Treatise on intake systems

[James Boughton]

Let me start over since I see the equation included by another
writer has affected our conversation.  If you consult Heywood's
"Internal Combustion Engine Fundamentals" you will find an
equation for tuning your intake lengths on page 312.  The equation:

	Tuned rpm = (955)(a)((A)^0.5)/(K)(((l)(Veff))^0.5)

where	a=speed of sound (Temperature dependent)
	A=effective cross sectional area of intake
	l=effective length of intake (basically length from back of valve to end of trumpet)
	Veff=Effective cylinder volume (typical cylinder volume with piston at 1/2 stroke)
	K=Fudge factor (typically taken as 2 until some data is in hand)

The speed of sound is simply (gRT)^0.5 where g and R are considered
constant.  If you wanted to be rigorous you could actually vary g and R
for temperature, but probably not worth it.  T is of course temperature.

Now reference 6.27 from Heywood is a paper by R.J. Tabaczysnki titled
"Effects of Inlet and Exhaust System Design on Engine Perfromance."  In
this paper a great deal of work was done to determine appropriate lengths at
different average cross sectional areas.  The various cross sectional areas
were actually shown as different intake velocities (since the velocity and the
area are inversely proportional).  What Tabaczynski determined (which I don't
actually agree with) is that the best intake velocity was between 180 - 200fps
(55 - 61m/s).

Now to calculate intake velocity the volume displaced during the intake stroke 
is taken as flowing during the time of the intake stroke.  This gives a volume
flow rate (length cubed per time).  If you divide this quantity by the intake area
(length squared) you end up with a (pseudo) velocity (length per time).  The 
volume is the cylinder volume - 500cc in your case.  Note that this displacement
takes place during half of a revolution.  If one revolution takes 1/rpm minutes then
1/(2*rpm) minutes is the length of time for a half of a revolution.  Since we are
going to divide the volume by the time we end up with V*rpm*2 as the volume flow
rate.  Then you simply divide by the area and the appropriate conversion factors
to get to feet per second or m/s whichever is preferred.

I hope this clears up some of the misunderstanding.  Please feel free to ask more
questions.

Jim Boughton