One thing puzzles me. Why is this test called a compression test in the aircraft world.
It is actually a differential pressure test or a leakdown test in the automotive world.
It is not a true direct compression test.
Does anyone do a direct compression test?
Eddie,
Its actually called a “differential compression test”. Why is it used in aircraft engines? Only legitimate answer is because it is the “recognized” test, and the one we’ve used forever in aviation.
the good news is that, properly administered, this test will pick up a lot of engine problems. That said, the specific numbers themselves (60 vs 70) are relative, and so not necessarily meaningful. Trends are more indicative of a problem, or, as others noted earlier, if you can hear where the Air is going, if the cylinder isn’t holding Air.
I once had an O-360 Lyc that ran hot, didn’t make much power (a REALLY relative function) etc, but it could pass a differential compression test. Just wiggle that prop and fiddle with it to get it to the right spot and it would hold pressure.
Finally, I called Lycoming Tech help, told them what I had and what we’d done, and first thing he said was go to NAPA and buy an automotive compression tester. Did that, and two cylinders would not pass. They could be made to pass a differential test, but that’s not how an engine works.
But, differential pressure is the aviation standard, and when properly applied, will reveal many cylinder problems.
MTV