I had a compression test done on my 67' Camaro.
I didn't think to ask the mechanic what it meant when he gave me a reading of all cylinders good and a pressure range of 125 - 138 lbs per cylinder.

Does anyone know exactly what those numbers mean?

Is there any way to convert those numbers to a compression ratio?

Thanks for your help!

Terry

the piston was at top dead center, TDC, and he measured compression, yours being fairly average for a street car. Those numbers are pretty good.
Those numbers along with what he said means that you have good compression. None of your cylinders, piston rings, valves or valve guides are letting air out, which would cause compression loss and severe los of power.
Sounds like you have a decent cam with those numbers, sounds like you have around 9.0:1 Static Compression.

Thanks for the help, that's what I needed to know.

I had a compression test done on my 67' Camaro.
I didn't think to ask the mechanic what it meant when he gave me a reading of all cylinders good and a pressure range of 125 - 138 lbs per cylinder.

Does anyone know exactly what those numbers mean?

Is there any way to convert those numbers to a compression ratio?

Thanks for your help!

Terry

Terry...those numbers your mechanic gave you came from doing a cranking compression test.You typically are looking for the high and low numbers to be within 10% of each other..to rule out a problem cyl.
A leak down test at tdc is a better indicator of cyl seal and usually given as a percentage of leakage vs psi with a cranking test.
The cranking compression psi will not establish a static compression ratio.
You would need to know the total swept volume in the cyl at bdc and then divide that by the volume above the piston at tdc.Obviously...this would require you know all measurements like bore& stroke/head gskt thickness and head chamber size,typically measured during the mock up and assembly process.
Lets say for example..your static ratio is now at 9/1 and 130 psi.If we closed the intake valve sooner with a cam swap..the pressure could go to say 150 psi with no change in the static compression ratio.Keep in mind,no cyl pressure can occur until the intake valve seats on the compression stroke.
See how not knowing all the engines internal measurements and valve timing events makes it impossible to zero in on the (as built in) static cr.
Hope some of this helps you.:beers:

Since compression ratio is a calculation of the ratio of how much an uncompressed volume of air in the cylinder is compressed down to and atmospheric pressure fluctuates around 14.7 PSIG doing a slow speed crank compression that nets 130 PSI and devided by the atmospheric pressure of the day calculates to around 8.84:1 compression ratio (130/14.7) now huge cams can bleed off cylinder pressure so theoretical compression ratio and dynamic compression ratio can be quite different - with most mild street cams the simple math gets you close to the static...