any of those maps will be optimized.

The tuning shop I am using is only charging me $250 to build an optimized map, which is a drop in the bucket compared to what I've put into both builds (Project MLG and The Rocketeer). Well worth the cost, IMO. He asked if I wanted to do pulls on the current map and I told him don't bother.

[whippers]

My bike is the stock 650.  It seems a simple way to get the bike to run a bit better.  I just don't have the knowledge of air flow to really nut it out more than "cut 'em off, stick bell mouths on 'em"  and looking at what I just wrote...perhaps I should stay out of the shed! The fuel x is mentioned on a tec bike youtube vid. Seems like a reasonable idea.
I must also consider, whilst I would like a bit more power the bike is running nicely, a little breathless up top is about it.  And I could manage to duck that up ;

You are far more likely to ruin it than make it any better if you attack the airbox trumpets the way you describe

[Intybe]

Rob posted an easier method to remove the airbox according to the manual - https://forum.classicmotorworks.com/index.php?topic=33218.0 However, once removed there's still the problem of limited access to make the cuts to shorten the stacks via the filter hole. Would cutting out the section above the filter to provide better access to cut and re-join the stacks in situ, then gluing the section back in place after the operation, be any worse than Rob's original method of removing the box, cutting the stacks out of the box and then replacing them back in the box once shortened? Cutting in situ certainly obviates the considerable hassle of removing the airbox 🤔 (Perhaps the master mechanics here could start a new thread recounting genius apprentice ideas 🤦‍♂️🤣)

Regarding gluing the bellmouths to the runners after the 2" section has been removed: Would NVDucati's suggestion of external silicon sleaves work or would the irregular external surface of the runners, i.e. the various stiffening ribs, make for a difficult unreliable seal? Alternatively, Rob, did you say a very slim stainless steel inner sleave could join the two pieces without interfering with the internal airflow, resonance, etc?
 
Ps, I doubt whether the bell mouths could be effectively glued back to the shortened runners without internal or external sleaves without removing them completely from box, as Rob did. 

[ideola]

There are tabs along the top of the box that hold the runners in place, so cutting up top will be complicated because the runners are attached up there, and you have to know exactly where to cut to do that.

The bellmouths I used don't need to be glued in. Once pressed firmly in place, they will not budge, and are VERY difficult to remove. The ribs on the outside of the runners would make it challenging to affix anything on the outside. The runners are flexible soft rubber, so getting a good seal from the outside without dissecting the airbox will be very challenging.

I think too much is being made of the "loss in diameter" using my method. The bellmouths I am using are, at most, only 1mm less in inner diameter than the stock runners. The surface area difference is 962.5 mm³ vs 1018 mm³, a difference of ~5%. All else being equal, is it enough of a difference to make a difference? Perhaps.

BUT we're not talking about all else being equal in this discussion. The gains achieved by shortening the runner not to mention having a much better bellmouth profile compared to the stock runner far offset any measurable loss in flow due to losing 1mm of I.D. My dyno results on Project MLG support my point here. I have only seen ONE documented dyno result that surpasses what my tuner achieved, and that was by Revelry themselves, and I expect to match or beat their number once I install bigger injectors. So I think it's hard to argue that my method has hurt performance.

I have a dissected airbox in the shop and I can show exactly the bellmouths I used. I will post photos later when I have time to get down to the shop.

All this to say, there are different approaches one can consider. Sure, removing the airbox and re-constructing the runners would allow for optimum design. But the method I used doesn't require that level of surgery, and can be easily accomplished by the home hobbyist and still yield improvements. It's a classic case of deciding which tradeoffs you want to make, and I would not be so quick to dismiss my approach. In fact, I will eventually do it again on the Interceptor after I get that bike back from the tuner (it's there as we speak). I might even consider doing before and after dyno runs. Worst case scenario, if I screw it up, I have extra unmolested airboxes I can swap back in.

[RecoilRob]

Ideally....we'd want a smooth internal surface with seamless transition from bell mouth to internal runner walls to get the best flow, but achieving that IS difficult.   I do think that the shortened intake trumpet will offer a lot more benefit than whatever small amount of turbulence you might create with a small mis-match on the bell to runner or slight reduction in diameter.   It's going to breathe better no matter how you move the openings back away from the airbox rear wall.

A push-on bell mouth that will stay put would be ideal if you can source them...I didn't find anything suitable with a considerable web search before biting the bullet and removing the tubes from a spare box and then doing surgery.   To section out a middle piece if both remaining ends have the stiffening ribs left they can be flattened and form a decent surface for gluing...but again doing this in situ probably not going to happen unless you have ET like hands.

I did mention the thought of a thin metal sleeve being used to join the two pieces after the cut...which would work fine (I think) if such a thing could be found.  It would need to be about 1.450" OD to fit snugly into the tubes and would lessen the precision needed to make the cuts as the ends wouldn't need to fit together perfectly...close enough would work fine I think.

The great results Ideola has gotten match well with my butt-dyno opinion of this mod...if you can tune the mixture it's a good thing!   Once he's got both bikes tuned optimally it would be interesting to push the redline up a bit and see how/if more power is available...which I think it is.  With the torque holding better at the higher revs there 'should' be more HP's, and with a 2" chop there's still enough bottom end to not notice any real reduction while riding down low.   I really, really wish the airbox stacks were something easy to modify like the air filter inlet...that would have been ideal, but sadly the reality of the design makes whatever you do inside the box a bit fiddly.

The 2" chop number was chosen because of the rib placement and the need to have enough surface area to reattach them and seemed to be a 'happy coincidence' that this dimension is also pretty close to ideal according to the intake tract calculators.   With a 4th order resonance right around the 3800'ish area it's excellent for highway cruise.  The stock length is down around 3000...which is fine and I do think the torque is getting a boost from the reflection off of the airbox wall, but at higher revs you really start to pay the piper with internal restrictions.  As the stacks get shorter the 4th order point moves upwards beyond what my normal highway cruise rpm's are...so this was intentionally avoided as it's nice to cruise where the airbox is happy for best mileage.  

[ideola]

it's nice to cruise where the airbox is happy for best mileage.

Excellent insights, as always.

And, did I mention, I managed 61 MPG on the 865 yesterday! And I was NOT babying it either!

[ideola]

I did mention the thought of a thin metal sleeve being used to join the two pieces after the cut...which would work fine (I think) if such a thing could be found.  It would need to be about 1.450" OD to fit snugly into the tubes and would lessen the precision needed to make the cuts as the ends wouldn't need to fit together perfectly...close enough would work fine I think.

