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Re: Fw: Fwd: Re: [T3] too much mind time

On 15 Aug 2001, at 0:55, sml214@casbah.it.northwestern.edu wrote:

> 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. 

I have always assumed that what was being tuned was the 
frequency of consecutive pulses down the pipe. If I've got this wrong 
please straighten me out. If I'm right, the cam duration has nothing 
to do with this, it would just be a matter of changing the Fourier 
distribution of frequency components above the fundamental, but 
you seem to be after something completely different.

> > 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: 

I've read this several time and I still haven't managed to get it 
through my head what you're doing. You're going to have to break 
this down into first year concepts, I guess. Are you just concerning 
yourself with events that happen withing the duration of a single 
pulse and ignoring the relation of each pulse to it's neighbor?

> 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... 

You're going to have to explain what you are doing to get these 
numbers and why. I still don't get it....

> 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? 

Define VE (Volumetric Efficiency?) please.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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