cylinder compression
 

I brought my 68 327/ 350hp to the mechanic to look over before letting her sleep for the winter.

mechanic said the cyclinders have 120psi even around the motor. I thought the compression should be at least above 140psi with a stock motor 327 (11.0:1)

he said motor should be rebuilt when compression nears 75psi

mileage says 50k but it has never moved since I bought it. the car could have 200k miles on it for all i know.

what is good compression?
When should a motor be rebuilt?

thanks

I've always heard 110 psi and below it need rebuilt. A good strong street motor should have about 180 psi. My drag car has 210 psi :O, and still runs fine on 93 octane pump gas. I had a 1978 Sea Ray boat with an old tired motor that had 115 psi. I'd say thats on the low side.

I would say cranking pressure is a relative (reference) number, when the motor is running its a whole different ball game.

cranking pressure

120 is a low number for an 11:1 engine unless you have a radical cam. Did he do the test with the throttle wide open?

I would also do a leak-down test as well - the two together tell quite a bit about engine condition. What's the static CR...is it 11:1? I would consider those numbers to be pretty low if that's the case...

For an OEM GEN I SBC, figure that the factory guides are starting to go south around 60-75K miles, and the shortblock is probably getting there around 90K+ miles. Certainly the engine will run out to 100K+...but the risk factors go up and the engine's making less power. Oil consumption is a good indicator as well.

This is just my opinion but...to me the main purpose of a static compression test is to look for differences between your cylinders. If he is saying you are getting an EVEN 120 then to me the important part of that statement is 'even'. There are other indicators, such as oil usage, smoking, etc. that may sometimes be better indicators of when a rebuild is needed. If you were going to go by compression test alone then I would have it checked by at least a couple or three different places. I suspect that type of test equipment could have a lot of variation in it and the number is really a relative value. Like I say, just my opinion.