Compression Test

[colinevans]

Hi Guys
I am just about to start pulling my S2 apart for a full body rebuild, so I thought I’d do a compression test to see if the engine needs surgery whilst its out. I haven’t had any major issues when driving in that respect and it runs nicely without smoking but my mileage has been limited.
Oil pressure is 60 cold, 50 warm at 3000rpm and about 42 hot at 3000. On a high tickover its 50 warm and about 35 hot (I’m pretty sure I have seen it drop lower than that on hot tickover in the past though)
Compression as follows;

1 118 Dry, 120 Wet
2 120 Dry, 125 Wet
3 118 Dry, 122 Wet
4 120 Dry and Wet
5 110 Dry, 115 Wet
6 118 Dry, 121 Wet

I don’t know whether these readings are good bad or indifferent for a 50 year old 4.2 but I would be grateful for any advice. I have plenty to do on the body but if these readings seem low it would seem sensible to sort the engine whilst I have the opportunity as this car is a keeper for me.
Cheers
Colin

[christopher storey]

Those look perfectly alright to me. It is consistency , rather than the absolute value, ( which can vary enormously with e.g. temperature of the engine, whether the throttles were fully open etc ) which you are looking for . If you had either piston ring or valve trouble , you would expect the affected cylinders to be markedly lower than the others . The only one which is a slight outlier is 5,( but even then it is within the expected 10% tolerance ) and it might be worth checking the valve clearances as a result

[colinevans]

Hi Chris, yes I though they were all pretty consistent. Engine was warm and throttle open. Thanks for the advice!

[malcolm]

I read somewhere that doing the test with throttle open just by pushing the pedal to the floor isn't adequate, and that you need to manually raise the pistons in the carbs when you test - is that right Christopher? (Or anyone else! )

[christopher storey]

Malcolm

A very interesting point which I've never heard of before . I imagine that in theory it will make some difference to the degree of filling , and therefore the effective compresssion ratio , but at the very low cranking speed we are talking about there is probably more filling lost by valve overlap than would be gained by lifting the pistons - and to do it on a 3 carb car would be a bit cumbersome

[malcolm]

You're right about the cumbersome bit! Need a team of three, 2 for the carbs and one cranking.

In the spring I might do a compression test and compare the two methods to see if there is a notable difference.

I suppose the theory is that without the engine running, flooring the throttle doesn't really do anything?

[mgcjag]

Depends who you listen to....some will say completely remove all restrictions...air filter etc....then the option hot, cold or warm?.......really your just looking for consistancy across the bores.....Steve

[abowie]

The actual compression numbers are a little low. You'd be looking for more like 140 to 150 psi on a fresh engine.

As mentioned above the consistency is probably more important than the actual number though. That, coupled with your good oil pressure and lack of smoking or excessive oil use suggests that your engine is in good nick.

So, were you planning to just continue driving the car, I'd leave it alone.

However if by "full body rebuild", you mean a ground up nut and bolt resto, I wouldn't do that. I have seen a couple of engines with OK numbers and no noises or bad behaviour, when stripped, to be totally shot inside.

Were it me, I'd strip the engine and get everything cleaned and measured professionally. If at that point bores or crank journals are out of spec I'd do a total rebuild.

Sure that's going to cost £5k but in comparison to what you're going to spend for a ground up it's cheap insurance. We charge 20 hours labour to remove and replace the engine and if you've just finished a resto you really don't want to be doing that with your fresh paint because you tried to cut corners with a tired engine.

If they are within spec then a rebuild with new rings, bearings, oil pump, timing chains and tensioners, crank sludge plugs, freeze plugs would see your engine go another 50 years.

I would also strip the head and have it inspected and probably have the valve seats recut at a minimum as you do have one cylinder that is a little out of the range of the others.

As above I'd also get the gearbox overhauled, fit a complete new brand name clutch and have the tailshaft balanced after fitting new U joints.

[colinevans]

Thanks Andrew. Its a tough one- thats kind of what I was thinking when I wrote the post. I wasn’t sure what the costs of a rebuild were likely to be here, I haven’t done anything like it myself before but I’m happy to learn, at least to a point where it needs professional skills. It’s all coming out as the tub is quite bad so it would seem sensible to at least get it checked over. I know the valves were done a while back with little mileage since but I can’t see any evidence/history of any bottom end work, or gearbox/IRS for matter so I think you’re probably right- its worth a look.

