Correct way to check compression - 84 V65

[Julian Sudano]

I now have all of the parts need to change out my valve springs to the later, small block single spring set up.
Before I do this, I want to check the compressions, but have read a few comments on other forums that suggested the engine should be run for several minutes before connecting the tester?

Seems odd to me as I've always done this on a stone-cold engine.

What is the best procedure, and what would be a good reading?

Thanks

 

[Kevin.NZ]

Do it on a cold engine and throttle wide open, Ive always worked on the assumption of Compression ratio X 14.6 (atmospheric pressure in lb/sq inch).

 

[Julian Sudano]

Do it on a cold engine and throttle wide open, Ive always worked on the assumption of Compression ratio X 14.6 (atmospheric pressure in lb/sq inch).

So, that would be 10 (comp ratio) x 14.6 = 146 lbs/sq.in as an acceptable reading?

Thanks Kevin!

 

[Kevin.NZ]

sounds good to me, no engine is perfect so acceptable would be 140. If its low you can put a small amount of oil in the bore, if the reading increases it indicated worn rings/bores, if it stays low then valves/guides/head gasket are a problem. then you proceed onto what is called a leak down test.

 

[scottmastrocinque]

Please let me add my 2 cents.

The most important numbers in a compression test is no more than 5% variance between cylinders.

This shows relative compression between the cylinders, just 2 in this example.

The actual number, 140, 130, 125, etc, shows the relative wear of the piston and piston rings.

So if the cylinders read 128 and 131, everything is just fine because the readings are within 5% of each other. Yes, it doesn’t have MAX compression but this is relatively unimportant.