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

On 13 Aug 2001, at 8:47, Shad Laws wrote:

> > My guess is that the difference is 1/8" or less, which really can't be
> > significant when the tube diameter is ~1".
> Only 1/8"?  I thought it was more like 1" or so... <shrug>  Also, I believe
> that the diameter is closer to 1.25" (I know the OD is slightly over 1.25",
> making silicone 1.25" hose a cheap, easily available, and good replacement
> for the stock boots).

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".  

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.  

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....  

I guess I'm surprised that it's so low; I don't expect it to have any 
real effect since all the resonances are crowded closely together.  

One thing I'm not sure of is how the speed of sound changes with 
pressure. If the speed increases with a decrease in pressure, then 
these resonances would all be shifted up, but I doubt if it would 
shift enough to help, and I suspect that it actually shifts the other 

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

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