Thread: Compression Ratio vs. PSI

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I realize this is a rookie/rube question but is there a direct relationship between expected CR & measured PSI. For example if you expected a CR of 9.5 what should you measure in PSI to confirm. Is there a table or site where I could find this info for stock SBChev engines.
Thanks Warren

You'll need to know where the intake valve closes. That's what dictates cranking compression. So, unless someone has dyno'd all the cam combinations, I have no idea if such info is available.

After some checking around I found:
assuming standard atmospheric pressure is 14.7 PSI the CR can be calculated from that info
for example if measured 135 PSI the CR is 135/14.7=9.18
This is only truly accurate for sea level but for what I wanted - expected ball park number to see if engine in good shape works for me. If I checked an engine that was supposed to have 10:1 compression I would expect ball park of 148 PSI if in top shape.

Thanks for the response techinspector.

Hmmm-
I just compression checked the used 350 in my son's 62 pickup at 162lbs average per cylinder ... when divided by 14.7 comes up to 11.02/1 YEAH!! We live at about 500 feet above sea level.
As Techinspector said, using this method would be dependent on knowing where in the stroke the intake closes. You'd lose some compression if the piston is already on it's way up when intake closes. Not sure your method could ever be mechanically accurate and I think your 14.7 figure might be a little generous, but this is an intriguing method of assessing used engines.

i dont think it will get a very exact idea re comp ratio. i think the cam and cam timing will affect the beed rate on a hp motor which may offset the high comp ratio. i know my 500 caddy had a high lb rating 165 + and really hit very hard on the 1st stoke which seemed strange but its a really big pump, a quart of air is being moved .