Compression test results... confusing???

[TomCat]

Alright so I compression tested my motor (rebuilt by someguy 46,000 ago) and it came up as 120 on the dot across the board. On the first stroke, it bumped up to about 90 on all and went from there. I tested it with throttle wide open, TFI disconnected and with all spark plugs out. Now reading on here, most people get like 140 or more? Confusing part is I asked my mechanic boss, who really knows his stuff, and he said that 120 is what 5.0s read at up here... keep in mind I'm at 4500 ft altitude, if that makes a difference.
Any input? Could it have been rebuilt with lower compression pistons??

Thanks in advance,

Brandon

 

[kj2833]

Other Compression Issues

There are some variables that affect the readings obtained from compression testing. They are cranking speed, altitude, temperature, worn camshaft lobes and high-performance, long-duration profile camshafts. The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed. There are factors to compensate for the different altitudes and the corresponding temperature differences. These are as follows: 1,000 feet = .9711, 2,000 feet = .9428, 3,000 feet = .9151, 4,000 feet = .8881, 5,000 feet = .8617, 6,000 feet = .8359, 7,000 feet = .8106, 8,000 feet = .7860. The equivalent compression reading for a cylinder that should be 135 psi by the data at 5,000 feet would be 135 x .8617 = 116.33.

so take your psi reading and multiply by like .8700, and that should get you the reading around 4500 alt

by my calculation your reading is above what they claim, so all seems well.

 

[Daggar]

Man.... I think I need to copy the compression test page out of the Ford shop manual so folks can see that raw numbers run on a cold motor mean next to nothing at all. Your readings will change dpending on the how you've run the test and whether or not the engine was warm. If course a warm engine will yield higher results. The ONLY thing that you need be concerned with are the numbers in how they relate to each other cylinder in your car. The limit is 25%. So long as your compression numbers are within 25% of one another then you are GOLDEN and your engine is healthy. If you're reading 120 across the board on a cold motor then I'd say that the thing is tight as a drum. Time to add some forced induction.

 

[TomCat]

Awesome man, thanks for that info!
lol, yeah I'd like to go FI... doing the Trickflow NA setup though