If bore and stroke is unknown?

Can I figure out what the compression ratio of a 400 cid motor is using a compression gauge? Is there a direct relationship between PSI and CR?

Thanks,
Steve

I don't think so, piston rings for example might have a lower compression reading on a gauge but the ratio is calculated the same

I'm not sure I get what you're saying... :think:

Are you saying that due to pressure loss around piston rings, the gauge might be inaccurate?

That, as well as cam overlap and other variables that can affect compression readings, but wouldn't change your compression ratio.

Compression ratio is the volume of cylinder when the piston at the bottom of the stroke to the volume of the cylinder with the piston at the top.

From smokemup.com:

The result is NOT the "compression ratio". The result is an indication of how well the compression process is being carried out. This should give you a rough idea, "ballpark", of the compression ratio of the engine. The only true way to determine the compression ratio is to calculate it. This calculation is not applicable to engines with long duration camshafts. Cams with overlap will allow cylinder pressure to bleed off resulting in an incorect reading.



I think you're on the track of calcuating it from:

say you get 147psi on a compression tester

atmospheric pressure is what, 14.7psi?

So in theory that's a 10:1 compression. But, a 2.0 DSM would get a 160psi on the gauge, when it's only 8.0 to 1.

I figured that the PSI reading wouldn't account for pressure lost due to cam duration...

Basically, I've come into posession of a 67 Camaro with a 400 cid motor that was clearly built to be a 1/4 mile car... (Roll bar, fuel cell, stripped bare for weight reduction, M/T drag slicks) but I don't have any specs on the engine internals... I have started it up and it will idle (VERY CHOPPY) on 93 octane, but I don't know if the motor was built for race gas only... This thing is in no way street legal....

Short of driving it hard and listening for knock... How can I figure out what fuel I should be running?

the only way to know for sure is by removing the cyl. head and mesuring the cc's of the combustion chamber and piston area at tdc. - of if you know the part # of the piston.
best bet run 50/50 racing fuel and prem.
good luck