compression testing

[mat82284]

i was going to do some compression tests on my car and i wanted to know is the compression the same for a cobra mustang than a stock one? and whats the stock compression ratio and the cobra one if there different. i just want to test and make sure compression is good before i add turbo.

 

[jrichker]

Compression ratio & compression testing are apples & oranges.

Compression ratio = amount of air in the cylinder at bottom dead center compared to amount of air in the cylinder at top dead center. Example: BDC - 40 cubic inches, TDC - 4 cubic inches. The ratio = 40 divided by 4, which is 10:1

With the engine warmed up, remove all spark plugs and prop the throttle wide open. Then crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.