[AzCal Retred]

I'm really feeling liberated by the feeling that there is nothing wrong with modifying these bikes.
Mac350 - It's great to see you jumping in and getting so much pleasure out of "fettling" this machine. It's great fun removing the lumps & warts and seeing the little performance gains. Big pieces that move real slow make for accessible technology!

Does that have an easy to calculate relationship to HP?
Nothing is ever that simple. Wave action has a lot more to do with exhaust scavenging. Every section change creates a reflection wave. Smoothing off that ledge cleans up the resonant path. Ideally there will be a negative pressure wave sitting in the exhaust port when the exhaust valve cracks open, evacuating the cylinder so that when the intake opens there will be a partial vacuum setting in there, allowing in more fresh charge. Big picture, tuned overall internal length determines the RPM range the exhaust scavenging effect happens at, longer is good for lower RPM. If it was just driven by "back pressure" from pipe diameter restriction we could get max power by removing the pipe entirely. Proper scavenging from tuned length increases mass flow thru the system and increases power output. It's a black art! 

[Mac350]

Wave action has a lot more to do with exhaust scavenging. Every section change creates a reflection wave. Smoothing off that ledge cleans up the resonant path. Ideally there will be a negative pressure wave sitting in the exhaust port when the exhaust valve cracks open, evacuating the cylinder so that when the intake opens there will be a partial vacuum setting in there, allowing in more fresh charge. Big picture, tuned overall internal length determines the RPM range the exhaust scavenging effect happens at, longer is good for lower RPM. If it was just driven by "back pressure" from pipe diameter restriction we could get max power by removing the pipe entirely. Proper scavenging from tuned length increases mass flow thru the system and increases power output. It's a black art! 

I remember a school friends old C90 that we used to hack around his large back garden and one day the manifold broke off and it ran terrible with no exhaust. That was the first time I heard mention of back pressure! And today is the first time hearing of scavenging.

Just looking it up more and I'm reading and I think understanding so far put simply there is exhaust gas moving out of the cylinder plus a faster pressure wave which presumably hits the gas in front from the previous combustion causing that to accelerate which further reduces the pressure in the area behind it which pulls the following expelled gases and so on.

So as long as the pipe is long enough for multiple waves to be within it there will be an increased vacuum effect caused by the pressure waves. Presumably then for high revving it can get away with a shorter pipe and low revving could do with longer. But then not so long that the gases aren't moving under their own steam before they exit. If they have to be pushed presumably that will mess things up further back.

Narrower pipe said to be better for low RPM. Makes sense as in a wider empty pipe the pressure wave won't have such an impact on less densely packed gases. Presumably that's where "back pressure" term comes from though I noticed some people sort of debunking that its even a thing. Maybe should be said as "Something dense for the pressure wave to hit."

I saw something about a pressure wave "rarefracting" from the exit of the pipe back to the valve but it made no sense and seemed unnecessary so i'll discount it unless someone says its legit.

Any restriction will slow down the pressure wave which will reduce the negative pressure in the area behind it that is pulling the next wave so mufflers and sharp kinks = bad.

Header design to factor in scavenging seems mostly related to multi-cylinder headers replacing a basic all in one manifold which makes sense. Not such a worry for our singles.

From what I can gather it seems a certainty that having a restriction from 30mm manifold to 25mm front section followed by a 30mm pipe will be bad in either low or high RPM. Seems that having a straight through smooth pipe with no disturbance is ideal. How mine was from the factory would have been about 20mm bearing in mind that the pipe couldn't align properly with the misaligned hole in the port. Bizarre to think it must have been a replicated error for as many heads that were made from that mould and presumably the moulds are made from moulds...

Seems like we don't need to worry about having a too big pipe for low range power, all the exhaust calculators online I tried think we need about a 2" exhaust pipe! So I'll be on the phone to Hitchcock's tomorrow then unless there are other options for buying an unrestricted header I should consider?

[Mac350]

You know how fresh eyes can sometimes be useful. Is a lot of whats talked about exhausts claptrap? This all sounds like junk physics - https://www.cycleworld.com/everything-youve-ever-wanted-to-know-about-motorcycle-exhaust/ "That negative wave enters the cylinder, at first extracting inert exhaust gas from the combustion chamber above the piston, then entering the intake system to cause intake flow to begin entering the cylinder even before the piston has started to move down on its intake stroke."

what is a negative pressure wave that can enter things?? Like a moving black hole? Can't he just say the gases moving away and the valve shutting behind it created negative pressure otherwise known as a vacuum and that will suck air through if it can when the valves are at overlap.

A state would exist if the vacuum's negative pressure is not equalised by gases coming through from the engine where some gases will be drawn back from down the exhaust once the forward momentum is less than the negative pressure of the vacuum. That could create a bit of back flow *of particles* but thats not at all "a negative pressure wave"

This person https://burnsstainless.com/blogs/articles-1/exhaust-header-theory says a rarefaction wave travels the whole length of the exhaust pipe only as the pressure wave exits. Its just not how it works. Handy diagram here. https://en.wikipedia.org/wiki/Rarefaction The pressure compresses and decompresses as it travels  He seems to then mix up that wave aspect with particle back flow that could happen to fill the vacuum.

A pressure wave (a wave of negative-ness no less) won't bounce back off an open end of a pipe where the pressure is released and all the way back up the pipe against the flow and into the cylinder to extract things. BBB

[Mac350]

I've just thought of how to convince anyone the rarefaction going back to the cylinder idea is nonsense. Because you might be thinking "But what if the pressure wave moves so fast it exits the end of the exhaust before it has decompressed the bit behind it, isn't there a vacuum (of sorts/equilibrium) from the pressure wave to be back filled?"