[malcolm]

I understand about consistency in the numbers being prime, but the Haynes manual states that for the 9:1 compression, the test should show 180, and for 8:1 it should be 150.
So doesn't that mean that if your figures are significantly lower than that, albeit consistent, then your compression ratio must be down? Which means there must be a problem? (rings?)(valves?)

[colinevans]

I guess Malcolm so but that’s why I asked the question, I know about consistency and the 10% rule but I don’t enough to know whether 120ish is particularly low or acceptable. Andrew makes a sensible point though, probably worth a strip down at least.

[Tom W]

Try a different compression tester before dismantling the engine. Maybe the gauge isn’t too accurate.

[mgcjag]

Hi Colin...once you start a strip down your inevitably into a full rebuild.....my thoughts are if it aint broke then dont fix it.....just a simple thing like undoing a head stud could cause it snapping off at the bottom of the block if there is corrosion......However how far do you want to go and what is your budget ....you say your starting a "full body rebuild" is this diy....have you priced it all up......once you go full body and mechanical rebuild it will probably cost more that the finnished car is worth.......just something to think about...Steve

[mgcjag]

Malcolm.....the Haynes figures are with engine at normal operating temp.....Colin...are your figures hot or cold...Steve

[colinevans]

Hi Steve
I’m lucky enough to have friends in the trade, so no not a total DIY job. Even at mates/trade rates I’ve done the sums and I appreciate what it’s going to owe me at the end, and it’s definitely more than the car will be worth, but its not about that as I plan to keep it. Also for me its about having a good honest car I can trust at the end of it. It started out as a repair but we’re chasing corrosion in lots of places. That being said there’s also some good stuff and a lot of things that has been done over recent years are good with many parts that were renewed still being decent, so its not strictly a full nut and bolt rebuild. I was always 50/50 about the mechanicals but I do think it would be nice to have peace of mind.
Compression test was hot btw.
Cheers

[mtnjag]

I may have missed it but i don’t see how many compression stroke hits you used for your compression numbers.
Is that 1 or 3 or 5 or cranked until it stopped going higher?
Assume same number of compression strokes for each cylinder.

[colinevans]

It maxed out at 4-5 strokes each time

[christopher storey]

I have a lot of difficulty in accepting that the figures given in Haynes bear a satisfactory relationship to reality. Simple Physics is involved in showing this .If the cylinder volume is taken as 706cc and the combustion chamber net volume taken as one - eighth of that for a 9:1 setup i.e 88.25cc then with the piston at the bottom of the stroke the cylinder and chamber contain 794.25cc. 794.25/88.25 =9 . So, in a sealed cylinder , when compressed the pressure will be 9 X ambient 14.7 lbs/sq.in = 132.3 lbs/sq.in

But, because of valve overlap , the cylinder is not sealed during the whole 180 degrees of crank stroke, there being 57 degrees at the start of the compression stroke when the inlet valve is open ( but gradually closing ) It is difficult to say how much outflow this constitutes, but at cranking speed it means that some of the theoretical pressure is lost

In a running engine the effects are not as pronounced as at cranking speed, because at running rather than cranking speed the full effect of inertia of the incoming gas causes the cylinder to continue filling even after the piston has started its upward travel . At cranking speed I wonder whether there is much of this effect, and I often wonder whether compression testers are calibrated to make some allowance for this deficit, hence thehigher readings often reported

[malcolm]

Interesting Christopher. I did wonder about numbers, because my engine was taken apart and reported as "in great, clean condition" etc etc, yet my compression test (warm engine) showed an average of 140.
Just to see if I could, I then took the head off ( ) and took it too the engineering firm that Angus uses. They said nothing wrong, but gave it a really good clean up and put in new exhaust valves. After putting it back together, compression averaged 135, so lower! Spoke to Paul, head honcho at the engineers, who said that with all the carbon removed from the top of the pistons (which I did) it would be lower! Makes sense I suppose.
So more to all this number stuff than meets the eye.

[mtnjag]

I always assumed i was at spec because at 5 compression strokes i am 175-180 on all cylinders, warmed up, throttle blocked open, all plugs removed for the test, no carbon build up.
Leak down test is at 95% which is perhaps a better indication of ring condition while not of actual cylinder compression.
This has been tested with two separate compression testers.
The eng has one rebuild, very light clean up skim to head, cc’d chambers, 9:1 pistons, higher lift cam but not radical.
And why would Haynes, and although I haven’t crossed referenced i would assume the factory manual, reference 180 for 9:1 spec?
With the logic placing normal compression for 9:1 around 140 I’m concerned I am way over 9:1?