What is compression testing with regards to piston engine airworthiness? The test yeilds a measurement of something over 80, with lower numbers generally being bad.

Thanks.

 

They basically blow air into the cylinder and see how much leaks out. The more that leaks out the lower your first number will be. 60/80 means you got a 25% leak rate. It could mean nothing, though, as a misaligned ring will blow your reading. I always want my compression checked when the engine is warm. Supposedly, the internal cylinder parts are at operating temp and will show a more accurate reading indicative of real life performance.

I'm pretty sure Lycoming recommends 60/80 as acceptable.

 

Continental engine numbers are lower than that. I believe the last time they published numbers as a minimum acceptable figure, it was 54/80. Then, i think, they said that the absolute numbers aren't nearly as important as their relationship to other cylinders - within a ten percent value, I think. I'm kinda fuzzy on the TCM engines.

The compression checks are important, but not an absolute testament of an engine's health. Low compression checks can point to cracked /unseated/warped valves (air blows out the exhaust or intake) or if the rings are unseated, aligned or broken.

There are other MAJOR factors that can't be studied with the engine intact. An engine that sits for long periods may lose its protective coating of oil on the camshaft and followers - the lobes then wear rapidly, and the engine loses power because the cam isn't pushing the valves fully open... yet, the compression test won't show this.

 

All 6 of the Seabee cylinders (lycoming) were between 74/80 and 78/80 on our compression check a few days ago. Made us very happy to see that.