Differential Compression testing.


One of the things I do to test motors after their first run is a quick differential compression test. The two gauges show the regulated supply air at 80 psi, and the other shows the pressure in the cylinder at TDC. there is a tiny calibrated orifice between the gauges. If the leak down is less than the tiny orifice, the cylinder is said in A&P slang to “Blow 80 over 80”.

In reality this is not common, and many manufacturers have service limits like 60/80 (said “sixty over eighty”) . These are the numbers you see in log books of planes at annual. My MOP manual discusses these at length, but here is a quick look at the test run motor blowing 79/80, a near perfect score, after a one hour break in run.

Other engine companies talk about what they can sell you, and that’s OK if you are just going to buy a motor, and honestly that is what the majority of todays builders want. But I’m not a salesman, I’m an instructor, and for this reason, I mostly speak of learning.


One Reply to “Differential Compression testing.”

  1. That is a thing of beauty! Isn’t it a joy when it all works as it should? Thanks for explaining differential compression once again.

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