What compression readings would you need to justify an overhaul?
What is the minimum number or range (psi)?

There are many things that affect cranking compression including cranking speed and cam timing. An obvious sign of something wrong is when all cylinders are not within 5 to 10 psi of the same reading. How about a few details on the Motor.

[This message has been edited by squarles (edited 07-18-2000).]

Thats kinda why I asked...the engine has a lumpy cam. Dont know the exact number right off hand (crane fireball II). But then I was thinking...the cylinder still has to compress to TDC and beyond anyway, so what difference would a "lumpy" cam make? Its a 327 which I have no idea of the compression ratio...probably stock. The starter turns strong. Im getting approx 100 psi on all 8

Sorry, I accidently hit submit before I was done.


[This message has been edited by squarles (edited 07-19-2000).]

[This message has been edited by squarles (edited 07-19-2000).]

The overlap on a "lumpy" cam bleeds off cylinder pressure at lower rpms to enhance upper rpm power. The bleed off of low rpm cylinder pressure is what affects the psi readings, it is also what makes for the "lumpy" idle. Advancing or retarding the cam in relation to the crankshaft will also affect cranking compression readings. Advancing will usually increase the readings and retarding will decrease. 100 psi does sound kind of low. Are you taking all the spark plugs out and cranking the motor over with the carb at WOT? You also want to crank the motor over a few times until you get the highest reading. A low compression motor with a lot of overlap will have low psi readings. Is your car soggy and lazy at low rpms?

<BLOCKQUOTE>quote:</font><HR>Originally posted by squarles:
The overlap on a "lumpy" cam bleeds off cylinder pressure at lower rpms to enhance upper rpm power. The bleed off of low rpm cylinder pressure is what affects the psi readings, it is also what makes for the "lumpy" idle. Advancing or retarding the cam in relation to the crankshaft will also affect cranking compression readings. Advancing will usually increase the readings and retarding will decrease. 100 psi does sound kind of low. Are you taking all the spark plugs out and cranking the motor over with the carb at WOT? You also want to crank the motor over a few times until you get the highest reading. A low compression motor with a lot of overlap will have low psi readings. Is your car soggy and lazy at low rpms?
<HR></BLOCKQUOTE>

Yeah, I checked the compression "by the book" with WOT and plugs out. The engine doesnt seem lathargic at low RPMs...but the 4.11 rear may be helping also. I really dont know the cam timing since I bought the car with this set up.
Is there no good way to tell piston/ring intergrity with a lumpy cam that has retarded valve timing?
Thanks for the input!

Does it use alot of oil? Does it smoke? Does it foul spark plugs? The bottom line is if it runs fine but has a low reading there is probably nothing mechanically wrong with the motor, especially since all cylinders show the same. It may just be a poor combination of parts. A rebuild with more compression will definatly pump up the horsepower.
I have seen people check for worn rings by squirting some light motor oil into the cylinder and see if the pressure reading goes up considerably. Just be careful not to put too much oil and hydralic the motor and do some real damage.
Just for grins you might want to borrow another compression gage just to verify accuracy before you spend alot of time and money.