Finding Comp. Ratio via comp. tester???
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
You could if there is no leakage past the rings and valves and there were no pressure drop in the air induction system. You can get a fair idea though. The compression gauge doesn't measure atmospheric pressure, so you have to compensate for that. The CR ideally should be the ratio of the absolute pressure at top dead center divided by the absolute pressure at bottom dead center. That would be pressure gauge + atmospheric divided by atmospheric. Atmospheric will be close to 14.7 lbs /in squared.
Burning Brakes
Joined: Nov 2001
Posts: 1,241
Likes: 9
From: Houston TX
absolutely not. the camshaft timing events have just as much (if not more) effect on the cranking compression pressure as static compression ratio does.
you can change cams, or advance/retard your existing cam, and get significantly different numbers.
also, depending on the camshaft timing events, many engines can easily have cranking pressures significantly higher than their corresponding static compression ratios. example, my stock LT4 would crank 195-205 psi, which corresponds to ~14:1.
-michael
you can change cams, or advance/retard your existing cam, and get significantly different numbers.
also, depending on the camshaft timing events, many engines can easily have cranking pressures significantly higher than their corresponding static compression ratios. example, my stock LT4 would crank 195-205 psi, which corresponds to ~14:1.
-michael
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
NO WAY!!!
jfb is totally off base on this one, which is VERY unusual. There is no reliable connection between the cranking pressure and the compression ratio. Of the many variables, the cam timing is the number one consideration in the cranking pressure. Even more so than the compression ratio.
RACE ON!!!
jfb is totally off base on this one, which is VERY unusual. There is no reliable connection between the cranking pressure and the compression ratio. Of the many variables, the cam timing is the number one consideration in the cranking pressure. Even more so than the compression ratio.
RACE ON!!!
Race Director
Joined: Jan 2005
Posts: 10,641
Likes: 13
From: Slidell Louisiana
With MSR and CFI-EFI, cam duration and overlap will affect cylinder pressure! I used to have a 12.5 to 1 compression big block that
would run on regular and I believe it was all due to the cam, the downside my brakes sucked at idle.
Mike