These are designed specifically for joining silicone intake tubing for automotive applications
https://www.siliconeintakes.com/turbo-piping/intercooler-pipe-fabrication-p-134.html


The 1.375" is the right diameter for an "insertion sleeve" in the OEM 36mm ID runner. Using one of these, you could cut off the end of the OEM bellmouth, shorten the runner as described elsewhere, and use these couplers to rejoin. You wouldn't even need clamps, as this is not in a forced induction application, and the bead will keep the coupler seated in the rubber tubes.

You will end up losing about the same surface area as with my method, though...

[Intybe]

Hi Ideola thanks for posting in response to my questions. 

There are tabs along the top of the box that hold the runners in place, so cutting up top will be complicated because the runners are attached up there, and you have to know exactly where to cut to do that. Rather than "cutting up top", I suggested cutting the left hand side of the airbox above the filter (see pic) to effectively double the size of the filter hole to get better access particularly to the LH runner. But it sounds like you shortened both runners without cutting the airbox at all, right? Cutting the RH runner looks pretty straitght forward but how did you manage to cut the LH runner, which is tucked up behind the LH cover of the airbox? Did you use a mirror? Did you have your hand inside the box when you cut it? You said you used a sharp blade. Would you mind posting a pic of the tools you used?

The bellmouths I used don't need to be glued in. Once pressed firmly in place, they will not budge, and are VERY difficult to remove. The ribs on the outside of the runners would make it challenging to affix anything on the outside. The runners are flexible soft rubber, so getting a good seal from the outside without dissecting the airbox will be very challenging. Agreed

I think too much is being made of the "loss in diameter" using my method. The bellmouths I am using are, at most, only 1mm less in inner diameter than the stock runners. The surface area difference is 962.5 mm³ vs 1018 mm³, a difference of ~5%. All else being equal, is it enough of a difference to make a difference? Perhaps. This maths/ engineering is way beyond me. Your use of push-in aftermarket bellmouths is an appealing simple solution and worked for you because you cut 4" off the runner, but when I cut  2", ie section X and Y in the pic, and reattach the bellmouths, they will be directly above the filter. As aftermarket bellmouths look to have a considerably larger diameter than the OEMs, using them would obstruct the insertion of the airfilter in my case. I note that the bottom longitudinal rib on both runners already has a section cut out so that the filter is unobstructed, so even the OEM bellmouth will cause some obstruction. I'm not sure whether it will be better to let the filter squish it into an oval shape (which is not possible with alluminium after market ones) or make indents in the filter to accomodate the bellmouth maintaining it's circular shape 🤔
...

I have a dissected airbox in the shop and I can show exactly the bellmouths I used. I will post photos later when I have time to get down to the shop.I'd definitely appreciate pics of dissected airbox to see what's going on inside.

All this to say, there are different approaches one can consider. Sure, removing the airbox and re-constructing the runners would allow for optimum design. But the method I used doesn't require that level of surgery, and can be easily accomplished by the home hobbyist and still yield improvements. It's a classic case of deciding which tradeoffs you want to make, and I would not be so quick to dismiss my approach. In fact, I will eventually do it again on the Interceptor after I get that bike back from the tuner (it's there as we speak). I might even consider doing before and after dyno runs. Worst case scenario, if I screw it up, I have extra unmolested airboxes I can swap back in.
I'm all for minor surgery, thanks for the confidence boost that it's doable in situ. It'd be great to see before and after dyno runs

[Intybe]

These are designed specifically for joining silicone intake tubing for automotive applications
https://www.siliconeintakes.com/turbo-piping/intercooler-pipe-fabrication-p-134.html


The 1.375" is the right diameter for an "insertion sleeve" in the OEM 36mm ID runner. Using one of these, you could cut off the end of the OEM bellmouth, shorten the runner as described elsewhere, and use these couplers to rejoin. You wouldn't even need clamps, as this is not in a forced induction application, and the bead will keep the coupler seated in the rubber tubes.

You will end up losing about the same surface area as with my method, though...

Thanks for the link, Ideola. Unfortunately they don't ship to Australia. An aluminium version is available here but OD is 35mm - OEM runner ID is 36mm so I guess the fit would be a little sloppy, but nothing that a little glue or silicon wouldn't fix, rightt?🤣 Can you see any issue with the aluminium?
https://www.ebay.com.au/itm/143674179580

[Intybe]

Hi Rob, thanks again for your insightful posts.

Ideally....we'd want a smooth internal surface with seamless transition from bell mouth to internal runner walls to get the best flow, but achieving that IS difficult.   I do think that the shortened intake trumpet will offer a lot more benefit than whatever small amount of turbulence you might create with a small mis-match on the bell to runner or slight reduction in diameter.   It's going to breathe better no matter how you move the openings back away from the airbox rear wall.

A push-on bell mouth that will stay put would be ideal if you can source them...I didn't find anything suitable with a considerable web search before biting the bullet and removing the tubes from a spare box and then doing surgery.   To section out a middle piece if both remaining ends have the stiffening ribs left they can be flattened and form a decent surface for gluing...but again doing this in situ probably not going to happen unless you have ET like hands. Just to clarify, did you remove sections X and Y and then join lateral ribs A and B, as in my photo? I wonder, if I kept both the A and B lateral ribs and could create a smooth mating surface around their respective circumfrances, with a Dremel, say, I could possible glue them and hold them in place with aligator clips at various positions around the joins and not need to use an inner sleave at all. By the way, what's the best glue to rejoin the OEM bellmouths to the runners? I would typically use Gorilla glue but it tends to foam out of joins so I'd have to be wiping the joins inside the runners while it dried. Otherwise, I'd use bicycle puncture repair glue, which offers a generously long working time before it goes off and it remains supple even then, though I'm not sure it's strong enough for this application 🤔

I did mention the thought of a thin metal sleeve being used to join the two pieces after the cut...which would work fine (I think) if such a thing could be found.  It would need to be about 1.450" OD to fit snugly into the tubes and would lessen the precision needed to make the cuts as the ends wouldn't need to fit together perfectly...close enough would work fine I think. Hmm, the aluminium sleaves I found are only 35mm OD, which is going to be a little loose, right? (Isn't that what she said?)

The great results Ideola has gotten match well with my butt-dyno opinion of this mod...if you can tune the mixture it's a good thing!   Once he's got both bikes tuned optimally it would be interesting to push the redline up a bit and see how/if more power is available...which I think it is.  With the torque holding better at the higher revs there 'should' be more HP's, and with a 2" chop there's still enough bottom end to not notice any real reduction while riding down low.   I really, really wish the airbox stacks were something easy to modify like the air filter inlet...that would have been ideal, but sadly the reality of the design makes whatever you do inside the box a bit fiddly. Ideola said he managed to do it, so hopefully he will share his method in more detail so that I can copy it. Otherwise, I'll cut the section out of the LH side and get better access that way.

