I'm looking at a car with 120-125psi compression on each cylinder and I've never thought of this question before so I'm askin' it here.

Does that PSI compression translate into a compression ratio? Like does 9.5:1 = XXX psi?

Thanks,
-Art

No , cam timing bleeds off or builds pressure , that 125 says it's probably less that 9 though .

There is no direct correlation but based on experience some people can give you a hypothetical approximation. There is a correlation though if you have 160 psi more than likely you either have higher compression or a small cam. This can not say that a 125 psi can not have higher compression but due to lots of overlap in the cam it is bled off at lower rpms (where you check compression) but in the higher rpms it doesn't have time to bleed it off creating the high compression at a higher rpm.

Basically you can get a relative reference but without knowing the chamber size the, the quench, piston (dish,dome) Deck Height, stroke and bore, and cam specs with installed centerline its impossible to calculate the compression ratio. The Cylinder pressure mainly just tells you the health of the cylinder with some relation to the Compression ratio.

Another way to think about it is you could have a 11:1 compression and the rings have a lot of blow by due to cylinder wear you might get 110 psi where as on a good motor it should be at 160psi (psi figures not exact just for example), that 11:1 with a long duration came could read at 125psi for a good motor and 90 psi for one worn out The PSI doesn't tell you the compression ratio but if you know the compression ratio and engine build you can see if the cylinders are good based on the compression test.

If the engine is all original then you can guess what the compression ratio is by reading the compression gauge.

On a race engine you would be guessing in the dark. 200 psi could be 12:1 with a small cam, or 15:1 with a big cam.

Cranking psi also varies dramatically with cranking speed, carb restriction, barometric pressure, etc.

When I do a cranking psi test on a race engine I warm up the engine, remove all plugs, remove the carb, and use a fully charged battery. That way I can get some half way consistent readings. But the readings will still vary on a cold day vs. a hot day.

Dad's Pontiac engine has 220 PSI cranking pressure and it is about 10.1:1. It has Rhoads lifters and those really goof the static numbers. My 340 that has true 10.8:1 and has cranking compression of 125 to 130 PSI. It has the older, more radical 340 manual trans cam. And my old 2.2 turbo with about 8.4:1 had 130 PSI. I had a relatively zero overlap cam in it.

So no, the numbers you get can't really tell you much other than how one cylinder relates to its brothers and sisters. :)

My 10.3-1 340 with a 474 cam has 165 psi cranking pressure on all 8 cyls!

Short answer:
the computer program
Engine Analyzer 3.2 does a pretty good job of taking every thing into account and calculating a cranking compression PSI

http://performancetrends.com/download.htm#ea3

Long answer:

If the camshaft closed the exhaust valve at top dead center and the intake valve at bottom dead center
And
The rings leaked no air
And
The cylinder head, bore walls and piston were perfect insulators
And
There was no humidity in the air the engine drew in
And
you knew the "ratio of the specific heats" of the air that is given the nickname Gamma,
... Then you could calculate the compression PSI
by hand with a modestly complicated "Power Law" math equation

Details of this at

http://en.wikipedia.org/wiki/Adiabatic_process#Example_of_adiabatic_compression

It is inaccurate to simply multiply
the 14.7 PSI of the air around us
by the compression ratio of the engine,
but that gets less inaccurate
the colder And drier the air is.
If you were working on snowmobile engines at the South Pole you would not be far off that way.

Thanks, as a rookie it's nice to know the details.
Its a stock engine and the published specs say it has 8.5:1 CR. Every cylinder was betw 120-125psi so it sounds like its reasonably healthy. I'm used to 140-145psi so I was concerned about potential issues before buying it. But i did! I will post about it when I get it on the road, it needs new headgaskets and some other work first.
Thanks, -Art

Compression ratio can be estimated/calculated from the cranking cylinder pressure if you know the intake valve closing point, and cylinder leakage.