Compression check or cylinder leak down check?

[NealW]

Our 21 touring seems to be running a little anemic lately compared to our 15 runabout. The runabout is is 200+ lb lighter and has a freshly rebuilt engine, so that is certainly part of the performance difference. The touring starts fine and runs well. I thought about doing a compression check to see if I've got a leaky valve(s), but there seems to be two ways that I can do it. The first is to use a compression tester and crank the engine over several times to see what the max pressure is in each cylinder. The second is to use a leak down tester with each piston at TDC and compare the pressure to a known input. That's how I do it each year on our homebuilt airplane to check if the compression is "weak" (the FAA definition). Normally airplane engines are checked relative to 80 psi. Obviously it's lower for T engines.

Since I need to buy either a compression tester or leak down tester to do the job, I was wonder what people's experience has been with their T's doing it one way or the other? It seems like with the leak down tester I could more easily listen for leaks at the different valves or even the rings.

Thanks,

Neal

[big2bird]

A leak down tester tells the whole story, and is far superior a tool.

[Les Schubert]

A burned #4 exhaust valve is not a uncommon problem after some miles. With modern valve material slightly less often.
I’ve only used a compression tester on my cars but certainly have used the leak test on my airplanes.
All the best

[Altair]

A leak down test will only tell you that there is a leak greater in one cylinder than an other but not what is leaking. Compressed air in to each cylinder will determine what is leaking. An exhaust valve leak will sound at the exhaust, intake at the intake or carb and rings will sound through the crankcase. If there is a greater leak down time in one cylinder, that is the cylinder to further check. All cylinders can be done with compressed air. Means will have to be devised to hold the crankshaft from rotating even tough the test cylinder is at TDC it may still rotate.

[Charlie B in N.J.]

To my mind the dry/wet compression test is the way to go. Choke off & throttle fully open. First dry then redo with a few squirts of oil in each cyl. before testing it. Write the #'s down. If your compression is low and the valves are bad the comp won't go up when oil is applied. It will go up if the rings are worn. I do T engines cold because they are low comp engines to begin with and it'll give you the compression when trying to start the engine cold which contributes to hard starting. It doesn't matter which valve is leaking or how many. If the test shows valves it's valves. If it indicates rings it's rings. Simple and easy. Pick up a cheap comp tester gauge and get to diagnosing.