Low compression

willdog · Post by **willdog** » Tue Mar 09, 2010 11:17 am

Hi all,
Did a compression test on the motor. Got 112psi evenly across all cylinders. I'm used to seeing around 150. Not this low! But it's even...

A search did bring up photoglossy mentioning minimum psi should be 129.

It raises the question as to whether it was rebuilt that way to take on High boost levels. Dunno about forgies but it has a copper head gasket coupled with an O-ringed block. Anyone kno wat sort of compression ratio it would be?

Will the psi increase by much u reckon if the engine is at operating temp?

Thanks,
-wiLL.

nz_n12turbo · Post by **nz_n12turbo** » Tue Mar 09, 2010 3:11 pm

copper head gasket would drop your compression significantly , if that was a cold test then i wouldnt be to worried as a hot engine will produce higher figures the 129psi min is assuming that the engine is stock as a rock .

just wind up the boost, it may be abit slugish off boost but thats just a side affect of a decompressed engine

willdog · Post by **willdog** » Wed Mar 10, 2010 12:56 am

Sluggish is a very good way to describe it. Despite spooling a small t2. i'm used to seeing full boost aroun 2800-3000 RPM. Its moved towards 3200RPM

I do hope its not close to some threshold that stops the combustion chamber from working at all. Any thoughts as to what such a figure would be?

For example i know that 50-60psi does jack-all and effectively renders the cylinder useless.
This has happened to me in cylinder #4 after what i believe to be a leanout. I must've detonated and busted the compression ring. I was running on three cylinders with the #4 cylinder essentially foating up and down.

Thanks for your thoughts,
-wiLL.

Kimmo · Post by **Kimmo** » Wed Mar 10, 2010 4:45 pm

AFAIK, these engines run a pretty low compression ratio in the first place; I doubt if going lower still is a good idea for the street.

willdog · Post by **willdog** » Wed Mar 10, 2010 11:26 pm

Is it a lemon? I'm thinking whoever rebuilt it could've botched it? With high boost, it doesn't neccessarily require forgies does it? As I recall the cast ones holding up fine in some quick ET's.

nz_n12turbo · Post by **nz_n12turbo** » Fri Mar 12, 2010 9:45 am

i have herd that you need 90psi to start a car , why are you thinking its a lemon ? does it smoke, use oil or run bad? or is it just because you done a compression test and got low numbers?

willdog · Post by **willdog** » Fri Mar 12, 2010 1:05 pm

Yeah just the low numbers. Factoring in wear/damage of the rings over time, 112psi doesn't seem much of a margin of 90psi.