Low compression on one cylinder of my Lycoming IO-540-C4D5D engine..anything to worry about?

nobbi wrote:

My CHT probe cable is running to the old factory CHT instrument. The EDM700 is connected by some backpack type to that probe. As a result the EDM 700 shows a 70°F lower value than the other cylinders.

Even if that might be the correct approach from a certification standpoint, it doesn’t make much sense to get a precision temperature sensor device and then introduce a huge error with that piggyback probe. Just remove the factory probe, tie it up safely in the engine room and insert a new EDM probe. That’s safe and sensible and your A&P/IA will agree. Guess what I have done myself…

@Achim … that “minusculus” change is on my agenda …

Finding out the reason for compression leaks should be quite easy to do: Set the suspect cylinder under pressure then listen at the exhaust pipe: No hissing – no exhaust valve leak. Listen at the carb intake: No hissing – no inlet valve leak. Then listen at the oil filler inlet: Hissing means leaking piston rings into the crank case. So this is no real mystery to do. Vic

I was going to say just what vic did: it’s quite quick and requires no special tools (eg borescope) to determine where the air is going, just your ears.

Personally I would always worry about excessive cylinder leakage. It might, just might be a precursor to something serious.

OK guys, please educate me. Why low compression does not effect power? I know that after O/H my engine I have gained 5kts for the same fuel flow, T/O run was shorter and climb became much better/steeper for the same airspeed.

Thanks

Continental has done extensive Tests of this matter, and they found out that even engines with compression as low as 20/80 did not suffer from ANY loss of rated power. The static leakage is test is something completely different than a running and hot engine – and cannot be compared 1:1.