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Old 03-15-2013, 11:42 AM
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The compression pressure (PSI) depends on several variables. The primary variable is the cam profile and valve overlap.

You can disregard variables and get a WAG at the PSI mathematically :

14.7 x CR = PSI

8:1 CR = 118 PSI
10:1 CR = 147 PSI

My 455 Pontiac had 12:1 CR with an average 215 PSI cranking (150 RPM). .
The hydraulic lifter camshaft was very mild CC-268H with only 39 degrees valve overlap. The short overlap quickly built up cylinder pressure for gobs of low end torque! That engine required at least 100% VP C-12 - 108 octane racing fuel.

A Ram Air IV camshaft with 87 degrees valve overlap in the same engine would produce an average 180 PSI. That much combustion pressure would be borderline on 93 octane pump gas with best performance with at least 100 octane VP racing fuel.
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