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unless you are running without sleeves you won't have a 4.25" bore, stock was 4.00, most are rebuilt with the overbore kit to 4.125". If you are looking for a psi that will depend on piston design, bore dia and combustion chamber cc. Put a compression guage on it and see what you have .
 


Like has been posted already. MANY factors affect compression.(Combustion chamber CC,piston dome/dish,cam timing,cam profile,ect.)

Probably MORE important is that all cylinder PSI readings are within a small percentage of each other.
 
Compression in PSI as in a compression check, or compression RATIO?

In PSI, more is better and consistent across all cylinders is more desirable than a high compression.

IIRC you need 90PSI for an engine to run decent.

>100PSI is an engine with a lot of life left in it.

>125PSI is a good strong engine.
 
That is true Mike72. On an old tractor engine a psi that is close to even on each cylinder is a good indicator of how healthy the engine is. But, the higher the cranking compression the more likely the engine will be powerful, which is what I think he is after. In automotive, a 350 Chevy will run fine with 90 psi even compression, but if you want a motor that builds big power you will want that number much higher. Cam timing and profile can change things drastically as you stated. I have a snortin 454 BBC here that has 185 psi cranking.
 
Me too. ;)


Thread is now "high jacked" lol
a83477.jpg
 
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