r-james@tamu.edu writes:
> The question is what pressure should be seen for a given
> compression ratio?
Good question.
> Suppose the compression ratio is 10:1. I assume that means
> the compressed volume (TDC) is 0.1 of the uncompressed (BDC) volume.
Partly correct. What we usually call "compression ratio" is really
geometric compression ratio, without any regard to valve timing (see
below).
> If so, the ideal gas law says the pressure is 10 times the original
> pressure. The original pressure (absolute) is 14.7 psi,
Only if you have the throttle plates fully open. With them closed,
you're drawing some vaccum during the intake stroke, and the initial
pressure on the compression stroke is somewhat less than 14.7 psi.
Also bear in mind that your formula is only correct if the intake valve
closes EXACTLY as the piston reaches BDC. Most cams, at least most cams
worth using, close the intake valve somewhat later, allowing the inertia
of the intake charge to slightly overfill the cylinder, increasing
efficiency and power. If you know the specs for your cam, you can
correct your formula to compensate.
--
Todd Mullins
Todd.Mullins@nrlssc.navy.mil On the lovely Mississippi (USA) Coast
'74 MGB Tourer waiting on an engine rebuild
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