Very odd compression test readings

antontsar · Post by **antontsar** » Tue Oct 30, 2007 9:05 pm

I fancied for the first time ever to do a compression test. What I discovered surprised me in that the readings on one head are all about 170psi (cold) but the readings on the other head are all around the 155psi level. This is a TVR Tuscan Challange engine using composite head gaskets. What could cause this odd discrepency?

ChrisJC · Post by **ChrisJC** » Tue Oct 30, 2007 10:32 pm

Two possibilities:
- One head has been skimmed. I don't know how many millimeters are required to give that sort of compression ratio difference......

- Or the heads are mismatched. Some heads have the valve seats more recessed into the heads, thus lowering compression ratio. I don't know how you'd tell them apart from the outside - I suspect you can't.

Chris.

Ian Anderson · Post by **Ian Anderson** » Wed Oct 31, 2007 8:35 am

Are you sure both heads have comp gaskets
Perhaps one is tin and the other composite

IAn

ian.stewart · Post by **ian.stewart** » Wed Oct 31, 2007 10:31 am

Sounds like time to take the heads off and have a looksie, as its commom to each bank I am with Chris, with his options, 170psi even sounds a bit low for a race engine, my street engine is normally over 200psi, you should be able to chek the gasket in situ to see if they are odd, probably with your fingernail in the gap, or a feeler gauge, Tins are 0.015" and comps are around 0.045"
Hope its the gasketsand not a dodgy head,
Just another thought, Check the tappet clearances and make sure thay are all correct. , unlikely but they could be set incorectly on one bank giving a longer bleed off period showing a lower compression.
Ian

mgbv8 · Post by **mgbv8** » Wed Oct 31, 2007 8:22 pm

Wrong pedestal shims on one side?
Assuming you dont have adjustable rods that is.