I was currently having problems before with my #7 cylinder not firing due to an oil soaked spark plug in my 84 z28 with a rebuilt 350. Repalced all the plugs and now that cylinder is firing, but i checked the plug and it is showing signs of premature wearing. I did a cylinder pressure check and got 120 psi on the #7 cylinder. I did a pressure check on the rest of the cylinders and they were also 120psi. I'm new to the small block chevy world, but this seems like a low cylinder pressure for a 350. Or maby i just have a really low compression ratio??
Here is my set up: 350 chevy, world product's rs heads, edelbrock rpm performer intake, edelbrock 600 cfm carb, summit headers, 3 inch exaust with summit mufflers. As for the cam I don't know what it is, but its definatley not real big. I do know that the motor was rebuilt so there might be a chance of an after market cam in there. The pistons and cylinder bore are also unknown.

 

Did ya have the throttle wide open while checking compression?
That will bring it up.

 

It really isn't too important what the pressure is (within reason) so long as all the cylinders are the same (within a few psi). Your next step if you suspect you have a problem, is to do a leak down test.

 

No i did not have the throttle open at wot when checking the pressure. What exactly is a leak down test?

 

leak down test = measure the amount of leakage in each hole, uses a test manifold and compressed air

 

If the throttle wasn't wide open then your numbers are garbage. There was this giant restriction (the throttle) preventing the cylinders from filling.

Repeat the test, with the throttle blocked wide open. Your numbers will be considerably higher.

Who's to say what a cam that's "not real big" might be; but I'd expect to see numbers in the 160-180 range with a typical mild street/performance type cam, those 67cc heads, small dish pistons, and the test executed properly. As was said, the absolute value of pressure isn't so important, as consistency across all cylinders. You might have a cylinder with bad rings, or a bad valve, or a head gasket failure somewhere; but it's beyond unbelievable unlikely that all 8 would be just exactly equally bad in exactly the same way, and produce exactly equal consistent bad numbers as a result. So if they're all equal within a 5% or less, odds are there's not a compression-related problem.

As to the oil-soaked plug, a very common cause of that in a new motor is a narfed intake gasket; remember that across the bottom of the intake ports, the port is next to the lifter valley, so a leak right there will allow the intake to suck oil directly from the crankcase. Also, a bad valve guide and/or valve guide seal can cause oil consumption. So there are other causes of that symptom that won't show up at all in a compression or leak-down test.