The 2" chop number was chosen because of the rib placement and the need to have enough surface area to reattach them and seemed to be a 'happy coincidence' that this dimension is also pretty close to ideal according to the intake tract calculators.   With a 4th order resonance right around the 3800'ish area it's excellent for highway cruise.  The stock length is down around 3000...which is fine and I do think the torque is getting a boost from the reflection off of the airbox wall, but at higher revs you really start to pay the piper with internal restrictions.  As the stacks get shorter the 4th order point moves upwards beyond what my normal highway cruise rpm's are...so this was intentionally avoided as it's nice to cruise where the airbox is happy for best mileage. Yeah, even with new cam, the bike doesn't have any go when trying to accelerate from 100km/h. I'm really hoping it's just air that's being restricted and this mod fixes that... Otherwise, I can always modify my expectations 😅

[RecoilRob]

Yes...you got the idea of where and what to cut.  If you leave the ribs on each piece it gives you a decently thick area to glue.

After a LOT of thought on this...I'm now of the opinion that removing the airbox to do surgery is probably the best way to go about doing this and much of my tribulations were caused by me trying to take the box out the side rather than straight out the back as the manual instructs.   Even with a larger inlet hole it's going to be difficult working inside the box to get a nice finished product unless a push-on/push-in stack of suitable size comes along.   And then you need to repair the box to be air tight

Getting the rubber tube ends joined isn't a real easy thing to accomplish either...I had to sleeve the joint with a plastic bottle to keep it aligned while drying...doing it freehand inside the box would likely end in disappointment and I'm not picturing in my mind just how it could be accomplished.   It's not hard to remove the rear wheel which then gives unfettered access to the mud guard which is part of the battery tray so once it's out you're at the box.   Doing it this way would have taken me about half the time...and 1/4 the cursing I'm sure.

With tubes in hand and surgery completed they need to be flexible to be reinstalled...so the only glue I had on hand that had a chance was RC56 though I'm sure there are lots of adhesives that can join rubber with a strong bond. (if you somehow can do this in-situ the adhesive wouldn't need to be flexible)  Once back in the box the shortened stack isn't really being put through much stress....hitting bumps would be the most force they'll see and the RC56 joints are strong enough that I couldn't break it by smacking the stack hard on the desk...so I'm pretty confident they'll stay in place.

The stock inlet trumpets ARE about as large as will fit above the filter...much bigger will be resting on the filter which probably isn't ideal.  Light contact probably wouldn't hurt but if they're making hard contact there's the chance that time will have them damaging the filter.

I did cut a sleeve out of a beer can to try that is thin enough to be left in place after gluing...so even if you can't find store-bought sleeves of the exact right diameter you can cut your own pretty easily.   Make it big enough to need a little pressure to push into the tube with the ends butted and it would hold the joint in perfect alignment for you.   The final and best solution will likely be a 3D printed push in trumpet that could be made of exactly the right size...I don't have a printer (yet!) but am mulling that option over.

Check your E-mail and we'll get the tune straightened out.   When I installed the TEC cans we got a nice little bump in midrange torque with the stock tune.  Changing to the large diameter head pipes offered nothing worthwhile with the stock tune and it needed quite a bit more fuel added via PowerTronic to make it pull hard...which it did...but we still had the drop-off at 6000.   S&S cam timing is more than adequate to eliminate that if the cam duration was the cause...which it wasn't...it's the stacks.   But another round of tuning enrichment was needed to feed the new cam timing and make it pull.   Once the stacks were shortened another couple clicks of fuel were needed so I'm guessing that whatever tune Revelry put in the PT isn't optimum for your engine.   Even if developed on a dyno...I'm not entirely happy with how they run them with such a fast rpm gain as I don't think it's loading the engine as much as you do when...say...pulling up a steep mountain.   I'm VERY fortunate to have one of the steepest mountains in the area right in my back yard and have made hundreds of runs up it over the years doing tuning.  Dyno's can be 'tweaked' to say what you want but you can't BS gravity!

There ARE dyno's that do 'step' pulls where the rpm is stabilized for a few seconds and measurements taken before being allowed to increase then held again the whole way to redline.   This by necessity takes a LOT longer to accomplish than what everyone seems to be using but I think it would give better results.   The usual inertial dyno's can't do this stepped pull...needs an extra water or eddy current resistance device which most Dyno Jet's don't have.

PowerTronic has a bunch of tunes available for whatever mods you have...but I found these to be just a starting point and FAR from ideal on my bike.  Don't worry....when we can look at the tune in your PT we'll have the answers.

[ideola]

I am not seeing the interference issue you are describing with the filter. I will need to create two posts to show you all of the photos of the dissected airbox.

This image shows the right half of the split airbox, with unmodified runner.


This image shows the left half of the split airbox (where the air filter inserts), with cut runner.


This image shows how the unmodified runner attaches to pins molded into the inside of the airbox.


This image shows the pin where the modified runner used to be attached.

[NVDucati]

One of the most informative websites anywhere is Master Bond https://www.masterbond.com/
They are also very generous with their tech and applications support.

[ideola]

This image shows the modified runner with the OEM air filter element installed. You can see there is plenty of clearance. Also note that I removed 3 "sections" of the runner, which is marked by the "rib" that is just before where the runner normal attaches to the "retaining pin"


This image shows the modified runner with my press-in bellmouth inserted. No problem.


This image shows the cross-sectional profile of the press-in bellmouth.


This image shows the ID of the bellmouth.


It strikes me after looking at these images of the bellmouth that if one were *really* concerned about the slight reduction in ID, one could purchase the next size up and simply cut off the end of the bellmouth before the end section that is intended to be clamped OVER a typical inlet runner. The rubber of the OEM runners is very "grippy", so forcing the bellmouth into the opening holds it firmly in place.

[Intybe]

Hi Rob and Ideola, you've reported slight differences inside diameter of the pipe 1.450 and 1.375/36mm respectively, which I guess is because it's flexible so it's hard to ensure it's perfectly circular when measuring and vernier calipers have their own +- error (I wouldn't trust my cheapies as the digital readout wavers all over the place).

Anyway, I've been doing a little more searching for sleaves and found this very thin plastic offering but OD is 1.474" Do you think it'd work.. with a little lube... just to align and support the two sections for gluing?

[Intybe]

Yes...you got the idea of where and what to cut.  If you leave the ribs on each piece it gives you a decently thick area to glue.

