CFM Continued...

[Greg Hermann]

>With what you say in mind, then WHY does a STOCK 426 Hemi SONGLE intake
>runner flow about 300 cfm each, from the factory??
>
>Just wonderin...
>
The Hemi runner may well flow 300 cfm at one of the two standard test
pressures--this simply means that it will flow all the air that the engine
wants at a far lower pressure drop than the standard test pressure.

Say you Hemi is turning 6500 rpm, is the stock 426 cubes, and is good for
94% volumetric efficiency at 6500 rpm (optimistic, but a Hemi might do this
well).

OK--426/8 = 53.25 x .5 x 6500 / 1728 = 100.15 cfm of air flow to each
cylinder. (Call it 100 cfm.) (But realize that this is an AVERAGE flow
rate. It is higher when the valve is at maximum lift, and lower at partial
valve lift, and non-existent when the valve is closed.

Say the cam is such that the valve is off the seat to a significant degree
33% of the time (240 degrees of duration at .050" valve lift).

Each intake runner flowed 300 cfm at a test pressure drop of 25" of H2O .
(with a CONSTANT test flow rate, and probably at a valve lift of .650".)

Well--first of all--the fact that the valve is only open 33% of the time
means that the flow through the valve would have to average 300 cfm DURING
THE TIME THAT THE VALVE IS OFF ITS SEAT, in order to achieve the AVERAGE
100 cfm flow calculated above.

Next--things get more complicated--one would really need to integrate the
area under the valve lift curve and the pressure drop in the port at
various valve lifts, and so on, in order to figure out exactly how much air
was flowing in the port at different parts of the valve lift curve, and at
what pressure drop, in order for the engine to draw in the amount of air it
consumes.

It IS important to remember that the net effect of pressure drop as air
flows through the inlet tract of an engine is less dense air filling the
cylinders. Which translates into less oxygen to burn fuel, and therefore
less power as the pressure drops increase.

The flow bench numbers are hardly an exact indication of what goes on in
real world flow inside an engine, but hopefully, the example above will
illustrate why the standard flow bench test pressures for heads (either 25
or 28" of H2O, depending on who is doing the testing) are fairly
realistic--that is to say--in the ballpark for the flow velocities which
occur at maximum valve lift in a real engine.

Regards, Greg