Any help appreciated on interpretation of my compression test results

Ascot Fawn

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Hey All,

Just did a compression test on the XK engine of my 1972 XJ6S1 that I received in June this year. Why? because I never did it before and just wanted to try it and because I think the engine is not super stable and calm (although I do not have any comparison!!)
I followed the general routine and a screw-in gauge. As the (matching) engine serial number has a '-S' suffix for 'Standard' i assume it is a 9:1 ratio engine. Mileage is unknown. 50.000km on the clock, but could be anything up to 350k off course.
Because i was getting lower results on 2 of the 6 cylinders i made a couple of so-called 'wet' measurements after pouring in 5 ml of oil.

Here are the results

Cyl 6 ; 160, 155,160, 190(w)
Cyl 5 ; 130, 120, 135, 140, 175(w)
Cyl 4 ; 155, 155
Cyl 3 ; 160, 160
Cyl 2 ; 150, 155
Cyl 1 ; 130, 130, 135, 165(w)

So, what do you guys think?
Does this make you happy? It it not very consistent i know, with variations around 20%. This might explain that the engine is a bit shaky?
Brand new 9:1 engines should read something like 170-190 psi, right ? So 160 psi is a good result for a 49 yr old car i guess.
Since the 'wet' measurements are 30 psi up from dry readings on the same cylinder, one might go for concluding that the piston rings in these particular cylinders are worn, right?
But since 'healthy' cylinder no. 6 also shows the same uplift in compression when using a bit of oil, does this one have the same worn piston rings?
and what if they all 6 suffer from the same weary rings; would you expect another reason for the realtively low compression in cyl 1 and 5?

any advice for next steps and general feedback would be great!
Thanks
Jan
 

JimboV8

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Personally, with those results, I would have the head off and replace the rings, just to err on the side of caution.
 

NM_OLD

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I guess it's down to how important it is to you to have super stable and smooth running? As Jimbo says above I'd look to have the rings changed but if you can source oversize pistons and rings it might be a good idea to get the cylinders bored out at the same time?
 

SV8Predator

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Further up the creek.
Did you have the throttle fully open during the tests?

Unless you're prepared to spend big money sorting this out, I'd leave well alone.

Is the car burning oil? Are there clouds of smoke? Do you put on a large mileage every year?
 

Ascot Fawn

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Yes, throttle fully open. No excessive oil consumption. No clouds of smoke. Own the car for 6 months and did 3000km.

I might decide do try first
1) a good valve clearance job first; If out of spec, i guess it can it lower compression readings?
2) a fuel cleaner additive to clean valves etc

and then revisit the compression readings. if the compression evens out on the 6 cylinders to 150+ i probably will not be bothered too much by the suggested weary rings. If unevenness in compression persists, i might need to take the head off first to check on gasket, valves and more?
 

Grubby mits

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Yes, throttle fully open. No excessive oil consumption. No clouds of smoke. Own the car for 6 months and did 3000km.

I might decide do try first
1) a good valve clearance job first; If out of spec, i guess it can it lower compression readings?
2) a fuel cleaner additive to clean valves etc

and then revisit the compression readings. if the compression evens out on the 6 cylinders to 150+ i probably will not be bothered too much by the suggested weary rings. If unevenness in compression persists, i might need to take the head off first to check on gasket, valves and more?
Personally I would leave well alone unless you intend a full engine rebuild - taking the head off will likely open a can of worms. Check the valve timing and clearances, rebuild the carbs, go through the ignition with a fine tooth comb and get it running as best you can and enjoy it for a few years. Then check the compressions at each annual service and see if they are changing. Some compressions you quote are a bit low and the variation between cylinders is greater than it should be but unless you are burning a lot of oil or suffering burnt out valves wait and see.

GM
 
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