Help Understanding Compressions...

[Ragsf15e]

Guys (and girls) - I'm going to be a new Mooney owner in the next few months... just gotta find exactly the right one to pull the trigger.  I've read a lot on what to look for, but I'm still wondering about the significance of the compression values.  I know basically what they are (compression in each cylinder), but what is that telling me about the health of the engine?  What if I see an aircraft that has about 1000 hrs SMOH and it's  71, 75, 78, 76?  Does that 71 mean the engine's not healthy?  Something's about to get expensive?

 

Thanks for your advice and expertice.

[PTK]

The compression test is in the FAR's and required. It is also unreliable in that it gives you non-reproducible results!

Here's some good info on more dependable and better ways to gauge engine health.

http://www.eaavideo.org/video.aspx?v=2149054014001

[Guest]

Cylinder compression testing should be used as a trend monitoring tool as opposed to an absolute. Generally Continental engines have lower values than Lycoming. Combined with crankcase pressure readings is more helpful.

Clarence

[Jeff_S]

The compression test is in the FAR's and required. It is also unreliable in that it gives you non-reproducible results!

Here's some good info on more dependable and better ways to gauge engine health.

http://www.eaavideo.org/video.aspx?v=2149054014001

 

+1 on all the Mike Busch webinar series. He really knows what he's talking about with piston engines, and he follows all the latest thinking and techniques for how to get the most out of your engine while also not spending any more than is necessary. One of his pet peeves has always been the compression check, which was used as a primary diagnostic for many years but now should just be one of the things that gets analyzed. For more of Mike's webinars go to his website at http://www.savvymx.com and go tot he Resources section.

[Marauder]

Guys (and girls) - I'm going to be a new Mooney owner in the next few months... just gotta find exactly the right one to pull the trigger. I've read a lot on what to look for, but I'm still wondering about the significance of the compression values. I know basically what they are (compression in each cylinder), but what is that telling me about the health of the engine? What if I see an aircraft that has about 1000 hrs SMOH and it's 71, 75, 78, 76? Does that 71 mean the engine's not healthy? Something's about to get expensive?

Thanks for your advice and expertice.

The compression on the Lycomings are usually in the 70s. If they drop in the low 70s, it usually means nothing. Sometimes the rings align themselves oddly and the compression is off a bit. Sometimes the compression test is done without a complete warmup. In the case of the engine above, I would have them do another test on that cylinder after running it a bit. The more important things to look at are the frequency of usage and making sure all the ADs were done.

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[aaronk25]

Also cylinders should be looked at as accessories because for $1-3k you can have one changed.

I'd be more concerned about time of inactivity such as less than 50 hours of operation a year, preferably the plane would fly at least 100hours a year.

I say this because sitting creates rust and when scrapped off after start the metal on the cam, lifters and cylinder walls becomes pitted.

Cam and lifters fail all the time in engines that sit. It's not that the cam and lifters are so expensive but it means the engine has to come off the plane and the crank case gets dissembles. It's 8k In labor before you even start.

One last note compressions tell you nothing about the condition of the lower 90% of the cylinder walls as it only is a indicator of how well the rings are matching up with the cylinder walls at the very top of the stroke. All the nasty rust and pitting almost always happens in the lower part of the cylinder wall...

Hope this helps.... Good luck!

[Ragsf15e]

Thanks for the comments and the videos - exactly what i was looking for!

[stevesm20b]

Cylinder compression by itself is not a very good indication of engine condition. I had cylinder readings all above 75. But had high oil consumption. Removed number 3 cylinder and found broken oil ring.

[jetdriven]

It helps identify a leaking valve or a cracked head but that's about it

[flyboy]

Cylinder compression by itself is not a very good indication of engine condition. I had cylinder readings all above 75. But had high oil consumption. Removed number 3 cylinder and found broken oil ring.

I have the same thing in my M20B. Why did you suspect # 3?

[PLN_FXR]

A compression test, as many have stated or alluded to, is one of several diagnostic tools to help determine engine health.  On a typical Lycoming, your readings will generally be 72 or more on a high-time engine, and 76 to 78 on a low-time engine ... assuming proper break-in and no problems, of course.  If your readings are above 78 (or possibly 79 with a properly calibrated differential compression tester), that cylinder is likely burning oil (the oil masks the leakage past the rings during the check).  A good technician will often learn more from listening during the compression check than from reading the numbers.

 

The above being said, a 60 reading is still a 75% cylinder, so I would have no hesitation regarding a reading of 71.  When you find an aircraft you are getting serious about, if you're lucky the current owner has a track record of oil analysis and decent engine records, which will help you determine engine health.  

 

Please keep in mind that as expensive as an engine is, it's really easy to spend more on avionics, or paint and interior.  If you find a clean, relatively low-time, corrosion free, hail free airframe with the avionics you want but a high-time engine, don't let the engine scare you away ... you can replace everything firewall forward on the right aircraft and end up with a real gem, but if you have an airframe that's corroded you have nothing.