cr750, Congrats on the the carbs find. I love it when I find just the right thing, but it was mislabeled, with a bad pic, etc. I regularly run eBay searches that include misspelled words for the expensive rare stuff I *need*.
I don't remember much, but there is a good thread somewhere around here discussing the tuning affects of stack length for the Mikuni RS34's. I bet someone here can link to it.
Here are some links that might help
HERE is a velocity stack calculator.
HERE is some good information about using velocity stacks to help tune big power out of a Yamaha V-Max - with lots of dyno info.
And you can always chat with Daniel, one of our local experts.
I don't remember the numbers, but the short run-down is that a shorter stack is supposed to make more power at high RPMs, and a longer one a lower RPM. A balanced one will give as much as possible through the mid-to-high range.
As you know, this is not exactly correct, because a tuned velocity stack is actually tuning to the pulse harmonics of the intake runner -- and there are 1st, 2nd and 3rd waves; Modern tuned race engines will catch up to the third wave, and requires pretty specialized equipment.
Basically, it must be very similar to a tuned 2-stroke exhaust. I am a bit of a vintage 2-stroke fan, and one thing I have observed with both my 2-stokes and 4-strokes is that there is quite a bit of visually observable harmonic pulse action -- stuff I can see with my naked eye. For instance, take a $1.00 bill (or $100, if you prefer -- because that's what you'll be spending...), and with the engine running, hold it up to each exhaust tip. The bill is blown out and sucked in toward the tip. Same thing with the intake -- remove the intake filters and you will be able to observe that the bill is both sucked in and blown out. These openings are commonly thought of as uni-directional, but there is air moving through them in both directions.
I am not able to precisely measure these things, but if I am able to visibly observe it, then there is a
significant loss in volumetric efficiency. In places where we want as much air to move as fast and with as much volume as possible, it is instead not even consistently moving in the correct direction. These directional changes represent a significant amount of energy which needs to be overcome -- or more accurately, this energy needs to be better understood and harnessed to work for us instead of against us.
I believe that a perfectly tuned exhaust would completely eliminate the "sucking" action at the exhaust tip, instead the suction would be completely moved up the pipe to help evacuate the exhaust gasses from the cylinder. Moden pipes begin to accomplish this with internal valving, etc.
Likewise, with the
intake, I have observed that the "blowing" pulses must carry raw fuel with them. This is one MAJOR reason hard-capped pod filters are evil-- they screw up these pulses, usually exasperating the pulse problem. On my 2-strokes I actually will loose raw fuel if my intake does not have enough volume before the filter to hold several of these pulses -- this is just one more reason why we have large volume (tuned) air boxes, plus velocity stacks in our stock intake. I believe this represents an early attempt to partially capture 1st and 2nd intake pulses.
-- I believe a perfectly designed intake will function like a perfect exhaust; it will be tuned to these pulses so that there is NO reverse flow blow-back, or it would take the blow-back harmonics into consideration and use this energy to its advantage. (This is what tuned intake runners and velocity stacks are supposed to do.) It also should capture at least two full pulse waves. Of course, the cam, cylinder head porting, valve angle, etc all has much to do with intake harmonics. Like a modern exhaust, The perfect intake design would also have to be automatically adjustable to compensate for changes in RPM.
Anyway, I woke up and was thinking about this over my Saturday morning coffee.
Peace and grease,
-Fang