Keep in mind the gases coming from the engine are a fluid flowing down a pipe. The pressure wave exits the cylinder faster than the gases and travels down the pipe through whatever is already there ahead of it while the gases from the same combustion follow behind.

A pressure wave moving through a fluid compresses the particles as it barges through them and as they are squeezed just ahead they are therefore a lower density/compression just behind (rarefaction), both sides are released once the wave has passed through, that releasing includes some particles going back away from the wave until they are are all back into their previous resting state.

The key thing to bare in mind is the only particles that will be released and can travel back upstream are the ones it has already compressed IE the ones the wave has moved through which were ahead of it when it left the cylinder. No matter how slowly the gases were moving, if they were flowing down the pipe at all then it is impossible for the release to be in particles at the cylinder at the time when the pressure wave exits the exhaust. It can't act on particles that came out of the cylinder behind the pressure wave or since. Only the ones it has passed through.

the pressure wave could bounce off a surface and move in another direction but that is something separate.

[Mac350]

Curiosity won and I tried to get the barrel off. Undid the nut, couldn't get the barrel to budge. Tried going around it with a leather faced hammer as much as i dared but gave up. The 5 through studs all move a tad when pushed so its just the gasket baked on. The 2 stud holes nearest the push rod tube are full of oil which seeped up.

I measured the bore in place and widest I found was 70.081mm against a 70.15 service limit. I only have telescoping bore gauges so not the most accurate way but got pretty consistent results in 5 places I measured. The top part where the piston rings don't touch was 69.896mm. There was a just discernable sideways movement of the piston but I have to assume its within the 0.3mm limit as i can't measure the piston.

If there is a trick to getting the barrel off please let me know!

I'd like to have a nice shiny piston top but I don't like the idea of brushing it off into the gap above the piston rings which seems inevitable if i do it in place... It seems like the least harm would be leaving it as it is

[Adrian II]

How about a little smear of grease around the edge of the crown with the piston at TDC, while you use a flat scraper to get the carbon off? Wipe clean, then rotate the engine a few degrees to mop up any residue off the top of the bore. Or soak the piston top in Redex.

To avoid the risk of damaging what sounds like an OK set of piston rings, I would leave the barrel on, but if it does need to come off, you might need to slacken off the top two crankcase studs nearest the crankcase mouth, but giving the cylinder base flange a good seeing to with a hot air gun or carefully applied blowlamp ought to be enough to soften any gasket goo. I don't think the factory used green Hermetite.

A.

[Raymond]

If you decide to remove the barrel. Yes to loosening the two crankcase studs that run near the barrel mouth. After that, and checking the nut above the tappet cover was removed, had to use a piece of wood and gentle taps against the barrel & lowest fin to get it to move. As soon as there was slight movement, it was wiggling the casting to and fro till it was happy to lift up, up and away. Catch the piston & rod as they come free.

[Mac350]

thanks for the ideas but yes will leave it in place for now. It does seem like everything has had only minimal wear so far so will just clean and put back together.

[Mac350]

How about a little smear of grease around the edge of the crown with the piston at TDC, while you use a flat scraper to get the carbon off? Wipe clean, then rotate the engine a few degrees to mop up any residue off the top of the bore.

that sounds like a very workable plan thanks.

[Mac350]

Happy days over here. I'm converting an extension into a workshop partly inspired or at least sped on by having this bike. Took the bowed and damaged chip board floor up expecting dirt same as rest of the house, but revealed an already concreted floor! Just need to grind it flat and paint it with something oil proof.

[tooseevee]

If you decide to remove the barrel. Yes to loosening the two crankcase studs that run near the barrel mouth. After that, and checking the nut above the tappet cover was removed, had to use a piece of wood and gentle taps against the barrel & lowest fin to get it to move. As soon as there was slight movement, it was wiggling the casting to and fro till it was happy to lift up, up and away. Catch the piston & rod as they come free.

     I remember when I had my barrel off I hammered & hammered (worrying about breaking a fin) until I got sick of it & my hands were killing me. So the next day I did what I did on the head. Sat & tapped a thin putty knife round & round into the gasket until it finally freed.

     The engine had Very low mileage at that time (2014) & I don't know what kind of cement they used, but it certainly worked. Spent hours cleaning those gaskets off with various chemistry experiments & careful scraping.   

[Mac350]

     I remember when I had my barrel off I hammered & hammered (worrying about breaking a fin) until I got sick of it & my hands were killing me. So the next day I did what I did on the head. Sat & tapped a thin putty knife round & round into the gasket until it finally freed.

     The engine had Very low mileage at that time (2014) & I don't know what kind of cement they used, but it certainly worked. Spent hours cleaning those gaskets off with various chemistry experiments & careful scraping.

I'm glad I didn't manage now, scrapping the gasket off worrying about bits falling into the crankcase or damaging the face, not much fun. I've got an old can of gasket remover spray but I remember it not doing much when it was new. Did you find anything helpful other than scrapping?

[tooseevee]

I'm glad I didn't manage now, scrapping the gasket off worrying about bits falling into the crankcase or damaging the face, not much fun. I've got an old can of gasket remover spray but I remember it not doing much when it was new. Did you find anything helpful other than scrapping?

    I used everything from acetone to gasoline & kerosene. Let it sit overnight. Mostly careful diligence to not gouging. Used putty knife & old wood chisel then emery cloth. Mostly I remember my hands hurting. I used grease only on the base & head gaskets & RTV around the push rod holes.

    No oil weeps later, but very low mileage. Never ever got near the 500 mile retorque.