How much oil for a wet compression test?

I just measured a cap full of oil and my Castrol bottle gives me just over a teaspoon which is 3-6 times less than the FAQ's recommended 1-2 tbs.

FAQ compression test

Eric S said:

I just measured a cap full of oil a

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I use a cap full as well.

Well I guess you would need enough to get to the rings, the tops of the pistons are kinda sunken so if you didn't put enough i think the oil would just pool in the middle of the pistons.

AXiS said:

Well I guess you would need enough to get to the rings, the tops of the pistons are kinda sunken so if you didn't put enough i think the oil would just pool in the middle of the pistons.

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You're right but after a few cranks of the engine I think the oil would spread around a lot.

The problem is that if you put in too much oil you reduce the volume of the cylinder and create an artificially high reading.

I don't even know what the ideal pressure should be. My Haynes book says the same as the FAQ at 164lbs for a 1G turbo but the machine shop who built the engine says an 8:1 ratio = 150lbs so 7.8:1 should be close to 145lbs. They also said that since my pistons are 20 over the pressure is reduced a bit because they make oversized pistons a bit lower profile to make up for the larger bore.

My mechanic says his shop manual specs the pressure at between 140-145psi.

I wish everyone could agree on something as straight forward as what the compression numbers should be on a rebuild and how to do an accurate wet test.

As mentioned, we have dish pistons and as such the wet test doesn't do a whole lot for us. Compression is artificially raised whether you add a little or a lot of oil, it doesn't matter. The wet test is inaccurate and really doesn't tell you anything useful. If you had a low cylinder and did a west test, the compression would be bumped up and you'd say "the rings are shot" and go to the trouble of rebuilding the bottom end when really the problem was a burnt valve or seat in the head.

Also as mentioned, your compression numbers are pretty even across the board, so no worries. If you really want to get specific, invest in a leakdown tester and do a leakdown test (requires compressed air). This is the only way to know what's really going on in your motor. I speak from experience.

nukefission said:

As mentioned, we have dish pistons and as such the wet test doesn't do a whole lot for us. Compression is artificially raised whether you add a little or a lot of oil, it doesn't matter. The wet test is inaccurate and really doesn't tell you anything useful. If you had a low cylinder and did a west test, the compression would be bumped up and you'd say "the rings are shot" and go to the trouble of rebuilding the bottom end when really the problem was a burnt valve or seat in the head.

Also as mentioned, your compression numbers are pretty even across the board, so no worries. If you really want to get specific, invest in a leakdown tester and do a leakdown test (requires compressed air). This is the only way to know what's really going on in your motor. I speak from experience.

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That makes sense. My 140psi compression numbers are even accross all cyclinders but the FAQ says 164psi is what I should have so it had me wondering.

Was the car at WOT and warmed up when running the test?

The factory "service limit" for 1G compression is 120. Ideal is 155. Your motor is in decent shape, but probably high mileage and in need of some love to reach its full potential in stock form. You could get away with re-ringing it in the car if you don't want to go to the trouble of pulling it, etc.

I've never seen a 1g with 160-165, not saying they're not out there, but I've never seen one.

If I was you and got 140-140-138-138, I prob would've stopped there. Assuming your bearings are ok and you engine makes decent oil pressure, you're not going to rebuild the bottom end because you've got ~140 on all 4cyl.

Rather than the oil test, just make your own leakdown tester. If you search you can find info. I spent about 25-30 dollars making mine if I recall correctly.

My engine is newly rebuilt with only about 1000km on it.

I think I remember my original pre rebuild compression being 155 in the best cylinder but not sure.

I think 140 for a new rebuild is low but it is bored 20 over and I have heard that that drops the compression because they make the pistons lower profile to compensate for the larger diameter.

I might try a leak down tester after a few more miles. Maybe I need to break it in more.

turbo8765 said:

I've never seen a 1g with 160-165, not saying they're not out there, but I've never seen one.

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I get 180 across the board. But I think that is from a ton of carbon build up...
carbon = bad

thegreatms said:

I get 180 across the board. But I think that is from a ton of carbon build up...
carbon = bad

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Time for MCCC.

turbo8765 said:

Time for MCCC.

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Done. Not much help.


I might try that more powerful cleaner stuff that there was a thread about a few weeks ago. The poster said it dropped his high compression numbers by like 10-15 psi from the carbon removal.
http://www.dsmtalk.com/forums/showthread.php?t=140475

I did the MCCC on my car when it had 165 compression. It didn't help that much. What ultimately did the trick was manually scrubbing the buildup off of the piston tops when I had the head off. After that it was a sweet 155 across.