Thread: compression test numbers

My car has been smoking since i got it back, so i ran a compression test on my motor today which has about 400 miles on it. Its a DE-T rebuild with 8.5 CR pistons. I ran the test 100% cold and got 139, 136, 150, 145. I saw what appeared to be fuel in cylinder 3 so i ran the test again after the car had been on a few seconds and got these numbers: 155, 151, 165, 160. I dont know which set i should go off of, but they are consistent with each other. Do you think my compression is good or should i be concerned?

I think it could be my valve stem seals but im really worried its my rings

I've wondered about forged aluminum pistons...since they expand when hot, I wonder if a compression test is best done warm, otherwise wouldn't air just leak past the pistons?

-G

Shouldnt the rings take the gap? Numbers will be lower but still enough to be a good test.

Compression numbers are supposed to be done with the engine warm. Your numbers are a bit off on 1 and 2 but still reasonable.

I assume your on factory pistons? Does it smoke any out of the breather while its running? If so you could possibly have cracked ring lands. Compression numbers will still turn up fairly decent just like yours for a while until the cylinder wall wears heavily from the broken lands. You will have smoke out of the breather and excessive oil blown out of the breather as while boosting.

idk? I would guess it would mess the compression test up enough to make sure you do it warm as ashton says above this post. I don't think the rings are supposed to be able to hold as much pressure as the gap machined into the block before the pistons expand. In other words, rings are not good at holding compression, pistons are.

-G

You can do a comp test hot or cold. I always do mine on a hot motor. You will see lower numbers on a cold motor. Nissan usually on lists the hot comp numbers in the fsm. I would do a leak down test if you have any questions about the condition of the motor. even with 400 miles on the motor most of the break in has already happened.

I have forged pistons, I will try to run another test with the engine warmed up, but probally wont be until next week. It doesnt smoke at idle only on deceleration.




if i can get hold of an air compressor i will do a leakdown test.

ok, cracked ring lands are probably out of the picture then, lol.

Could just be bad ring sealing. Just never know. A leakdown will tell you what you need to know.

Interesting that the hot comp numbers are what's listed in the fsm. If that's the case, then I think everyone w/an SR20 should be doing hot compression tests to get an accurate comparison to what Nissan says compression #'s should be.

Having said that, I'd guess it'd be more of a requirement to do a hot test on a forged piston motor than a cast piston motor due to the increased piston to wall clearances and the expanding of the pistons when warmed up.

-G

So what are some comparable cold numbers to hot numbers? ANyone have a ballpark guess? 100? 80?