by 13brv3 » Thu Apr 27, 2023 10:55 am
Either test will give you a reference number. The differential compression test will be more consistent, since it doesn't rely on how fast the engine is turning during the test. It also give you information about where the leak is if you have one. While holding pressure, you will hear the air leaking at either the carb, exhaust, or crankcase breather. That tells you which valve is leaking, or if the leak is past the rings.
The test units have an orifice that is sized for the amount of leakage that would be considered normal for the piston. For "aircraft" units, there are at least two orifice sizes to choose from depending on whether the piston is larger or smaller than a specific size (5" I think). That's important to make the engine manufacturers specs meaningful. I'm not sure how automotive units chose an orifice size, but I've never seen a spec for those anyway. A larger or smaller orifice just makes your "normal" reading a bit higher or lower.
If you get one, I'd just get an automotive unit, since they're cheaper, and work just as well. I've compared the "aircraft" units to automotive units on the 912 engine, and either works fine.
Rusty
Onex- Rotax 912 (130 hours and counting)
Fixed wing, gyroplane, A&P