ok i threw a compression testor on my 383 stroker and i'm wondering what the compression ratio is on it? i got readings of about 155 PSI on all 8 cylinders, anyone know what that is for a ratio? do i just take that number divided by standard atmospheric pressure(14.7 PSI)?thanks!
Dustin

 

The Ideal Gas Law says PV=nRT. If you are dealing with an ideal gas and no heat transfer or leakage takes place, (and if I could do subscripts) you could say say P1V1=P2V2, where P1 is atmospheric pressure and V1 is the BDC cylinder volume, and P2 is the compressed pressure and V2 is the TDC cylinder volume.

However, the atmospheric pressure probably wasn't 14.7 psia that day, and air isn't an ideal gas. And, you've got two unknowns and only one equation, so you can't do the math.

Regardless, the only "real" way to calculate static CR is to know BDC and TDC volumes and divide the former by the latter.

 

um that's a pretty vague/open ended question, you don't know pistons/heads you got? if you had 12:1 compression with a big cam with lots of overlap I'd say 155psi is about right, i know it doesn't answer your question fully

 

wow... i think thats a bit high... i hope so anyways. well the heads are still a stock 75cc chamber with 1.96/1.5's... the pistons are flat-top -7cc dished rated to produce 10.9:1 with 64cc heads so i figured roughly with the 75cc heads it'd be about 9.3-9.5:1 but i think 155 psi is like 10.2:1 or something... i'm not sure. the cam is 234/244 and .488/.510 lift. soon i'm puttin a set of edelbrock 64 cc 2.02/1.6 aluminum rpm heads. i just found out that my block was already zero decked before i bought it becuase the numbers on the deck are no longer there becuase' they've been shaved off.... and five7kid, the crank should be 3.75" stroke and the block 4.030" bore with 75cc chambers and a standard .045" felpro head gasket. dunno if that helps at all or not...

 

someone will plug that stuff into their desk top dyno and pop out some answers, I'm only 10:1 with 64cc chambers and -4cc flat tops, surface decked, it shaved off some of the vo numbers, so I'm a tiny bit over 10:1 without getting too scientific, pistons sit maybe .010 in the hole, they come up pretty far.

 

Here's a cool comp. ratio calculator I found on the web:

http://www.csgnetwork.com/compcalc.html

I came up with 9.51:1 now and 10.67:1 with 64cc heads using the #'s you posted. Did you measure piston depth in the hole? If you know the part # of the head gasket, you can probably get the correct specs to punch in the calculator (compressed height, gasket bore).