After a LOT of thought on this...I'm now of the opinion that removing the airbox to do surgery is probably the best way to go about doing this That's not what I wanted to hear!and much of my tribulations were caused by me trying to take the box out the side rather than straight out the back as the manual instructs.   Even with a larger inlet hole it's going to be difficult working inside the box to get a nice finished product unless a push-on/push-in stack of suitable size comes along.   And then you need to repair the box to be air tightMaybe I'm being naive but I don't see that restoring the cut section of the airbox accurately and airtight after shortening the runners is a big problem - it's a simple cut that can be glued or plastic welded accurately back  in place with clamps and even taped and then sanded with new Dremel 🤗 to perfection

Getting the rubber tube ends joined isn't a real easy thing to accomplish either...I had to sleeve the joint with a plastic bottle to keep it aligned while drying...doing it freehand inside the box would likely end in disappointment and I'm not picturing in my mind just how it could be accomplished.   It's not hard to remove the rear wheel which then gives unfettered access to the mud guard which is part of the battery tray so once it's out you're at the box.   Doing it this way would have taken me about half the time...and 1/4 the cursing I'm sure. Easy for you/ some; what could possibly go wrong for me, the guy who is still messes up on his torque wrenches 🙄 I've got more experience and confidence fixing broken china so naturally I'm leaning towards cutting and gluing in situ.

With tubes in hand and surgery completed they need to be flexible to be reinstalled...so the only glue I had on hand that had a chance was RC56 though I'm sure there are lots of adhesives that can join rubber with a strong bond. (if you somehow can do this in-situ the adhesive wouldn't need to be flexible)  Once back in the box the shortened stack isn't really being put through much stress....hitting bumps would be the most force they'll see and the RC56 joints are strong enough that I couldn't break it by smacking the stack hard on the desk...so I'm pretty confident they'll stay in place. I'll investigate best adhesive - thanks for link NV

The stock inlet trumpets ARE about as large as will fit above the filter...much bigger will be resting on the filter which probably isn't ideal.  Light contact probably wouldn't hurt but if they're making hard contact there's the chance that time will have them damaging the filter.

I did cut a sleeve out of a beer can to try that is thin enough to be left in place after gluing...so even if you can't find store-bought sleeves of the exact right diameter you can cut your own pretty easily.   Make it big enough to need a little pressure to push into the tube with the ends butted and it would hold the joint in perfect alignment for you.   The final and best solution will likely be a 3D printed push in trumpet that could be made of exactly the right size...I don't have a printer (yet!) but am mulling that option over. If you or anyone else has a design for a trumpet I've got a friend with a 3D printer who I'm sure would be keen to give it a go.

Check your E-mail and we'll get the tune straightened out.   When I installed the TEC cans we got a nice little bump in midrange torque with the stock tune.  Changing to the large diameter head pipes offered nothing worthwhile with the stock tune and it needed quite a bit more fuel added via PowerTronic to make it pull hard...which it did...but we still had the drop-off at 6000.   S&S cam timing is more than adequate to eliminate that if the cam duration was the cause...which it wasn't...it's the stacks.   But another round of tuning enrichment was needed to feed the new cam timing and make it pull.   Once the stacks were shortened another couple clicks of fuel were needed so I'm guessing that whatever tune Revelry put in the PT isn't optimum for your engine.   Even if developed on a dyno...I'm not entirely happy with how they run them with such a fast rpm gain as I don't think it's loading the engine as much as you do when...say...pulling up a steep mountain.   I'm VERY fortunate to have one of the steepest mountains in the area right in my back yard and have made hundreds of runs up it over the years doing tuning.  Dyno's can be 'tweaked' to say what you want but you can't BS gravity!

There ARE dyno's that do 'step' pulls where the rpm is stabilized for a few seconds and measurements taken before being allowed to increase then held again the whole way to redline.   This by necessity takes a LOT longer to accomplish than what everyone seems to be using but I think it would give better results.   The usual inertial dyno's can't do this stepped pull...needs an extra water or eddy current resistance device which most Dyno Jet's don't have.

PowerTronic has a bunch of tunes available for whatever mods you have...but I found these to be just a starting point and FAR from ideal on my bike.  Don't worry....when we can look at the tune in your PT we'll have the answers.   Thanks for your explanations and help, Rob - much appreciated 🙏

[Intybe]

I am not seeing the interference issue you are describing with the filter. I will need to create two posts to show you all of the photos of the dissected airbox.

Hi Ideola, thanks for your excellent detailed photos from inside your airbox - they're most helpful! 🙏
It's clear that the aftermarket bellmouth you used fits without interfering with the filter. I only queried this because I notice what looks to be a cutaway of the longitudinal rib right where the filter inserts, one obviously designed into the mould. It's a little hard to see in your pics but I've posted a pic of Rob's (from OP) and mine to show what I'm seeing. I figure the manufacturer/ designers would have made this design change for a good reason. 

[RecoilRob]

Hi Rob and Ideola, you've reported slight differences inside diameter of the pipe 1.450 and 1.375/36mm respectively, which I guess is because it's flexible so it's hard to ensure it's perfectly circular when measuring and vernier calipers have their own +- error (I wouldn't trust my cheapies as the digital readout wavers all over the place).

Anyway, I've been doing a little more searching for sleaves and found this very thin plastic offering but OD is 1.474" Do you think it'd work.. with a little lube... just to align and support the two sections for gluing?

I'm sitting here with one of the chopped out sections of my trumpets and it measures 1.470'ish inside using calibrated Mitutoyo calipers so those sleeves would likely work.

[Intybe]

.... It strikes me after looking at these images of the bellmouth that if one were *really* concerned about the slight reduction in ID, one could purchase the next size up and simply cut off the end of the bellmouth before the end section that is intended to be clamped OVER a typical inlet runner. The rubber of the OEM runners is very "grippy", so forcing the bellmouth into the opening holds it firmly in place.

Hey Ideola, I really admired your work on your bikes, including your attention to detail. I'm also greatly appreciative that you're so meticulous with your documentation and generous with your sharing - Kudos! 🙇‍♂️ You're clearly a perfectionist who goes to great lengths to achieve your vision. So please don't take this as a criticism when I ask about your thinking for using the smaller aftermarket bellmouth that causes a step in the flow of the intake tubes when you couldv'e purchased the next size up and created a seemless transition? That strikes me as just the kind of thing you would've been concerned about? I'm on a steep learning curve about ICE intake runners and their their fluid dynamics, but it seems clear (ok, cloudy) to me that while there'd be no great restriction on air being sucked in, there would be a problem with even a small step down in diameter when the air pulse/ wave bounces back in the other direction. My very amateur understanding is that that back and forth of resonant waves is what runners and airboxes are trying to facilitate as it can have a significant impact on power and performance. 

[GravyDavy]

Hey guys,

If you're going to split open your airbox, any halfway competent fabricator can easily make one-piece metal velocity stacks to whatever dimensions and surface finish you may desire. No need to mess around with cutting apart and patching together rubbery pieces that will inevitably have rough transitions and inconsistent cross-sections, and no need to try to 3D print something smooth that won't deteriorate from fuel mist standoff. Trying to smoothly together bits of the sort of roundish OE rubber tubes seems like the hard way.

