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

 

Five ml of oil is a huge amount - enough to affect the readout results. One squirt from an oil can is sufficient.
The suffix "S" denotes an 8:1 engine - S being for standard. Compression for these engines should read around 155/160. I would say your guage is reading high due to the 190 psi reading which even a 9:1 engine would not show. Also the amount of oil used could improve the sealing of the valves.
Lumpy idle can be from a lean setting on the carbs. I run my car this way, for fuel economy. My spark plugs are white when I pull them out. I can get the car to idle better by lowering the jets (HS8 Carbs) Try an adjustment on you carbs to start.

 

Thanks Mekon.

Are you sure about the S suffix denoting a 8:1 engine?
From the BL Jaguar XJ6 service manual Edition 155/3: page A.5
"Compression ratios 'L' or S' are specified for both (2.8 and 4.2) engines, the difference in compression ratios being obtained by varying the crown design of the piston. The CR of an engine is indicated by /8 or /9 early cars; 'L' or 'S' later cars, following the engine number"
My interpretation would be that S stands for 9:1. I can try do some more research on that.

(My spark plugs are hazelbrown and looking just fine, I tried runnig on the rich side a while but did not improve (much) on rough idling)

 

What I would do next would be a cylinder leakage test. You will need a cylinder leakage gauge from OTC tools and a air compressor. Start at either end and put the piston on TDC. Install gauge and add 100 pounds of compressed air. Any air loss can now be considered a percentage. Air leaking from intake or exhaust would be poor seating of valve. Air leaking at radiator cap would be a head gasket. Air leakage from oil cap is rings. 7-10 percent is normal worn leakage.

Thank you
Larry Louton

 

Jan,

I agree that the Service Manual does indeed say what you have quoted - I have one and checked. So I could be wrong - which will be great as my engine is an "S" engine and I have always considered it to be an 8:1 engine. Perhaps this dates back to my E Type days where engines were either S or H, the H being the 9:1 version.
Only thing is I have had my engine apart and the domes on the pistons do not look to be as high as the later series 3 pistons whcih are all 8.7:1 I would welcome more information on this subject.

 

I've never seen a worn out piston ring. I've observed many worn cylinder bores. I have also seen rings stuck in pistons of old engines that have sat a long time. In your case since the compression is just a bit low so consider using a bore scope to examine the cylinder bores and then a leak down test. Scuffed bores in one or both of those cylinders can be observed and account for low compression.

1. Get a bore scope and compare the bores of cylinders with low compression to ones with high compression.
2. If the bores are scuffed, nothing to do but enjoy as is or rebuild.
3. If the bores look good you may want to consider a leakdown tester to try to figure out where the leakage is. When a week cylinder is under pressure you can hear where pressure is escaping.
Pressure in crank case = rings. Pressure heard in the intake manifold = intake valve. Pressure heard escaping in to the exhaust = exhaust valve. Bubbles in the coolant = head gasket.
4. If the pressure goes out the crank case and the bore looks nice then you may have a stuck piston ring. A healthy dose of any "rust remedy" such as my favorite, Liquid Wrench, may fix it. I have seen piston rings so stuck that they have to be broken off to be set free but this only happens if the engine has sat for a long time.

Interesting to me that cylinder 1 and 5 are next to each other in the firing order and if this engine sat it may have stopped with the valves in those cylinders partially open and moisture may have entered and rusted the bore or stuck the rings.

 

Did some more research on the CR of certain engine numbers because there seems to be confusion about engines having a CR of 8:1 or 9:1 when the XJ6-XK engine number ends with a ‘S’

From the BL Jaguar XJ6 service manual Edition 155/3: page A.5:
“Compression ratios ‘L’ or S’ are specified for both (2.8 and 4.2) engines, the difference in compression ratios being obtained by varying the crown design of the piston. The CR of an engine is indicated by /8 or /9 early cars; ‘L’ or ‘S’ later cars, following the engine number”

My interpretation would be that S (in 4.2Xk engines fitted in XJ6 cars) stands for 9:1.
But others state S stands 8:1; The latter makes sense if you look at the XK engine’s in Etype’s. These engine numbers would (sometimes/always?) have a L - S- H suffix, being Low, Standard and High compression, representing a CR of 7, 8 or 9.

Now i scratch my head…let’s check the XJ6 Haynes owner workshop manual: on page 12 it says:
"Compresion… … … ratio; 8:1 (L) 7,8:1 (S3/3.4), 9:1 (S), 8.4:1 (Series 2/3)

So my Engine no. 7L60937S (and all other 7L engines with the suffix S) is a 9:1 !!
(And earlier XK engines fitted in E-types with a S suffix are 8:1 indeed)

--Please don't hesitate to 'challenge' the conclusion of this limited desktop study --

Cheers
Jan

 

Very interesting!
My 1975 XJ6C 4.2 engine # 8L40403L doesn't have any letter after the L. So I don't know what its ratio is. And the pistons are concave in the middle (see pics).

Pulled spark plugs (Champion N12NY). Plugs color looked light brown; fuel burn seems okay. Ran warm compression check - Unpluged fuel pump at dash left/right tank switch, removed pistons out of carbs, all plugs out at same time, Accelerator to the floor on all tries; two tries xx/xx:
1 133/133
2 134/133
3 135/135
4 128/129
5 127/127
6 126/128

The engine seems to be running fine. I just lapped in all the valves. But the compression ratio is definitely low compared to what you all say in the above messages. Anybody have any ideas?

 

Hi Bill,
As quoted above the Haynes manual suggests that all XJ6 4.2 S2 and S3 have a factory CR of 8.4 : 1.A nice compromise it seems :-) .Your compression values seem perfectly fine to me and (even more important) the variation within 10%; which is great.
Did you have the engine rebuild? CR might be different now then the 8.4 factory value, if pistons are replaced or cylinders have been re-bored. Any clue there ?

 

Jan - I don't know if the PO did an engine rebuild. I bought the car 17 years ago and just recently did a valve relapping job. Yes, they are within 10% but definetly lower than the 160ish as reported above. That concerns me.

 

It probably does not have to concern you Yachtman. As my engine is a 9:1 and your XJC engine seems to have the standard 8.4:1 compression common for (all/most?) series 2 and 3 engines. Values of around 130 are reported many times for well preforming 8:1 engines. I do agree that newly fresh 8.4:1 engines probably have slightly higher values than 130. Slight wear of the bores (--and not so much the rings, thank you icsamerica-- ) and thus sligthly lower compression values, should therefore be legitimate. Values of around 155 are probably more common for 9:1 engines.