Hi Guys,

Looking into a prebuy for a 95 XJS v-12. I was reading Kirby's book about performing a compression check. But does anyone know what are good numbers that I should look for from the mechanic? Thanks...

 

@Manny

When I got my compression test done my mechanic told me that the absolute values were not all that important, it was the range of values that mattered. (Assuming the car is actually running, which removes the possibility of all cylinders having a problem, which I guess you could get if someone had been messing with the heads, or just rebuilt it badly, or something). He said he had seen cars give quite different absolute results compared to each other, but each had a very small range and was deemed to be good (This was subsequently proven to be the case). Maybe others will chime in with a definitive answer, but I was happy with this explanation and used this data to help make my final decision to buy or not

 

Your version is a 6.0, so I am not sure of the number required. Mine was about 128-130 all around, however it is a 5.3L HE. As stated above, delta is important.

 

From my 1995 X300 service manual, 6.0 engine section, you shoud see 13.6 to 16.3 bar or 197-236psi.

As far as I know there were no significant changes in 6.0 spec as used in the XJS so I'd go with those numbers.

Cheers
DD

 

Good that they're even but that sounds awfully low. The service manual shows 200-240psi for the HE engines. I expect the lower end of the spec if the the 11.5:1 engines and the hiigher end for the 12.5:1 engines

Cheers
DD

 

I am not sure either, maybe that Chinese gauge is off, will check again.

 

Yep 200 to 240psi.

More important is the delta between the cylinders make sure they are all within ±5psi. Also if you do the test yourself make sure it is done with the throttle wide open, if not you will get lower readings.

 

That I did not do, I kept cranking with it closed.

 

This is most likely why your pressures were down. With the throttle closes you wll be getting less than 5.3L of air into each cylinder.

CR is the ratio of volume of air at BDC to TDC in other words how much the engine compresses the air. So a 10:1 CR will compress 6.0l of air ( a 6.0l engine) into 0.6l so if we say we have a combustion chamber volume of 10cc 0.6L and 10cc gives say a pressure of 200psi

If you only fill the cylinder with 3.0L of air when the engine compresses the 3.0L into 0.3L in 10cc the pressure will be 100psi.

 

Warren, you are right but your maths are wrong.
5.3 litres is the displacement for all 12 cylinders.

displacement = Pi/4 x bore squared x stroke x number of cylinders

So the 5.3 will be 0.785 x 81cm sq x 7cm x 12 = 5.34cc

Therfore only 445cc = 0.445 litres of wind in each cylinder.

 

I know I messed up, no need to put a number on it!

 

I realise that the numbers were only to make easy for people to follow