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

[sml214@casbah.it.northwestern.edu]

Hey Jim- 

> 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. 

Quick note: everything I describe in this response is at best a fairly accurate 
approximation.  For dead-on accurate results, one must play with expensive 
engine simulation software... anyway... 

We can idealize the runners' scenario by saying this: there are two very 
different events that occur in the runners.  One is the actual intake of air.  
The other is the time in between actual intakes.  For the runner length 
calculation, we'll assume that the former lasts a fixed number of crank degrees 
and, more or less, sharply ends.  This isn't a bad approximation if the 
effective (i.e. at 0.050" lift) cam duration is used.  The latter lasts the 
remaining of the 720 degrees.  So, in the calculation I performed, this is 210 
and 510 degrees, respectively. 

The other factor in the idealization is that immediately after the intake valve 
sharply closes (thanks to our idealized "digital" cam :-), a pressure pulse is 
generated at the back side of the valve.  This "positive" (i.e. higher than 
mean plenum pressure) pulse travels to the plenum, is reversed, travels back to 
the valve as a "negative" (i.e. lower than mean plenum pressure) pulse, 
reflects, travels back to the plenum and is reversed again, then travels back 
to the intake valve as a "positive" pressure pulse.  This scenario is what I 
termed the 1st harmonic.  The 2nd harmonic is when this sequence happens twice 
and the 3rd is thrice, etc. 

A graphical way to look it is this: If you measure pressure right behind the 
intake valve during that 510 degree period, it will look like a cosine wave 
offset by the plenum pressure (i.e. the "y" of the cosine wave has the plenum 
pressure added to it and "x=0" is when the intake valve closes). 

The total timescale goes from x=0 to x=A.  A=[(720-ECD)/360]*[60/W] where ECD 
is the effective cam duration in degrees, W is the engine speed in RPM, and x 
is in seconds. 

A quarter-wave is one travel through the runner.  So, T=4*L/V where T is the 
period in seconds, L is the effective runner length (i.e. runner length plus 
port length plus 1/2 the diameter of the runner) in inches, and V is the speed 
of sound in hot air in inches per second. 

The goal, obviously, is to make that timescale an integral multiple of the 
period of the wave.  So, A=N*T where N is our reflection value. 

Result: 
V*(720-ECD)=24*W*N*L 

Let's plug this in: 
L=20.625 in 
V=15000 in/sec 
ECD=210 degrees 

For N=1, W=15455rpm.  For N=2, W=7727rpm.  For N=3, W=5151rpm. 

Of course, there are deviations from this idealization.  Quite frankly, the 
intake valve doesn't sharply open and close and the pulse isn't suddenly 
generated... but it's not too far off... 


> 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? 

Yes.  My idealization assumes that the actual time the intake valve is open 
pretty much resets the whole game. 


> > 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. 

You're correct.  Q=A*V=VOL*VE*T, where flow is Q, area is A, velocity is V, 
cylinder volume is VOL, volumetric efficiency is VE, and time (i.e. the amount 
of time the intake valve is open) is T=[ECD/360]*[60/RPM].  Again, this is an 
idealization at best, but I've read that with this idealization, estimating 
V=180 ft/s actually corresponds somewhat well to dynoes. 

Take care, 
Shad 


> 
> - 
> ******************************* 
> Jim Adney, jadney@vwtype3.org 
> Madison, Wisconsin, USA 
> ******************************* 
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