Compression test questions

[win]

Just did a compression test on the '96 Expo.5.0 and I'm not sure what the results should be.

Engine was stone cold and outside of the TM headers, box stock.

Throttle body wedged wide open and all 8 cylinders were between 130 and 135 psi.

I'm pleased the results were so even but are those results within reason?

Basic math has about 140 psi the max the cold 9.5CR 302 should produce but my (probably fading) memory had me expecting more like 150 to 180 psi.

Yea, I know a compression test should be done hot but I'm simply to much of a wimp to try the test on a hot engine in such a tight engine bay. I'll leave the deliberate branding to the masochists.

 

[RonD]

The way you tested is fine, I never test warm because the test results are to find abnormal cylinders not maximum compression.

Compression psi has alot to do with crankshaft speed, so battery condition and starter motor has an effect.
And all spark plugs should be removed to get maximum crank speed.

Than being said, I also would expect above 150psi across the board.

But because all cylinders are within 10% of each other I would say the test shows no ring or valve issues and probably good head gaskets.

I have found the simple formula of compression ratio X 18psi will give approx. good compression test numbers for mid-mile engines, use 18.5 for rebuilt/new.
'96 Explorer 5.0l should be about 9.0:1 compression ratio, actually 8.8:1 they had the large chamber heads
9.0 x 18 = 162psi

If it was 9.5CR then I would expect 171psi, 9.5 x 18 = 171

The 18 is based on air pressure at sea level, 14.5psi + 3.5psi for heat pressure when compressing the air.
If you were testing at 2,000ft above sea level then use 17 for the 1psi drop in local air pressure
9 x 17 = 153

4,000ft
9 x 16 = 144

No wonder the people living in Denver all want turbo chargers, lol.

 

[adsm08]

You know, my sister keeps telling me that she looses coolant when she goes to Denver. She already has a turbo though.