Good compression vs Bad compression
 

Whats your opinion on whats good and bad on a 454 or 502 motor? And whats you method of checking compression?

a stock merc 454 or 502 150-160 would be normal with no more than 10%variance,throttle open,ignition disabled.6 or 7 cycles per cylinder,it won,t go higher even with 50 cycles.

this is what Merc recommends and still currently below in their service bulletin. They list in all the current service manuals -- bare min of 100 psi

anyways FWIW

http://www.boatfix.com/merc/bullet/97/97_25.PDF

8,75 - 1 static c/r and the whimpy cam in a stock 502 mag that only has 100 lbs cranking compression is a sick puppy.wtf is mercury marine thinking?

the old Harleys said rering when front cyl was 110 and rear cyl was 90. they knew the rear cyl got hotter and burned up sooner. this merc bulletin ALSO says all cyls within 70% not 90% of highest. they are saying if you get a lil water and stick a few rings, the engine is still acceptable and not their problem...

What are you working on or looking at? There are some variables that can come into play. Static comp CR, Stock, rebuilt, how long has it sat, etc, As mentioned rule of thumb is within 10 % of each other. Doesn't mean if there all low it's ok. Build depending but 130-150 I'd think.