XJS high compression results.

[NicholasXJS]

Just done a compression test on 1996 XJS USA model. It come out at an astonishing 207 lbs per square inch or about 145 kg/cm square. I thought the gauge was at fault until cylinder number 4 which came out at only 150 lbs.

Anyone got any thoughts on this - perhaps it should have a thicker gasket.

It maybe causing problems with the oxygen sensors and the cat.

[PaulGover]

Which engine? Probably a six, as you'd say which bank for number 4.

The gauge is duff, someone's fitted the wrong pistons, someone's skimmed (skimmed? more like carved) the heads for performance, or there's an incredible amount of carbon clogging the chambers. 207 psi is a compression ration of 15:1, which is nearing diesel levels; my guess is you'd be suffering terminal knocking if it were true. My vote is for the gauge being wrong, or you're using it wrong (though I've no idea how).

AFAIR, the highest CR was the early V12 HE engine at an astonishing 12.5:1, made possible by Michael May's cylinder heads, but all later ones were 11.5:1 or lower. the sixes being around 10:1.

Another thought:
Were you trying to test for leaking piston rings by squirting oil into the bores before testing? If so, you might have put too much oil in, and thereby increased the CR, except on the 4th cylinder (which therefore is the one with the duff rings).

[steve_m]

These are my readings from a couple of years ago, on a V12 obviously.

1A 201 2A 199 3A 200 4A 202 5A 201 6A 204
1B 198 2B 210 3B 202 4B 195 5B 205 6B 200

So 207 looks right in the ballpark to me.

[poprivet]

There are many variables with compression testers and the technique used. The key thing here is the odd one out rather then the values.

[leo_denmark]

The air will heat up during compresssion and it's therefore quite normal to see a higher pressure than the compression ratio should be able to create.

[Paul_1959]

For information:

Jaguar specifications for 6.0 V12 (1993 to 1996) engine are:
13.6 to 16.3 bar (197 to 236psi)

With maximum difference between two cylinders
1.38 bar (20psi)

Standard compression ratio is 11:1 for 6.0 V12