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Hey Jim- > Okay, I thought it might be worth measuring, so I pushed a length > of brazing rod thru each side and measured 15.5" and 15.81". So > the length difference is ~5/16" (8mm). It's still a pretty small > difference. The OD was as you remembered: 1.25". Very groovy info! > If we take into account the length of pipe contained in the intake air > distributor and in the head, the total runner length comes out to > ~20". For that, we can calculate the resonance: V=freq x > wavelength and the lowest resonant wavelength will be 80" (4 x 20", > as this pipe will be a quarter wave pipe.) V is the speed of sound or > 1100 ft/sec = 13,200 in/sec. > > So freq = 13,200/80 = 165, but a given pipe gets an intake pulse > every other revolution so the engine rpm that corresponds to this > pulse rate is 330 rpm. Yes, but one detail: the pulse doesn't get 720 degrees - it gets 720 minus the time that the intake valve is open. I don't have the specs for the stock cam offhand, but I'm guessing that it's about 210 degrees at 0.050" lift. > The next harmonic is at 3 times this, when the pipe is a 3/4 > wavelength pipe, so 990 engine rpm, and so on for all the odd > multiples of 330 rpm: 330, 990, 1650, 2310, 2970, 3630, 4290.... Not really. First off, the runner length is ALWAYS a quarter-wave (the relative pressure of the wave as compared to the plenum pressure reverses at the plenum, so you need two reflections to get back to the valve the same "positive" pressure pulse the wave left with). The only difference is how many wavelengths you travel during those 510 or so degrees of rotation. The first harmonic is when it takes the full amount of time for the wave to travel to the plenum and back twice. The second harmonic is when it takes the full time for the wave to travel back and forth four times. The third is six times and so forth. Also, I added 3/4" to the runner length since typically the runner is "lengthened" by a hemisphere out of the intake runner into the plenum and increased the speed of sound to 1250 ft/sec. due to temperature. This corresponds to the following RPM sequence: 15454rpm for one, 7727rpm for two, 5151rpm for three, 3864rpm for four. Most systems are designed for the second or third pulse. The first is nice, but requires VERY long runners. Beyond the third pulse, the signal becomes more and more damped... BTW, interestingly enough, at a runner air velocity of 180 ft/sec. (a good number for a rough maximum speed...), a 1.25" runner cooresponds to about 5100rpm if the engine is at 75% VE. But then again, when was the last time you still got any decent amount of VE at 5100rpm on a stock cam with stock heads? Take care, Shad ------------------------------------------------------------------- List info at http://www.vwtype3.org/list or mailto:help@vwtype3.org