20 replies to this topic

[BaronVonchesto]

Had a coolant leak the other day and had to drive home some 30 miles.  Engine was running for maybe 10 or 15 minutes (low speed, low revs) before I got to the nearest petrol station for an inspection and refill. Steam everywhere in the bonnet, and had to top up close to 2 litres of water. Then drove home slowly stopping every 5 or 10 minutes top up the water.

 

Cause of the leak has since been identified and fixed, and everything seems to be running just fine, but I figured no harm in doing a compression test. Here are my results:

Cold, Dry:
No 1: 92.5 psi
No 2: 105 psi
No 3: 105 psi

No 4: 105psi

Cold, Wet (sprayed some WD-40 down the each chamber before testing it)

No 1: 97.5 psi
No 2: 105 psi
No 3: 105psi

No 4: 107.5psi

 

These numbers seem a little low to me. Engine was rebuilt in 2019 and has done less than 20k miles since then. At the time i fitted a brand new C-AHT88 head, and had it and the block decked to raise the CR up to about 8.5 if memory serves correctly. yet the Wet compression test numbers suggest the CR is between 6.6-7.3. Pretty awful really.

 

The engine runs well and I wouldn't have thought anything wrong if I didn't do this test, though I feel like it used to have a bit more oomph shortly after the rebuild.

 

edit: now i know these numbers are pretty darn low. my compression tester is a cheap one, and the head has screwed into the cylinder head by hand as there isn't a hex head to use a socket. so i guess these numbers aren't reliable?

Edited by BaronVonchesto, 07 October 2022 - 04:54 AM.

[hazpalmer14]

8.5 is quite low, I'd expect that sort of compression ratio to be used on a forced induction engine with big boost

[BaronVonchesto]

8.5 is quite low, I'd expect that sort of compression ratio to be used on a forced induction engine with big boost

IIRC the 998's factory compression is about 7.6?

[Lplus]

 

8.5 is quite low, I'd expect that sort of compression ratio to be used on a forced induction engine with big boost

IIRC the 998's factory compression is about 7.6?

 

The lowest CR I can find in the Haynes manual was 8.3:1.  '83 to '87 manuals were 10.3:1, '88 onwards were 8.3:1 or 9.6:1

 

Just to confirm, compression tests are usually done hot, with all plugs removed and the throttle wide open.

Edited by Lplus, 07 October 2022 - 09:06 AM.

[Ethel]

Can't remember the figures off th4 top of my head, but compression was lower on base models before the A+ at the end of the 70's, the expectation was they'd run on lower grade fuel.

 

You could still be seeing higher numbers, BUT it's not an exact science. More air can leak out on a slow spinning starter than when it's running under its own power. I'm not sure WD40 is a great choice either it's as much solvent as lube and too thin to fill the ring lands.

 

At least they're all similar, which suggests there are no specific faults.

[ACDodd]

This is where a compression test can be misleading, if the readings are low across all cylinders it can mean the engine is fine and the gauge is out of calibration, or it can mean you have issues on all 4 cylinders. In this case I would follow that test up with a cylinder leakage test to determine if indeed all cylinders are sealing or not.

Ac

[BaronVonchesto]

 

 

8.5 is quite low, I'd expect that sort of compression ratio to be used on a forced induction engine with big boost

IIRC the 998's factory compression is about 7.6?

 

The lowest CR I can find in the Haynes manual was 8.3:1.  '83 to '87 manuals were 10.3:1, '88 onwards were 8.3:1 or 9.6:1

 

Just to confirm, compression tests are usually done hot, with all plugs removed and the throttle wide open.

 

aah you're right. i was a number down. checked my old notes. I was targetting a CR of 9.6 not 8.6 when doing my calculations for how much to skim the head 

[BaronVonchesto]

This is where a compression test can be misleading, if the readings are low across all cylinders it can mean the engine is fine and the gauge is out of calibration, or it can mean you have issues on all 4 cylinders. In this case I would follow that test up with a cylinder leakage test to determine if indeed all cylinders are sealing or not.

Ac

Yea thats a good point. I strongly suspect that my gauge is off. The engine honestly drives fine. If the compression were that low there would be serious power problems I think. Might take it to someone who can do a leakdown test. or borrow a better quality compression tester. one which can be tightened with a socket, instead of by hand

[Cooperman]

Please confirm that the test was done with all the plugs out and the throttle fully open and with a fully charged battery.

WD40 is not ideal and some air-tool oil or 3-in-1 would be more ideal.

If they still look a bit low, do a leak-down test and, if that comes up not too good, take the head off and have a more detailed look.

[Homersimpson]

Even if it is low if it drives ok I would carry on with it for a bit and see how it goes and if its using oil or water.

 

Given the OP's location its possible that the engine had a lower compression design to cope with a lower quality petrol that might have been available locally when the car was new.

[nicklouse]

I strongly suspect that my gauge is off. The engine honestly drives fine. If the compression were that low there would be serious power problems I think. Might take it to someone who can do a leakdown test. or borrow a better quality compression tester. one which can be tightened with a socket, instead of by hand

That could well be the issue.

[Ethel]

Hand tight should be good enough it it has a rubber O ring.

 

 

The obvious thing would be to try it on another car. Though if it runs ok it probably is ok: lack of power, oil consumption, poor acceleration & mpg would also be present with excess blow by.

[BaronVonchesto]

Please confirm that the test was done with all the plugs out and the throttle fully open and with a fully charged battery.

WD40 is not ideal and some air-tool oil or 3-in-1 would be more ideal.

If they still look a bit low, do a leak-down test and, if that comes up not too good, take the head off and have a more detailed look.

yes test was done with all plugs out and throttle fully open. battery was also fully charged. but i did the test with the engine cold.

 

Will try to borrow a better quality compression tester that can be tightened with a socket (unlike mine which has to be hand tightened). or perhaps just go straight for a leak down test for peace of mind.

[BaronVonchesto]

Please confirm that the test was done with all the plugs out and the throttle fully open and with a fully charged battery.

WD40 is not ideal and some air-tool oil or 3-in-1 would be more ideal.

If they still look a bit low, do a leak-down test and, if that comes up not too good, take the head off and have a more detailed look.

yes test was done with all plugs out and throttle fully open. battery was also fully charged. but i did the test with the engine cold.

 

Will try to borrow a better quality compression tester that can be tightened with a socket (unlike mine which has to be hand tightened). or perhaps just go straight for a leak down test for peace of mind.

 

Even if it is low if it drives ok I would carry on with it for a bit and see how it goes and if its using oil or water.

 

Given the OP's location its possible that the engine had a lower compression design to cope with a lower quality petrol that might have been available locally when the car was new.

i doubt the CR would be lower. For one, i changed the head when i rebuilt the engine in 2019. Secondly i decked both the block and the head to raise the CR.

[BaronVonchesto]

Hand tight should be good enough it it has a rubber O ring.

 

 

The obvious thing would be to try it on another car. Though if it runs ok it probably is ok: lack of power, oil consumption, poor acceleration & mpg would also be present with excess blow by.

 

yes it has a rubber o ring.

 

Thats what puzzles me. I mean the car drives fine. It does feel slightly sluggish compared to when i first rebuilt it, but overall it is still a treat. I just took the car on a 900+ km trip to Thailand and back and it drove perfectly well. No signs of oil consumption, poor mpg or acceleration. Which is why the more i think about it, the more i suspect the compression tester to be off.

but perhaps i should try a hot compression test next.