V6 Essex - correct engine compression

[duncscz]

Decided to test out my Christmas present and check each cylinder compression - I had in mind a figure of 120 psi as a reference. However, I found a range between 115 and 150 (trouble afoot, I guess). Actual readings were 130, 145, 115, 150, 130, 126. Tester suggested a too high reading could be fluid in the cylinder and/or excessive value clearances. Too low, the usual culprits - piston rings, valve setaing, leaking head gasket.
Engine runs okay and the plugs (except 1?) looked cleanish. Doesn't seem to use much oil, but then it's only weekend miles. Engine has done 100K + miles though.
Frist things first - what should the pressure be?

[Pies]

IIRC about 120

[njhucker]

If it runs Ok, & doesnt use much oil, then don't worry.

Gave mine a top end overhaul at 109,000. Found very little wear in the bores. Had to replace one pushrod, as slightly bent, and renew valve stem oil seals, other than that was all good. I forget the exact compresion redings, but they were all within 20%.

[GreenV8S]

Do you know what the nominal compression ratio is? Assuming you're somewhere around 9.5:1, I'd expect a healthy engine to be up around 150 - 160 PSI. So you have at least one good cylinder there! Big variation between cylinders is a sign that you have a problem, but at the low rpm these tests are carried out at, it only takes a small leak to have a big effect. Don't know what order you tested the cylinders, but I was advised the reading would get lower and lower the longer you crank the engine, something to do with bore washing or oil draining away or something, I forget. So if you notice the cylinders reading low are the ones you tested last, consider running the engine up and then testing them again.

[Clive W]

Correct compression ratio for Essex V6 was 8.9:1,Turbo version reduced to 8.0:1. Don't know how it converts to psi!

[HarryW]

As Peter said I would have thought a static compression of around 150 is a good 'normal' figure. Have a look here ,it may help understand what compression testing is doing and also gives some suggestions as to why you have imbalance.
As a layman I've always worked on the premise that the static compression will reduce with wear and age of an engine, with the main concern being that they are roughly even (+/-10%).

Harry

[duncscz]

Thanks - I'll try the more comprehensive test at the weekend and see if that sheds any more light on the differences. So far I've come across various articles which predict a figure of anything from 120 to 170 psi! - gut feeling says 120-130 psi for my engine, so those cylinders higher and/or lower will need checking further.