Compression test results - advice?

[malcolm]

As my car uses oil (about 300 miles on a pint), I would appreciate views as to whether or not the following readings are indicative of a problem. I did two tests on a warm engine, one "dry" and another with a small cap of oil added into the bore. The engine is a 4.2 series 2, with a 9:1 compression ratio.

.........................DRY......................OIL ADDED

No. 1..................137......................148
No. 2..................137......................147
No. 3..................129......................143
No. 4..................137......................149
No. 5..................143......................153
No. 6..................138......................144

No. 3 seems to stand out as both the lowest dry figure, and biggest increase when oil is added. Others all seem pretty similar, with No. 5 being slightly higher. Am I due major costs with a re-build, or just keep adding the oil?!

[christopher storey]

Malcolm : I see nothing in those compressions to be concerned about . Furthermore, nearly 60 years' experience of various XK engines suggests to me that 300 miles per pint is certainly nothing out of the ordinary and , indeed, better than many XK engines usage. IMHO It is only if it gets below 100 mpp that it is suggestive of a real need for overhaul

[flatfloor 3.8]

Hi, i have a 3.8 and the readings are up around 175 to 180.

[JJC]

Malcolm : I'm afraid to say that after reviewing your compression test results, I believe you may need a re-build in about 250000 miles !! LOL Drive the hell out of it.......thats what the E was built for. Enjoy !

JJ

[malcolm]

Thanks for that all; I'll carry on chucking the oil in! Just hope that when I do my next track day, I don't suddenly end up in a cloud of blue smoke! I never get quite up to the red line, max out at about 4700, so guess I shouldn't get a sudden piston ring break up etc?

[JJC]

Malcolm, my good man.....my late Dad, another car guy ,had a wonderful theory. He stated that every engine has x amount of rpm's. The faster you use up that unknown number of rpm's, the quicker your engine heads to the overhaul shop. On a 50 year old engine, do you really want to be thrashing about at anywhere near 4700 rpm's ? No one loves E'type's more then I, and it is a fantastic handling car, but now that your baby is in its golden years, why not cap your rpm's under 3800 ? Hate to see your beautiful Jag separated from its engine for 6 months while , even the best of shops, reaches into your pocket for at least $15K for a rebuild, plus labor.

One silly man's opinion.

JJ

[malcolm]

Understand JJ, and on the road I don't, but I like my track days too much, and can't keep my right foot up any more than I could my right wrist when I was riding bikes at silly speeds. So it'll just have to put up with it, as will my wallet probably!

[christopher storey]

Just make absolutely sure that your tachometer is accurate . As they age, there is a tendency to underread

[malcolm]

OK, thanks Christopher. Seems spot on, checking it against sat nav speed.

[PeterCrespin]

OK, thanks Christopher. Seems spot on, checking it against sat nav speed.

That's an interesting concept - checking the tach against satellite data. Useful when you miss a gear and the revs go stratospheric I suppose :-)

[malcolm]

Like it peter. What I mean is, I know that at 2850 revs, with my diff, and my tyres, I should be doing 70mph in top gearproviding the revs shown are correct. At those revs, the hand-held tom tom and smart phone app both confirmed I was doing 70 at those revs.