Motor Compression

[I_R_DA_ONE]

I just checked all the cylinders compression on my '86 with a pressure gauge...but I have no idea what it should read to be good......The average of all cylinders was 163.325 psi.....The lowest was 157psi and the highest was 170psi ....is this good or normal,,, what should be good for a standard L98:confused

thanks in advance
Mike

 

[G Winter]

The amount of diff. from cyl to cyl is what is important.In my opinion the spread you have isn't bad and your pressure is pretty good. You can check ring seal by injecting a little oil into the spark plug hole against the top of the cyl wall. then take another test and compare to the dry test on each cyl. If you get a large diff. It can indicate a poor seal of the rings. The cranking speed has a lot to do with the amount of cyl pressure you have. So how good your starter is and how strong the battery effects the amount of pressure.

 

[Ken]

Those are good numbers.

With the lowest number reading at 157 psi and the highest at 170 psi, that's works out to 8 percent difference, which is perfectly acceptable for a stock engine, especially one that has a few miles on it, as I imagine does yours, being a 1991 model year.

However, if you were talking about race engines, there could be some confusion.

A race engine, especially a block that has been used for many years, will gradually develop larger piston-to-wall clearances due to the many rebuilds. They may begin to show terrible leakdown numbers, as much as 30 percent, but only have 2 to 4 CFM on a blowby meter while under full load, such as on a drag strip.

The bottom line with race engines is that leakdown numbers are not the final word in ring seal quality. Leakdown is static, while a blowby meter is dynamic. In performance applications, what counts is when the engine is running.

 

[WhalePirot]

more, in this link

FWIW: http://www.motorage.com/motorage/article/articleDetail.jsp?id=16170