Sometimes I'm glad I'm not looking for ways to make my Interceptor faster. I get the appeal, but it's not what I want these days.

[Intybe]

Last question, Ideola: Did you make this cut via the air filter opening when the box was intact? If so, would you kindly post an account of how you did it - eg did you use a mirror or just go by feel along the latitudinal rib? Did you use a hacksaw blade or a knife? How long was the blade? And any advice to make it easier for those of us who wish to replicate what you did? Thanks in advance.

This image shows the pin where the modified runner used to be attached.

[Intybe]

I'm sitting here with one of the chopped out sections of my trumpets and it measures 1.470'ish inside using calibrated Mitutoyo calipers so those sleeves would likely work.

Bingo! Just need 85 members for an order... Give me a call if you wanna make 1200% profit on a $1.50 investment!" 🤣
https://store.visipak.com/1-3-8-x-4-thin-wall-round-clear-plastic-tube/

[RecoilRob]

That's great!   Must buy a box of 86 tubes...each 4' long...when you need about 4" per bike.   That calculates to a box holding enough to do 1032 bikes!   Surely someone somewhere must have a similar tube in a slightly more realistic quantity that might work for us.

[ideola]

Last question, Ideola: Did you make this cut via the air filter opening when the box was intact? If so, would you kindly post an account of how you did it - eg did you use a mirror or just go by feel along the latitudinal rib? Did you use a hacksaw blade or a knife? How long was the blade? And any advice to make it easier for those of us who wish to replicate what you did? Thanks in advance.

That was my sacrifice / experimentation airbox, so I made no effort to get a clean cut. The airbox was already split (I bought it that way from a salvager on eBay) so I didn't have to sacrifice a good box.

For the "production" box that went into the CGT 865 build, I bought another salvage airbox from eBay that was so clean it could have been factory fresh. The original air filter element was in place and looks brand new. I have no idea what happened to the donor bike, but it must have been very low miles 

Anyway, for the production version, the airbox was obviously out of the bike. Once I knew where I wanted to cut, I used a hacksaw blade (detached from the hacksaw itself) and just held it by hand. I accessed the runner thru the air filter opening, and was able to get about 3/4 of the way through the near side runner with the blade. I then used a pair of needle nose pliers to pull the runner off of the retaining tab (it's just an interference fit), and twist the runner down and around into a position where I could finish snipping thru the rubber with a pair of heavy duty shears. I then repeated the process on the other side. The key is knowing where that tab attaches to the pin, and working the runner off of it so you can take advantage of its flexibility to get it into a position where you can finish the surgery.

It was a bit tedious, but patience and persistence always pays off. Obviously easier to do with the box out of the bike so you can change your angle of attack from the outside, but certainly not impossible to do by leaving the airbox in the bike.

In my case, the airbox was going to come out anyway because of the 865 build, bigger throttle bodies, etc., so it was a no-brainer to just buy a couple of salvage boxes to experiment on. I think all-in, I spent $50 on the two salvaged airboxes.

{EDIT}
The other advantage of having the airbox OUT of the bike is that I was able to wrap it in self-adhesive heat shield material designed specifically for this purpose. That part would be near impossible to do a very good job of with the airbox in the bike.

[Intybe]

That's great!   Must buy a box of 86 tubes...each 4' long...when you need about 4" per bike.   That calculates to a box holding enough to do 1032 bikes!   Surely someone somewhere must have a similar tube in a slightly more realistic quantity that might work for us.

😁
ClearTec has a more budget friendly minimum order of 15 for $37:
https://store.cleartecpackaging.com/clear_en/utw.html?Item=PRT00091&Nominal_Size=1.37500&Length=48.000

Alternatively, we could hit manufacturers up for a sample... Hmm, what's our our business? Florists, right? Legit, specialising in long stem roses for bikie molls 😎😎 Sturgisrose.com!
https://www.visipak.com/request.html
https://cleartecpackaging.com/plastic-tubing.html
https://www.petropackaging.com/plastic-tubes/clear-plastic-tubes-sealed-bottom/open-end-plastic-tube/

But sweetest of all, for less than a reimbursable tenner, one of our UK valentines, secretly admiring our endeavor, might like to indulge us 💕🙏😍
https://www.sinclair-rush.co.uk/categories/visipak-packaging/round-clear-packaging-tubes/?pn=32661

💐🌷🌸🌹🌺🌻

[Intybe]

That was my sacrifice / experimentation airbox, so I made no effort to get a clean cut. The airbox was already split (I bought it that way from a salvager on eBay) so I didn't have to sacrifice a good box.

For the "production" box that went into the CGT 865 build, I bought another salvage airbox from eBay that was so clean it could have been factory fresh. The original air filter element was in place and looks brand new. I have no idea what happened to the donor bike, but it must have been very low miles 

Anyway, for the production version, the airbox was obviously out of the bike. Once I knew where I wanted to cut, I used a hacksaw blade (detached from the hacksaw itself) and just held it by hand. I accessed the runner thru the air filter opening, and was able to get about 3/4 of the way through the near side runner with the blade. I then used a pair of needle nose pliers to pull the runner off of the retaining tab (it's just an interference fit), and twist the runner down and around into a position where I could finish snipping thru the rubber with a pair of heavy duty shears. I then repeated the process on the other side. The key is knowing where that tab attaches to the pin, and working the runner off of it so you can take advantage of its flexibility to get it into a position where you can finish the surgery.

It was a bit tedious, but patience and persistence always pays off. Obviously easier to do with the box out of the bike so you can change your angle of attack from the outside, but certainly not impossible to do by leaving the airbox in the bike.

In my case, the airbox was going to come out anyway because of the 865 build, bigger throttle bodies, etc., so it was a no-brainer to just buy a couple of salvage boxes to experiment on. I think all-in, I spent $50 on the two salvaged airboxes.

{EDIT}
The other advantage of having the airbox OUT of the bike is that I was able to wrap it in self-adhesive heat shield material designed specifically for this purpose. That part would be near impossible to do a very good job of with the airbox in the bike.

Thanks Ideola, that's an excellent and helpful account of how you cut the tubes. Could I trouble you for a detailed pic of the retainer tab attached to the pin, please? The tab on the runner is on the third section back from the end of the OEM bellmouth, right, so I'll also lose it when I remove sections two and three. I guess there's no great risk of the runners flopping about in the airbox unattached, as they can support their own weight with the help of the bellmouth resting in it's new place on top of the filter, right?
Ps, speaking of bellmouths, you mentioned one could size up if one wanted a seemless transition from bellmouth ID to runner ID, but judging by your photo, which shows a very snug fit of your aftermarket belmouth between the roof of the airbox and the filter, I don't think a larger one would fit, would it?

