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Old 07-06-2003, 11:09 AM
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cylinder psi vs compression ratio

Hi, guys. After searching through this site as best I can, I am coming to the more knowledgeable for help. I am a newbie so please go easy on me.

In searching for a new camshaft for a mild engine build (Chevy 350), I find that most cam manufacturers have a minimum compression ratio for a specific cam.

My 350 is not original to the vehicle and it seems as though it would be easier to measure cylinder pressure than compression ratio.

Is it possible to convert a psi reading to compression ratio?

The engine will be in a Chevy shortbed 2WD with 3.73 limited slip rearend. As with most people, I want all the performance I can get for the buck. My intentions are to change the cam, lifters, springs and timing chain/gears. Also will install headers,exhaust system, intake and carburetor. A new torque converter is OK if I can run a big enough cam with my compression ratio.

I would also appreciate any recommendations from those of you with more knowledge than I, which is almost all of you.

Many thanks in advance.

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Old 07-06-2003, 12:01 PM's Avatar
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There are many things that effect cranking pressure, engine temp, type and condition of ring, valve seal, atmospheric conditions, position of throttle plates,etc. but the cam can cause low or high cranking pressures by the way it is ground and set up, long duration high overlap cams have low cranking pressure and short duration cams give higher cranking cyl pressure. So compression ratio can't reliably be determined by cranking pressure.

The local circle tracks use a P&G cylinder for checking cubic inch and compression ratio, what a joke, not even close, i've seen the same motor blow(CHECK) three different specs on three different nights. NHRA quit using them years ago because they aren't accurate.
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Old 07-06-2003, 02:45 PM
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if the engine is still in the state it left the factory (e.g. no other performance work has been done / hasn't been re-built, bored etc.) probably the easiest way will be to find the casting no.s on the block and heads and post them, we should be able to get enough info from various sources (like to give a best guess.
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