So, my 2001 xk8 has somewhere around 100k mi.

I don't trust the carfax report. And going to measure the cylinder compression. Is my primary concern balance between all cylinders? Or some absolute value?

What value would I look for to consider a ring change?

 

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Ken:
There are rules of thumb, but the compression ratio times the atmospheric pressure is the theoretical "spec". So, an XJ8 has a compression ratio of about 11 to 1, I THINK, so you might expect a compression test value of about 160 psi.
Oops- I left off the specific heat index- actual maximum stsic pressure is the beginning pressure (1 bar) times the compression ratio raised to the power of the specific heat index (1.4) for air. So , for 11 to 1, the maximum ststic pressure would be narly 280 psi!! In fact, that is lowered by the valve overlap portion of the compression cycle. And the actual compression ratio of the AJ26 NA is 10.5 to 1.
For standard design engines, after factoring the compression and the valve overlap, the measured compression pressure is 15 to 20 times the compression ratio. So, the AJ-26 should be between abot 150 to 200 psi. Most important is balance between cylinders.
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All cylinders should be within 10% of each other. If your high is 170 PSI then the low should not be any lower than 153 PSI, anywhere in between is fine. Make sure and do it with the throttle wide open.


Wayne

 

Wow. On a V8, I guess the delta between cylinders can be quite high.

How would the throttle opening affect the compressing value?

 

Quite high compared to motorcycles I should say.

 

If two adjacent cylinders are both low then the gasket is probably blown between them. Also if a squirt of oil increases the pressure a lot then the rings are probably bad/worn.

 

With the throttle closed the engine rotation creates a vacuum behind the throttle plate and resulting in lower compression results. I stick the handle of a screwdriver in the throttle body to keep the plate open. The thought is that you don't want to use anything will 1) fall into the intake manifold and 2) deform the edge of the throttle plate.

At 100k miles 150 +- 10.

 

A compression test should be done on an engine that is 'AT OPERATING TEMPERATURE' to get a true indication. It is designed to run at a certain temp and that is normally where it spends most of the time operating.

A cold engine reading will be different but an engine does NOT run very long COLD!!!!!

Most people perform the test COLD because it is more comfortable (less likely to burn fingers) and you don't need to hurry to keep it COLD. It will remain ambient as long as you want!!

bob gauff