Compression test rerults - analysis?

malcolm · Post by **malcolm** » Mon Nov 23, 2015 3:58 pm

So, did my compression test today.
Series 2 4.2ltr, UK car
Engine was hot
Results were, reading 1 to 6,
145, 150, 140, 135, 140, 140
Do those seem ok, or should I worry about number 4?
Also, Nos. 3 and 6 were initially read as 125, but I re-did them (after engine turning to test other cylinders, with plugs out, so no fuel build up) and got the 140's.
Any views appreciated

cactusman · Post by **cactusman** » Mon Nov 23, 2015 4:09 pm

Hi. My 3.8 with 9:1 pistons showed 185-190 psi qcross my six pots. Your look pretty consistent to me? If your pistons are 8:1 then probably your reasons are about right. I would not worry...but there will be people with better expertise than me.....Julian

Post by **mgcjag** » Mon Nov 23, 2015 5:07 pm

Hi Malcolm....150 is good for a 9.1 engine.(what is yours 9.1 or 8.1)....was it really up to full temperature...135 is still not too bad......what condition is the engine in..how many miles.....why did you do the test was you having problems... if the car is running will i wouldnt worry about it.. Steve

malcolm · Post by **malcolm** » Mon Nov 23, 2015 5:35 pm

Well, engine is original 9:1 uk, as shown by matching numbers (checked)
It's done 90,000 miles, but had a lower end re-build about 20,000 miles ago, and cylinder head re-build by Rob Beere about 12,000 miles ago. Engine runs very well, car quite quick, but uses 1 pint of oil every 350 miles'ish. You can see small amounts of grey smoke when you cane it (4500 revs + a bit)
Just driven it 1700 miles over the Spanish Pyrenees withouit a single hiccough, so I know there's nothing terribly wrong, but thought I'd have a look see!
Pressures nowhere near those of Cactusman, but a quick dirty reckoning with PSI to atmospheres suggests I'm getting about my 9:1? Yes?
Engine was hot; I took it for a 15 minute run, temp guage up to normal
Thanks for replies by the way! Always good to hear from people who know a lot more than me - and that's just about everyone here!

kingzetts · Post by **kingzetts** » Mon Nov 23, 2015 8:25 pm

You can't directly convert from compression test pressures to compression ratio because of the effect of adiabatic heating of air when compressed (think how hot a bicycle pump end gets when you pump tyres up). A perfect 9:1 mechanical compression ratio would lead to considerably more than 9x atmospheric pressure due to the increase in gas temperature.

Having said that, I'd not be concerned at the results you report. The fact that your engine is performing well and the results are consistent between all 6 cylinders is a good sign. Also, comparing results between different people is fraught with danger as the quality and therefore accuracy of compression testers is notoriously variable.

abowie · Post by **abowie** » Mon Nov 23, 2015 10:14 pm

Tiny bit low and consistency is important too but at 90,000 miles if it was mine I'd just drive it.

180 plus is high; too much skimming of head.

christopher storey · Post by **christopher storey** » Mon Nov 23, 2015 11:23 pm

Absolutely nothing to worry about in those figures

Moeregaard · Post by **Moeregaard** » Tue Nov 24, 2015 1:02 am

I'd be happy with those figures. If you want to turn it into a real science project, squirt some motor oil into the bores and repeat the test. If there's little change, there's some leakage around the valves; significant change in results indicates worn rings. From your numbers I don't see a problem. The oil consumption isn't a big deal either, especially when driving the car hard. I assume that when the head was rebuilt you installed shorter guides with seals on the intake side.

mooney1el · Post by **mooney1el** » Tue Nov 24, 2015 2:22 am

Because of the vagaries already mentioned, I am not a big fan of the stand alone compression test. In addition to doing one, I also use what we call a "leak down test". I am sure a simple google search will turn up multiple hits on this test and the recommended procedure. I do them as a matter of course to monitor engine "health". And for what it is worth, I listen in the exhaust, the intake, the crankcase and the cam cover for any tell-tale leakage.

That said, to me, your numbers show no concern.

Richard

malcolm · Post by **malcolm** » Tue Nov 24, 2015 10:01 am

Thanks all for your replies. I'm re-assured, and will carry on as is.
Want to find another car to test now, 'cos it was fun!

MarekH · Post by **MarekH** » Tue Nov 24, 2015 10:45 am

Dear Malcolm,

There may not be much to be learnt from that. The value of the test is that you use the same equipment, do the test the same way, on the same engine cylinders and look for disprepancies between them.

For example, there would be no fair comparison by using a different gauge second time around, or testing done at different altitudes or different engines etc.

The best advice would be for people to get a reference reading after they have just rebuilt their engine or when it is running fine, so that they have a baseline figure to compare to when they later suspect something may be amiss.

kind regards
Marek

malcolm · Post by **malcolm** » Tue Nov 24, 2015 11:23 am

Thanks Marek. Actually, all I meant was that I wanted to try out my shiny new guage on more cars! Not for any scientific analysis, just for the sake of using it. But thanks again

cactusman · Post by **cactusman** » Tue Nov 24, 2015 2:18 pm

Head never skimmed according to a vast pile of receipts going back 20 years. Starts on the button. Runs like a dream and goes like a train so 180+ I am happy with. :) julian

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