Ash911

Dear Users,

Would someone be able to kindly advice me what values of an engine compression test would be considered good for 1991 3.3 Turbo ?
I just had the test done for a car I’m considering buying but have no baseline to compare the results to.
Thanks,

Megatron-UK

That seems awfully low... that's averaging what? 75-78psi?

Here's a thread with a posting claiming 130-135psi.
Here's another claiming 124-128psi across all cylinders.

Have they tested it properly??? Those other threads translate to almost 9 bar.

Ash911

Thanks for that .. online I see the new 3.3 turbo

engine should be 7.0 .. but that value seems very low.

Megatron-UK

Yes, compression ratio is 7:1 as standard, so you're never going to see a high reading like a modern, normally aspirated engine... but sub-100psi is very, very low by my estimates. Surely that test has been taken wrong?

Would a 3.3 at 75psi even run right? Wouldn't you be getting loads of oil past the rings?

I'm no expert on the engine... but it doesn't seem right to me.

Black_Hat

You need a leak down test done with engine cold then warm

same for oil pressure at idle and 2500 RPM both warm and cold

compression testers are notorious for not giving accurate numbers across different brands what you are looking for is no or a small compression difference between cylinders combined with the leak down results and oil pressure



Stefan

Metal Guru

I don't put a lot of importance on absolute values because they are affected by atmospheric conditions a lot. Also, if your compressor's only putting out so much pressure, that's reflected in the leakdown numbers.
Looking at the relative numbers, they look good. No cylinders seem to be abnormally low.
A compression check would provide some more useful data to the picture.