[Intybe]

I now have three potential options for the internal sleaves to reconnect OEM trumpet to the shortened runners:

1. Aluminum hose connector, 38mm OD, which is a little large, but worth a try: https://www.aliexpress.com/item/2052245035.html?spm=a2g0o.order_detail.0.0.1553f19cSZjUZ

2. Clear thin walled 1 3/8" ID plastic tube - Absent a UK forum valentine willing to repost a section, I persisted with the parent company and learnt they have an Au manufacturer (which didn't come up in my searches). On the end of the local number was a helpful customer service lady who agreed to send me a sample for proof of concept work 🙄 The local minimum order was significantly larger than the parent comany's. When she asked for my company details, I gave the name of the high school where I work 👀 Fortunately, she didn't enquire further 😅 She rang back a week later to inform me the item was currently out of stock and that they'd need to manufacture a batch to send me a sample: It wouldn't arrive til June 30. Would that be OK?  Bless her! 🙏

3. 1 3/8" OD Stainless steel exhaust section, custom stretched locally to exact OD

Before I start the hack, I want to ensure that whatever dust and debris taht results from my cutting and sanding of the intakes inside the airbox doesn't cause problems elsewhere. Obviously, I'll stuff the intakes themselves with rag so rubber dust/debris doesn't end up fouling the FIs or heads, but I also intend to seal with gaffa tape # 1, 2 and 3 in the attached pics. Have I missed any openings that should be taped off?

Ps, #4 is not listed on the manual's airbox diagram and I can't feel a hole when I reach inside airbox with my finger. Does anyone know what it is and whether it also needs sealing? 

[RecoilRob]

#4 is just a drain hole and already plugged so no worries trying to block it from the inside.   I think it's there in case you submerge the airbox and some vehicles have a little rubber one-way valve to allow water to flow out but block airflow back in.   At first glance the clear flexible plastic plug appears to be such a valve...but I just pulled my spare off to check and it is not open.

Just vacuum out the box when you're finished and it'll be fine methinks.

[Intybe]

Thanks Rob, that's good to know. When I've all my bits and bobs assembled, I'll get to work.

Does anyone have experience of an of a adhesive for rubber, plastic, metal that acts as lubricant when wet, so I have the best chance of sliding the tight fitting sleave into the runners, where it can then bond? Hairspray is often used to ease tight fitting grips onto bicycle handlebars, so this is my natural choice. But as ideola mentioned, it gets quite hot inside the airbox... Does hairspray liquify under heat? I wouldn't want the trumpets blown off in midsummer. Ladies, and blokes with comb-overs: How does your hair hold up under lights, in the tropics, etc?   

Ps, Various 3D printer forums discuss hairspray as cheap hack, but it was all Greek to me: https://3dprinting.stackexchange.com/questions/11182/why-does-hairspray-work-as-an-adhesive-for-abs 

Pps, Am I revealing too much by admitting I found model railfans' discussion of hairspray to set tree foliage, grass and various flocks quite soothing? https://www.railpage.com.au/f-t11396625.htm

[Cazzy_R]

 I've used hairspray to good effect on both the rubber to metal and rubber to plastic clutch side and throttle side handlebar grips respectively. Works well.
 Not sure how it would be affected by heat and fuel vapour though.
 If a bellmouth did work loose it would be an inconvenience rather than a disaster as they couldn't be injested. Could they?
 Could you use a zip tie to provide some additional clamping force? Or.A bead of superglue by the bellmouth/ inlet hose transition?

[Hoiho]

Sika Body Sealant. Good glue that stays fluid long enough to move it around

[NVDucati]

Thanks Rob, that's good to know. When I've all my bits and bobs assembled, I'll get to work.

Does anyone have experience of an of a adhesive for rubber, plastic, metal that acts as lubricant when wet, so I have the best chance of sliding the tight fitting sleave into the runners, where it can then bond? Hairspray is often used to ease tight fitting grips onto bicycle handlebars, so this is my natural choice. But as ideola mentioned, it gets quite hot inside the airbox... Does hairspray liquify under heat? I wouldn't want the trumpets blown off in midsummer. Ladies, and blokes with comb-overs: How does your hair hold up under lights, in the tropics, etc?   

Ps, Various 3D printer forums discuss hairspray as cheap hack, but it was all Greek to me: https://3dprinting.stackexchange.com/questions/11182/why-does-hairspray-work-as-an-adhesive-for-abs 

Pps, Am I revealing too much by admitting I found model railfans' discussion of hairspray to set tree foliage, grass and various flocks quite soothing? https://www.railpage.com.au/f-t11396625.htm

The short answer is that the propellant evaporates leaving only the sticky stuff behind. Often that propellant is LPG / propane. That is why hairspray cans make handy little blow torches. Here is a short but informative look at aerosol propellants: https://www.mosil.com/blog/aerosol-and-its-propellant/

Considering the effort required ...  some side-by-side bench testing might be in order. Get some sample materials and hang weights on the different method choices, add heat.  One question with adhesives is always, do you ever want to remove the object in the future?  Flammable (not all natural) hairspray may very well end up being your choice. Or it might be as simple as soapy water if the fit is tight enough that a full sized person needs lubricant to get it on ... and the materials used don't rot ...  and one of the parts doesn't expand in moderate heat ... it might just be easy.

[Intybe]

Thanks Cazzy and Hoiho. Super strength hairspray is apparently available at dollar shops and Sika is cheap at Bunnings. Justonemore also suggested enamel paint, which reminded me of childhood brushes stuck to benches 🙄 Speaking of which, DV, side by side tests, a la Project Farm, sounds like my kind of fun 🤓

https://www.youtube.com/watch?v=SEqG6GPY4b0

I'm taking a couple of weeks break - and no, I won't be giving my mobile, not because I'm too busy but because I don't want my emptiness interupted. Anyway, I trust you can figure out problems quicker without me.* But I will leave you with a pleasant ride through quintessential aussie back roads on modified twins - both cammed and tuned, one an HC 650, the other an 865. Enjoy!

https://youtu.be/yrNjA5jo8Fk

* So glad to have you back, Phil. Reading the forum last night was the best! 🤗

[Intybe]

Just a quick update: I had some pancakes for breakfast, which inspired me to get to work on the intakes. But cutting in situ requires odd body positions and quite a bit of core strength, think pilates, so best done on an empty stomach.

Apart from gut busting, the hardest bit is trying to use one's hands/ tools through the small opening, iniside small irregular box, and see what they're doing. I used a sharp kitchen pairing knife to cut the tubes (and my hands 😢) then trimmed the edges with a small pair of side snips. I cut in sections, ie the near side trumpet came off first, then its middle section, then far side trumpet, then its middle section. It took an hour or so and I still need to trim a 15mm section of the circumference of the near side of the near side trumpet (obscured in the pic by the facing side of airbox). It's frustratingly hard to get too with snips and I can't see what I'm doing. 

