Compression Test Results

TVRTASMIN · Post by **TVRTASMIN** » Mon Sep 24, 2007 9:43 pm

Turned the 3.9 engine over i just bought tonight, and the results after 5 compression cranks were between 90 and 95psi.
The engine was flat cold when the tests were carried out as it is not plumbed in yet.
All the plugs were removed for the test.

Do these results look ok? Engine is a 9.35:1.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Sep 25, 2007 12:54 am

I would say that 90-95PSI is very low and would expect something in the 150-160 PSI in a 9.35-1 for an engine in good condition.

If you have just put the engine in the car without a rebuild then at that age it could have done anything between 100 and 150.000 miles so is likely to be a little tired.

Have you made sure you have oil pressure, as the oil pump may need re-priming and its likely that the bores are fairly dry, a good squirt of oil down each plughole would probably help matters. Good sign is that the cylinders seem fairly similar so it could improve with some use although I feel it would be worth doing a rebuild with some fresh bearings, rings and a hone in the near future if the crank and bores are within spec.

What was the old oil like and is the top end of the engine sludged up, if it is fairly clean it would be a sign of reasonable maintenance and you should get some oil pressure showing on the guage just spinning it over on the starter motor.

Kevin.

TVRTASMIN · Post by **TVRTASMIN** » Tue Sep 25, 2007 7:18 am

I would have expected the compression pressure to be around what you say if the engine had been run and was warm, but i wanted to do a few basic checks before committing myself to the expense of tearing the motor down just at this stage.

The battery i was using only turned the engine over slowly, but i did see 15psi oil pressure.

I intend just running it as is for the time being.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Sep 25, 2007 9:36 am

Well that's a good sign then, try doing the compression test again with a little oil in the cylinders and a bit more charge in the battery.

Kevin.

Pocket rocket · Post by **Pocket rocket** » Tue Sep 25, 2007 8:59 pm

Book value is 130 psi - presumably that is for a new engine

TVRTASMIN · Post by **TVRTASMIN** » Tue Sep 25, 2007 9:19 pm

Well after draining the oil and noticing crud come out after putting my little finger into the slud in the sump pan, i decided to tear the engine down!

Plan now is to fit all these bits i've been buying on Fleabay as part of the rebuild process!

jefferybond · Post by **jefferybond** » Wed Sep 26, 2007 10:37 am

TVRTASMIN wrote:The battery i was using only turned the engine over slowly, but i did see 15psi oil pressure.

Yeah, cranking speed can make a big difference. Also, did you hold the throttle wide open while doing it?

Jeff

TVRTASMIN · Post by **TVRTASMIN** » Wed Sep 26, 2007 4:36 pm

The inlet manifold was off so it would have got plenty of air.