On Wed, 2 Jul 1997 09:32:53 -0600 phil sims <phil@wilsongriak.com>
writes:
>OK, Is it this simple?
>
>Is the correct way to determine compression ratio to simply divide
>your
>measured compression (in my case 140lbs each) by 14.7(standard ambient
>air
>pressure)?
>
>Or is there more black magic to the equation?
>
>Phil Sims
>Wilson-Griak,Inc.
>(612)377-7900
>phil@wilsongriak.com
Sorry Phil, but the compression pressure has little relationship to the
compression ratio.
Compression pressure is the measurement of the pressure rise at
cranking speed, and as such is affected by the state of piston rings,
state of valve seats and faces, lift and overlap of the cam, speed of the
starting motor, etc, etc.
Compression ratio, on the otherhand is a mathematical measurement of
the ratio of the volume of one cylinder plus the combustion chamber
volume when at bottom dead center (BDC) and the volume of the cylinder
plus combustion chamber volume when at top dead center (TDC). Compression
ratio will not change with pressure of the air, indeed remains constant
even in a vacuum.
In you example 140 lbs of pressure at 14.7psi ambient air pressure,
would lead to a compression ratio of about 9 or 9.5 to one. But suppose
you had a couple of broken rings on that cylinder and the pressure drops
to say 90 lbs. The compression ratio has not changed, but the pressure
did.
One some highly tuned racing engines, using high compression ratios (13
or 14 to 1) may actually have LESS compression pressure at cranking speed
than a stock engine. Not due to any mechanical defect, but because the
extreme overlap of the cam profile doesn't let the pressure build up
because one or more valves are still partially open when the piston
starts on the compression stroke.
Sorry for such a complicated answer to a simple question, but I hope it
helps
Rick Morrison
72 MGBGT
74 Midget
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