Ok, just dropped in my .040 over 350 and haven't taken it for a ride yet, but I just checked my compression on one cylinder and got 185 psi. I was expecting pretty high, but dont know how much compression I've got going by psi. Anybody have any conversions or ideas?

 

there is no conversion for this. too many variables.

 

Well, there is a way, but you have to know the volume when the piston is down all the way, and the volume when the piston is up all the way.

But, if you know the volume when the piston is down all the way and the volume when the piston is up all the way, you don't need to know the cranking cylinder pressure to figure out the compression ratio, because the former divided by the latter IS the static compression ratio...

 

What zippy said.

 

Only way to tell for sure is to measure before the engine is put together, different cam duration's and designs greatly affect the cranking compression. But 185 PSI is very good so I don't think I would worry about the real ratio unless it pings badly with the correct ignition timing.

 

There is no way to create an equivalence, for a number of reasons. One, the valve timing affects how much of the static compression remains "captured" in the cylinder, which causes the reading to be lower than it otherwise might; and two, when you compress a gas it heats up, which raises the pressure beyond the static amount (if you could compress the cyl and let it cool back down this error would go away but obviously that's not practical) which makes the reading much higher.

But, 185 psi is good, if all cyls are that. One cyl that high probably means a bad intake cam lobe, loose valve, or other cause of short intake duration. Look for consistency in compression readings, not the absolute value.

So, like the other post said, too many variables.