Bono

I have seen a few random posts and thoughts on cylinder compression tests, but the PSI data seemed to be all over the place, and I'm hoping to get fresh/definitive thought on how my 1989 944S2 should test out.

I have a very nice Snapon compression tester, and tested all 4 cylinders this past weekend and got PSI's of 150, 145, 145, 150. After doing some additional research I realize that I did not do a "wet" test by adding oil to each cylinder before testing, and also, I did not have the throttle wide open ("WOT") while testing.

I'm going to retest this weekend and will make sure the cylinders are wet, and also that I have the throttle wide open.

My question is what is a stock/good/rebuilt engine reading for PSI. I have seen some posts where people say their cylinders have tested out above 200PSI? Seems crazy high relative to my dry, closed throttle test, but I'm hoping my numbers will be higher.

Any information you can provide on what cylinder PSI should be will be much appreciated.

951and944S

In a perfect world, with throttle open, at sea level.

Atmospheric pressure is 14.7psi X compression ratio, 147 psi @ 10:1 CR.

I think the S2 has the same 10.9:1 CR as the 944S.

160.23

Wetting helps seal the cylinder but will also raise compression equal to the mass that the liquid displaces.

T

PaulD_944S2

"Wetting helps seal the cylinder but will also raise compression equal to the mass that the liquid displaces."

Volume, not mass. But we got it. But you might be right since more mass would occupy more volume, or something like that!

mikey_audiogeek

Worth getting a leakdown test done? Usually more revealing than a compression test.

Cheers, Mike

Eric_Oz_S2

Sounds about right. Expect around 150 engine cold, and around 170 engine hot. Don't worry about oil, just test with engine hot. You should be more concerned about differences between cylinders. Absolute values don't normally mean much.

Bono

Thank you for the responses. I like the mathematical explanation of atmospheric pressure (14.7) X 10.9 = 160. In any event, none of you gave an explanation that would support crazy high numbers i have seen in forums like 200psi.

I will retest this weekend wet and with WOT and will report back on results. I will also make sur engine is warm.

Thanks for the quality answers.

PaulD_944S2

Bear in mind that carbon accumulated on the piston and cylinder head can and will raise the compression pressure.
The big picture is for the cylinders to be close to each other and the calculated pressures.

Bono

So I was back in the garage this weekend, re-did my compression tests, and thought I would share information that I think will be helpful to others on this topic of compression tests. I thought I should also mention at this time that I ran these tests on my 944S2 race car.

As was the case with my tests the first time around, I warmed up the engine, but this time I was sure to test with a WOT (wide open throttle) and I also introduced a small amount of oil to each cylinder to do "wet" tests as someone previously suggested.

The first most valuable thing I can definitively say, and to my surprise, is that you can absolutely get psi readings greater than 200 psi, and it would seem that introducing a greater amount of oil to the tests is what causes these spikes. I added too much oil.. maybe 1/2 ounce to the first cylinder I tested, and with a WOT and fresh battery I got a reading of 205 psi in the first cylinder toward the front of my 944S2. So I think it is very important to make sure you add the exact same amount of oil to each cylinder to control test results.

What I also found is that I seemed to be chasing an engine that was cooling off and also a battery that lost some charge from cylinder to cylinder. To address the battery issue, I used the start function on my battery charger to make sure I got a good crank for each test.

Bottom line after I felt I had all three factors (engine heat, battery strength and amount of oil) evened out as best I could, is that from front to back my compressions were +180, +182, +165, and +180.

My third cylinder from the front is definitely a bit lower than the other three, and the additional of oil didn't seem to have as much of an affect on this cylinder. Based on what I have read, I will probably have to go to a leak-down test to further determine what might be going on with this 3rd cylinder.

But, with all 3 cylinders above +165 psi and with all being within 10% of each other, I'm going to leave the car as is for the coming season and not do anything major. I may ask a professional to do the leak down test just out of curiosity.

Thanks to all for your comments and feedback above. It was a good learning experience to see how just a bit too much oil volume can cause psi's in excess of 200.

Any remaining comments/thoughts/observations will be appreciated.

FHMotorsport

I have tested both of my S2 engines and both had between 215-220 psi on an average per cylinder. Both engines are "healthy".

BMillar

Took my S2 to an independent shop recently, the motor has about 3,500 miles on a "stock" top-end rebuild. The shop owner tells me that my compressions are all above 255. I've not seen any posts or technical data with numbers that high. Is that possible?