Ace, your mod is very inexpensive and could be fun to tinker with for a while. But is there any way to test these tubes on the bike to generate some data to see if the effort is worthy? Maybe hook up a vacuum gauge and take some readings? Maybe add a fan to simulate a forced air condition, then compare it to stock figures and so on? Having data to prove the value of something is half the fun of the chase to improve something.
Well, we start off by doing some calculations.
Here are some of the basic formula calcs
Pulse in these calcs is the same as reflected wave.
Intake Tract Tuning for RPM = (1100 x Half Intake Cam Duration x 0.960)/Length = 2nd Pulse
(1100 x Half Intake Cam Duration x 0.705)/Length = 3rd Pulse
(1100 x Half Intake Cam Duration x 0.538)/Length = 4th Pulse
Intake Tract Tuning for Length = (1100 x Half Intake Cam Duration x 0.960)/RPM = 2nd Pulse
(1100 x Half Intake Cam Duration x 0.705)/RPM = 3rd Pulse
(1100 x Half Intake Cam Duration x 0.538)/RPM = 4th Pulse
The earlier pulses like 2nd pulse are strongest, and then get progressively weaker as they increase to 3rd pulse, 4th pulse, and so on.
Nascar tunes to 3rd pulse.
This gets you some intake tract lengths to look at for starting points, and the challenge is to try to find where you are going to put all that intake length once you figure out how long it's going to be. And it IS gonna be long. the lower the rpm that you target, the longer the tube must be. And they get real long at the low rpms that we're targeting.
So, then we just probably try to get 4th pulse if we can, and there's realy not enough room for anything that looks relatively "normal" for that length either, so we "could" just try for as long as we can fit. That's what I did.
I do have a full-length 4th pulse tube here that I'm going to strap on the bike and see what it gets me. Just to see. Just so you know what it is for a 4th pulse on the Bullet at 3500 rpms, the inlet tract(this length includes everything from the back of the valve head all the way to the tube air inlet opening) is 22.3" for the half of 264 duration calc,
Anyway, our goal for the Bullet would be to tune the pipe for the torque peak, which is somwhere between 3250 rpm and 4000 rpm, depending on what the mods on your motor are. For a Bullet equipped with performance exhaust and 28mm Mikarb, it's probably going to be around 3500 rpms for torque peak. Larger carb will have higher torque peak.
The other kind of goal is to tune at higher rpm for the horsepower peak. That's a shorter pipe. But it's only good at your rpm at peak hp.
So, you make the call. I think boosting torque peak is better for us, because with higher torque peak, all the hp figures above torque peak also get boosted. Downside is a real long tube.
Cam duration for the intake cam on a Bullet is 264 degrees at .012" lash, or about 230 degrees at .040" lash(which is where the valve curtain is unshrouded). Some people design one way, and other people use the other figure. It's up to you.
So, that gets you somewhere in the neighborhood.
You can't use a flow bench for this, because we're not increasing flow. We're making wave tuning modifications to extend flow longer after BDC which doesn't show up on a static flow bench.
And unless you have a dyno, you have to use the "seat of the pants dyno".
If it's working, you'll know it, beause you'll have more torque, and everything above the torque peak(hp) will be stronger.
A fan won't help with anything. It's all based on engine filling during the intake period.
Vacuum gauge won't help anything either.
If you really want hard numbers, you have to do it on a dyno.
This is well-proven technology. All serious racers do it.
Most racers use high-revving engines, so their tubes don't need to be that long.
Our difficulties come from our low target rpms, and not having alot of room to put the tube. It can be curved gracefully with shallow radius curves, if that helps any.
I think we need to make a compromise between what we can get for length that fits into the appearance of the bike.
But, it's fun to experiment!
Any inlet length increase is going to be picking up some wave reflection at some rpm. It just depends on what it happens to be. 7" inlet tract, which is what we have from vavle to carb mouth, is too short for any wave pulse to be strong enough to really do anything for us at our low rpm range. It's just too short. So, anything "could" possibly be a help at some rpm. The longer, the better. You'll run out of room to put the tube before you hit the optimal lengths. But it's worth a try to get something.
A BSA Gold Star racer I know uses a 15" length pipe on his Gold Star carb. With a big fat K&N filter on the end. He has a support bracket for it coming off his rear shock mount. It goes that far back!