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Compression test numbers - what is normal?


carreraberry
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lower numbers (130's) are not necessarily a bad thing. What you want to see is realitvely similar numbers (165 with the rest in the 130's is not a good thing) This is what I have always been told but take in mind im not expert on compression/ mechanics. I remember reading somewhere that a properly rebuilt fresh engine should be in the 160's on all cylinders, though I may be misrembering.

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Did you test with the engine hot? Throttle wide open? if not try again. Compression testers do vary, so the comparison between cylinders is more important. Usually look for no more than a 10% variation. You should also do the test dry and then with a small amount of oil in the cylinder to check the rings.

I normally rely on a leakdown test as they tell you more.

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+1 on the Dry + Wet test.

+1 on a max variation of 10%.

In your example, the #1 seems like it's an inaccurate result. #4 is the cylinder most likely to be the weakest, generally speaking. But #1 is especially strong compared to the rest.

Was the head torqued prior to the testing?

How many miles on the motor? On the last valve job?

I realize you may not know the answers, but for a car you're considering purchasing, if it were me, I'd think it warranted further investigation.

Right now, any issues are his, but they become yours once you buy it.

No real shortage of '02s out there - Caution should be a watch word.

Cheers!

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It looks like your motor is going to need a rebuild. Cyl 1 looks good but that’s about it. Cyl 2 is real weak with Cyls 3 & 4 in the poor to normal range.

A strong 2002 motor should have all four cylinders around 150 psi or above. The normal rage is about 135 – 149 psi. Poor range: below 130 psi.

All cylinder readings should also be close and yours is not.

Rebuild it now while it’s still running. It will cost more if you wait and it breaks.

Best of luck.

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The most critical element in the M10 compression test (apart from the obvious warm engine, wide open throttle and fully charged battery) is correct valve clearance. A couple thou variation in the valve lash makes very large changes in your compression readings.

Don't rebuild your engine until a qualified mechanic/engine builder deems it necessary.

A little knowledge is a dangerous thing. I'll bet money that if you reset your valves to proper clearance your #s will even up, and save you about 3 grand on an engine build.

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compression test 1502 - 2002tii

good = above 10.5 bar........... (149.3 lb/in)

normal = 9.5 bar....................... (135.1...149.3 lb/in)

poor = below 9.0 bar.............. (128 lb/in)

test procedure

use calibrated tester,

with battery fully charged,

engine at operating temperature

and throttle fully open.

Turn engine over at starting speed.

ref BMW Repair manual vol. 1

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Odd, CD, that there is no variance given for compression ratio,

as an M10 could range from 8.1 to 9.5 and above,

and we all know that the same cam went in most of them...

But yeah, on the face of it, something's funny with #1.

Beyond that, all it shows is that the motor will run, and reasonably well.

I've had engines with better numbers than that have terminal mechanical

problems (broken rings)

and the motor in my street car right now (pretty strong, if a bit smokey)

was lucky to make 120 on any hole if I cranked it a bunch.

Lash valves, leakdown (good valve test, not so great for ring or bore)

and then maybe try again...

hth,

t

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