Cylinder Compression Readings (Leak Test)

[GeneralT001]

Hi,

Just trying to learn

Am I right in assuming that a reading of 80 (psi?) is perfect the cylinder leak check? I think I typically see reading of the 4 cylinders as something like 76/78/78/74 and slight variations to that.

I did see one example which was 72/78/76/78....would the 72 be a cause for concern or as long as they are reasonably close to one another then its okay? At what reading do you start to worry about work needed?

[PeterRus]

https://www.aopa.org/news-and-media/all-news/2023/february/13/aircraft-maintenance-a-modern-approach-to-compression-testing

[N201MKTurbo]

The compression tester has a regulator that supplies 80 PSI to the cylinder. There is an orifice between the regulator and the cylinder hose. If there is no leakage the test gauge will read 80 PSI. If there is leakage, it will read less. 
 

There can be leakage past the rings, the intake valve, the exhaust valve and the spark plugs and tester connection. 
 

You can tell where it is leaking by listening to the crankcase through the oil filler for ring leakage, to the exhaust pipe for exhaust valve leakage and to the carb or fuel injector servo for intake leakage.

If you have ring leakage, you can get a bit more info by moving the prop so the piston is on the up stroke or the down stroke. This will seat the rings on their top surface and bottom surface respectively. You can also check the sealing at different places in the piston stroke. There is information beyond TDC.

The readings are an indicator of cylinder health, but have to be taken with a grain of salt.

A reading of 72 is no reason to do anything. I have found little correlation between compression readings and engine power. There is a correlation between compression and oil consumption.

[GeeBee]

56 is a concern for Lycomings. 40 for Continentals. Still don’t do anything on a low reading until you fly it again for at least 45 minutes then recheck warm. If still low, borescope first before pulling the cylinder to ascertain why the compression is low.

As M201MKTurbo points out even a low cylinder does not greatly affect power. Continental filed down the ring gap on all 6 cylinders to the point of zero compression and the engine still turned full rated power.

Compression checks are like an EKG on human. Sometimes indicative of problems, often is not but a bad one requires further inquiry as to why and where.

[EricJ]

Just to add a little bit to the wisdom already mentioned, these are not precision tests.    If you get a low reading you can often just fiddle with it a bit to get the compression to come up, by moving the prop a little, spraying a little oil in the cylinder, flying it a bit and trying again, etc.    Unless you suddenly get a very low reading on a cylinder, which happens sometimes, they're really mostly good for a general health test and for long-term trend analysis.    If you get a cylinder that starts to consistently have trouble making compression, then it's good to keep an eye on that one and on oil consumption to see where it's going.

Compression tests combined with borescope analysis and sending off samples for oil analysis when you get an oil change, all together give you pretty good insight over time on how your engine is aging.   

Another thing to be aware of on compressions tests if you want to do them yourself is that they can be dangerous if you're not careful with the prop.    The compressed air at TDC can generate a fair amount of torque and swing the prop pretty quickly if it gets away from you.