Pontiac Grand Am
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March 2010 ROTM
March 2010 ROTM
Joined: Mar 2008
Posts: 973
From: Canada
ROTM Winner's Club
Pontiac Grand Am
Ok, so, my best friend's got a 1990 Pontiac Grand Am, with 2.5L Iron Duke, 3-speed auto, and 270,000 miles. For some reason he loves it like a baby. 
Anyway, he is demanding I ask you guys a question about compression testing or something.
I'll just paste what he said here since it's easier.
Anyway, he is demanding I ask you guys a question about compression testing or something.
I'll just paste what he said here since it's easier.
*According to this one guy, the pressure that a cylinder in a gasoline engine should compress to should be x*14.7, the atmospheric pressure at sea level. x being the compression ratio of the engine. In my case 9:1, in yours er...9 or 9.5:1.
*14.7x9=132psi-ish. minus some for leakage, since the seal for piston rings isn't perfect. So we'll figure ~128psi for a 9:1 engine.
*If this is true, my engine is still sealing exceptionally well. one of the cylinders even hit 137psi.
*So, your project for tonight is to go onto the automotive forum you frequent, and ask how the hell one would get the proper reference pressure for compression testing :v
*14.7x9=132psi-ish. minus some for leakage, since the seal for piston rings isn't perfect. So we'll figure ~128psi for a 9:1 engine.
*If this is true, my engine is still sealing exceptionally well. one of the cylinders even hit 137psi.
*So, your project for tonight is to go onto the automotive forum you frequent, and ask how the hell one would get the proper reference pressure for compression testing :v
Tech Droid
Joined: Aug 2005
Posts: 9,215
From: North East PA
Its not that easy, remember that air heats up as it is being compressed causing the pressure to go higher than his simple calculation shows. Also valve timing gets involved which usually causes pressure to be lower due to valve opening overlap. Also a lot depends how fast the engine is being cranked and how many times it is spun through the compression stroke. I have seen engines with 10:1 compression register around 200 PSI on the gage.
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