what is correct compression?

[NicBowman]

Hi, trying to find my misfire, starting with compression test, I get 6 around 110 to 120psi, one at 140 and one at 150psi. It is a precat 4.3BV with a racier cam than usual.

Does this sound right?

Runs well enough, lots of power, just seems to misfire a bit on constant low throttle opening, esp at 2000 rpm.

Ta

Nic

[Bluebottle]

A freshly rebuilt engine should have 150psi plus across the board, I was told to take the mean value (so long as it is above 125psi) and if all are with 5psi of this you are ok, your range is like mine was before the rebuild.
You need to have a leak down test which will give you a better idea af where the loss is..could be leaking past the piston rings (mine was) could be leaking past the exhaust valves (mine was), could just be the head gaskets but i would hazzard a guess that the valves need re-seating for starters.

Edited by Bluebottle on Thursday 28th June 20:55

[NicBowman]

Ta, sounds like I shouldn't panick.

Nic

[350Matt]

Try squirting a bit of engine oil down the plug hole before taking the reading, if it goes up compared to no oil then the rings or bore are worn if no change and still low compression its valves or head gasket

Matt

[dnb]

Mine was 145 to 155 on my old engine. It leaked slightly through the exhaust valves in the end, but that was it. Power felt fine, but since it was a gradual degredation the power loss was not noticable until I drove another car.

The new build is around 180 for all cylinders.

I'd worry a bit if I had cylinders as low as 120 PSI, but not too much as it's not a huge job to sort out this sort of problem, and the chances are that if it's caught early it won't be a huge bill. As everyone says, do the oil test and see if that changes the results - you'll know a lot more then.

[rev-erend]

Those figures do not sound good.

Mine were all a lot higher than that and I have a wild cam (404 Mechanical) and as has been said before this test should show that all cylinders shows a very similar value (+/- 10 per cent) ..

I it was my car and I was happy that the test was done well - then I would start budgeting for some remedial work or an upgrade (5.3 )