The good news for part 2, rejoining the trumpets to the shortened intakes, is both the plastic tubing and the alu sleeves fit perfectly with just the right amount of resistance. Not yet sure which to use. The alu sleeves are tighter, mostly due to their lips, but if I use them I'd cut those off to improve airflow. Given the trumpets are lightweight, 20mm sleeves glued in, should be sufficient, right?

[RecoilRob]

Well done lad!   Seeing what you're working with, that plastic tubing looks pretty sweet.   If you cut it into 2" sections, then glue into the loose stack mouth about an inch....let dry, then lube up with glue and shove it into the body of the stack still in the box until the stack sections touch.   The aluminum would also work just fine even with the ribs left on.   I doubt you'd notice any difference with the rolled rib removed as the airflow right along the wall has an area of stagnation right along the surface.   It's mostly the unshouding of the inlets at work here so minor interior imperfections shouldn't be a big problem.

I'm getting really excited to hear what you think about this mod...

[Intybe]

Thanks Rob, I went with the plastic sleeves, which are fixed into the stack mouths with non-foaming clear Gorilla glue and curing overnight. A camfered inside sleeve edge makes for a super smooth transition from the rubber mouth.

Ps, I tried to tidy up the rough edge on the circumference of the near side of the left hand shortened runner, but just couldn't reach it. I'll have another crack tomorrow - nail clippers might work. But if still no joy, I'll cut a mirror section out of the mating edge of the stack mouth for a flush fit. (Thanks, Kevin, for this suggestion 🙏)
 

[Intybe]

Hi Rob, I'm not sure if you're up (nearing midnight where you are; 2pm here) but I have a quick question:
 
OEM bellmouths each have a cutaway on ~1/8 of their circumference. When the runners are stock I believe these cutaways face the top of the airbox (so they can clear the airbox ceiling?) However, when you rejoined your LH bellmouth to the runner you rotated it 180 degrees, so that the cutaway is now facing down (see #1 in attached image, which one you attached to OP; I can't see the RH bellmouth clearly enough in your images to determine whether you did the same to it, as well? see #? in image). Did you do this intentionally, to ensure the filter had sufficient clearance and doesn't snag on the stacks?

I can also see that you cut away some of the downward facing longitudinal rib on both LH and RH runners (see #2 & #3). Was this done for the same reason, to improve clearance and prevent the filter snagging? Is it necessary?

I'm reluctant to do a test fit (ie without glue) on mine because they're a tight fit and I'm afraid I might not get them out again; and once I glue them in place (for better or worse 😅) they're definitely not going anywhere.

[Intybe]

Nevermind previous request, Rob. I went and had another go. I pushed the bellmouths in a few mm with the flat edge down and without glue, then replaced the filter. It didn't snag. So I used my finger to rub clear Gorilla glue around the exposed part of the sleeves, then pushed them all the way in to mate. Job done!

As mentionied, the top and nearside edge of the LH runner is the hardest to make a clean and accurate cut on because it's impossible to see and reach with tools I had at hand, including nail clippers. (see Ideola's pic below; I dare say, it would be a moment's work for a dentist.) The rough edge protruded a 1-2mm rearward of the lateral mating rib, so I cut a 1-2mm into the corresponding trumpet mating edge for a neatish fit, which no one will ever see, so lets just say, it's perfect!

I'll leave it overnight for the glue to cure and take it for a ride tomorrow, if the weather's fine. Hopefully, there'll be an opportunity to do phase 3 roll-ons with the GT soon (see https://forum.classicmotorworks.com/index.php?topic=33548.0). Kevin's also agreed to ride both bikes back to back and give his impartial assessment regarding whatever differences he percieves.

[RecoilRob]

Hi Rob, I'm not sure if you're up (nearing midnight where you are; 2pm here) but I have a quick question:
 
OEM bellmouths each have a cutaway on ~1/8 of their circumference. When the runners are stock I believe these cutaways face the top of the airbox (so they can clear the airbox ceiling?) However, when you rejoined your LH bellmouth to the runner you rotated it 180 degrees, so that the cutaway is now facing down (see #1 in attached image, which one you attached to OP; I can't see the RH bellmouth clearly enough in your images to determine whether you did the same to it, as well? see #? in image). Did you do this intentionally, to ensure the filter had sufficient clearance and doesn't snag on the stacks?

I can also see that you cut away some of the downward facing longitudinal rib on both LH and RH runners (see #2 & #3). Was this done for the same reason, to improve clearance and prevent the filter snagging? Is it necessary?

I'm reluctant to do a test fit (ie without glue) on mine because they're a tight fit and I'm afraid I might not get them out again; and once I glue them in place (for better or worse 😅) they're definitely not going anywhere.

The stacks are hard against the airbox roof because they're at an upward angle and the flats there for that reason.  Shortened back to where we're going has about the best clearance all around that we're going to get in that airbox using the stock filter design.  If the mouths were symmetrical they won't hit the air filter but it for sure wouldn't hurt to put both down.   My one stack is that way just because my cuts weren't totally square and it just fit better that way...not for any design reason.

Just had a funny thought:  The Commies are (in)famous for copying things and this would be just the kind of thing they'd dutifully reproduce without having any idea why it's like that.  

[RecoilRob]

Nevermind previous request, Rob. I went and had another go. I pushed the bellmouths in a few mm with the flat edge down and without glue, then replaced the filter. It didn't snag. So I used my finger to rub clear Gorilla glue around the exposed part of the sleeves, then pushed them all the way in to mate. Job done!

As mentionied, the top and nearside edge of the LH runner is the hardest to make a clean and accurate cut on because it's impossible to see and reach with tools I had at hand, including nail clippers. (see Ideola's pic below; I dare say, it would be a moment's work for a dentist.) The rough edge protruded a 1-2mm rearward of the lateral mating rib, so I cut a 1-2mm into the corresponding trumpet mating edge for a neatish fit, which no one will ever see, so lets just say, it's perfect!

I'll leave it overnight for the glue to cure and take it for a ride tomorrow, if the weather's fine. Hopefully, there'll be an opportunity to do phase 3 roll-ons with the GT soon (see https://forum.classicmotorworks.com/index.php?topic=33548.0). Kevin's also agreed to ride both bikes back to back and give his impartial assessment regarding whatever differences he percieves.

Well done!   Neat job...those plastic sleeves look to be the ticket!   And you have a forum's worth in your sample...   Very curious to hear your riding impressions as this is not just about making more horsepower on the dyno...it really changes how the throttle works as the stacks are having a LOT easier time getting air.  One of the things we've done is increase the volume of the airbox by the amount of tube we've removed, and every little bit helps when it's a limitation.   

