Re: Engine compression adjustment question
I'm no professional engine builder, but I realize that dynamic compression is just static compression, with intake valve closing calculated in. Dynamic compression will always be lower than static. I know I am not 100% correct to interchange "cylinder pressure" with "dynamic compression", but they are certainly related.
DjD, the initial post said the guy was told to use 106 octane. Well, if he's got cranking compression of only 160 or 170 psi, then why in the world would he ever want to use 106 octane? He doesn't need it. Variations in chamber design, quench, gaskets, pistons, etc., as well as rear gearing and vehicle weight will affect detonation, and maybe that's what's allowing you to run 220 psi on 91 octane with no detonation. I'm no expert in that area, but that number sounds very high for detonation-free running on 91 octane.
All I'm saying is; if a guy has an engine in his car (built with unknown components) and he's just trying to get a quick feel for cylinder pressure and what gas to use, then a cranking compression test is a good first step.
Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
Last edited by Chevy-SS; Dec 11th, 05 at 03:43 PM.