Top Hat - Not a direct answer to your question, but http://autospeed.drive.com.au/cms/A_108913/article.html has a discussion about rings and compression. An extract (deals with motor vehicles but it reads across to aero engines in basic principles)

During operation of an engine, blowby passing through the ring gap is under very high pressure and it reaches what is termed "sonic velocity". At this point any increase will not result in any increase in gas velocity (hence the rate of escape of the gases). From this point of view, leak-down tests are of little or no value in determining engine condition, as they measure leakage under static and low pressure conditions.

The gas which does escape the piston rings, and which is returned to the intake charge, is termed blowby. The control of blowby to an acceptable level is important because excessive blowby means loss of engine power, leads to ring sticking and an increase in emissions. That is why manufacturers tightly control piston ring shape and flatness, two factors important for good gas sealing. Blowby is measured under laboratory conditions by sealing off the engine's crankcase and attaching an accurate gas meter by means to the breather on the oil filler cap. A typical 2-litre engine can expected to have a blowby figure of 20-30 litres per minute under full load conditions.

A reasonably good measure of trending any HP degradation is available to the operator of a fixed-pitch prop aircraft, and that is to keep a log of static runup RPM at home base. Record the OAT, wind, and QFE along with RPM and you'll get a good picture of what's going on. Every 50 hours might be a good interval for these checks, at least until you have a good database.

Unfortunately a variable (constant-speed) prop complicates the matter, unless there's a torquemeter like the R-2800 has.

Indeed, BMEP tells all...mostly.

Brian Abraham,

You post is interesting but we need to take care in interpreting it.

The words

"At this point any increase will not result in any increase in gas velocity (hence the rate of escape of the gases)."

Mean that increasing cylinder pressure will not increase the leak rate. This is just basic high speed duct flow theory.

But it does not mean that increasing the size of gaps between the rings and the cylinder wall will not increase leak rates.

Larger gaps will result in larger leakage rates, lower cylinder pressures and ultimately lower power output.

Many thanks for all of the replies.

The original question was raised because you see a lot of aircraft for sale with the seller stating that the compression is "x" per cylinder.

This got me thinking abt the actual engine performance and other directly related issues and whether the performance figures in the POH are correct for a new and an engine that is obviously mid life and has lower compressions.

"The original question was raised because you see a lot of aircraft for sale with the seller stating that the compression is "x" per cylinder."

Which is a reference to the static differential compression test performed during the required inspection(s). :)