Test riding is one of my favorite activities....make a change then you get to ride the heck out of it 'for test purposes only' ...officer.   Never worked before...but worth a try if you need it.

[Hog Head]

So did anyone ever 3D print a slip over bellmouth?

[RecoilRob]

Not as far as I know.   That would simplify the mod for sure if something like that existed.  I've been resisting the temptation to get a 3D printer just because when something new comes into the house I end up with 3 or 4 of them and just don't have the room right now.

[Quest]

Not as far as I know.   That would simplify the mod for sure if something like that existed.  I've been resisting the temptation to get a 3D printer just because when something new comes into the house I end up with 3 or 4 of them and just don't have the room right now.

Actually, some years back I bought one for a college kid I knew. It was one of the cheap "erector set" type. We played with it as the whole 3D thing was new. I still know him. He is out of college and now has current issue, multi material, printer and is always interested in making something new.
I'm not suggesting that was a strategy ... 

[danketchpel]

Thinking about cutting the tubes, has anyone tried a hot wire or hot knife approach?

I agree, 3D printed belmouths would be the ticket.

[Intybe]

Sorry but this is just for Robert, aka Hog Head.
I'm posting here because the forum doesn't allow DMs to include attachments.
Here's a pic of what's in the box.

[danketchpel]

So did anyone ever 3D print a slip over bellmouth?

It just so happens that I bought a nice 3D printer for my plastic vacuum forming business and am intending to print some bell mouths for my bike. I modeled this up just from imagining the actual size but I'd adjust it to suit once I have my intake tubes cut. I'm going to see if I can do it using a hot knife approach.

I would start with the stock bell mouth geometry but if anyone has any insights on bell mouth geometry please speak up.

My intention is they will slip over and be glued to the cut rubber intake tubes and the inside diameter will match the stock tubes for a smooth transition.

[RecoilRob]

There is very little room above the air filter for the inlet bells...best you can do is have them almost touching the filter and in contact with the roof of the airbox.   The radius of inlet doesn't make so much difference just so it's smooth and lets the air flow in without creating much turbulence.

The biggest advantage of the shortened tubes is the additional airbox volume you create....the stock airbox is really small so any volume increase will help it breathe easier.   And having bells that just fit over the cut tube means you don't have to cut back so far as I did to get surfaces able to be glued.   Just cut back 2" and however thick the bells end up which will be easier to reach for sure.   Good luck!

[danketchpel]

I decided to pickup a used complete airbox off eBay to play around with.

I want to see if I can develop a "fairly straight forward" method for cutting the intake tubes, then work out the 3D printed bell mouths.

I reviewed all the hot knife cutting blades I could find and none really have the right shape to cut the tube closest to the filter opening. It seems like you need a curved blade similar to a linoleum/carpet knife to be able to reach up there. It might require just using a std carpet knife and heating the blade with a torch, something like that. They are cheap so I don't care about ruining the blade temper etc.

Something I observed, there is room for the upper rear portion of the airbox wall can be pushed out towards the rear over the battery, but I don't think that modification would be any easier in the end.

[Quest]

I decided to pickup a used complete airbox off eBay to play around with.

I want to see if I can develop a "fairly straight forward" method for cutting the intake tubes, then work out the 3D printed bell mouths.

I reviewed all the hot knife cutting blades I could find and none really have the right shape to cut the tube closest to the filter opening. It seems like you need a curved blade similar to a linoleum/carpet knife to be able to reach up there. It might require just using a std carpet knife and heating the blade with a torch, something like that. They are cheap so I don't care about ruining the blade temper etc.

Something I observed, there is room for the upper rear portion of the airbox wall can be pushed out towards the rear over the battery, but I don't think that modification would be any easier in the end.

You might consider a piece of wire (thickish) or the stem from a pop rivet. Slip it into the end of your electric soldering gun and tighten. You can bend it to suit and it will cut the tubes in a single slow stroke.

[danketchpel]

You might consider a piece of wire (thickish) or the stem from a pop rivet. Slip it into the end of your electric soldering gun and tighten. You can bend it to suit and it will cut the tubes in a single slow stroke.

Yup, very much along the lines of what I've been thinking.

I hoping to work out something that others could reproduce easily with the airbox in the bike.

[danketchpel]

Update, I received my spare airbox. It's in nice condition and complete.

A friend of mine suggested using a cable saw and that may have potential. I picked one up from Home Depot meant for cutting plastic pipe. I'll play around with just threading it in and see how the motion and alignment might work out. I suspect I'd need some sort of metal protector for the opening of the airbox so it doesn't get cut. I also picked up a carpet/linoleum knife to see how that shape might work out.

Today I did a test print of a sample Bellmouth that I'd modeled just to see how that shape might print. I know it's not the final size and quite possibly the shape. I tried Nylon filament with 100% fill to see how sturdy it might feel. The printing profile is not there yet and this was printed in a rather coarse resolution to speed things up. I'm still considering the best material for this application. I'm pretty new at 3D printing and have learned I have a lot to learn, LOL.

[Red Rider]

So I may have missed it, but what is the advantage of this over, say, removing the airbox and adding pod filters?

[Hog Head]

This image shows the right half of the split airbox, with unmodified runner.

How did you split the airbox, as mine seems welded together and I am a bit reticent to use a brute force approach

[Racer57]

https://gearhonest.com/do-velocity-stacks-work-on-motorcycles/

[TSTM]

Reads like a Tellsell commercial 
And how about this claim?

It keeps the intake valve open during the piston begins.

[Racer57]

Reads like a Tellsell commercial 
And how about this claim?

I saw that also. No idea wtf their trying to say.  LOL

[Quest]

How did you split the airbox, as mine seems welded together and I am a bit reticent to use a brute force approach

I just saw this. He isn't very active on the forum, sadly. I'm pretty sure he used a band saw on a extra airbox so as to calculate his plan.
Are you going to remove the box? Or do it in situ?

[Johnny Ro]

This is on my list to do, after suspension. I will buy a used airbag assembly and figure it out without interrupting bike rides.

[Hog Head]

I just saw this. He isn't very active on the forum, sadly. I'm pretty sure he used a band saw on a extra airbox so as to calculate his plan.
Are you going to remove the box? Or do it in situ?

I removed the engine for the 865 kit - yes I know it can be done in frame, but not that difficult to remove, makes the top end work easy, and lets me do a few other things at the same time.
Easy out the front when nothing is in the way.........

[Quest]

I removed the engine for the 865 kit - yes I know it can be done in frame, but not that difficult to remove, makes the top end work easy, and lets me do a few other things at the same time.
Easy out the front when nothing is in the way.........

+1