compression's on piston motors, lyc and cont

[climbhappy]

someone told me that compressions on cont motors are judged differently than ones on lyc.

example: on an io 470, that compr in 50s or 60s are okay while the 70s are the norm for lyc.

what's going on here?

 

[IFollowRoads]

Never heard that one. Ask the person for a reference. I always use PSI/80, and warn the owner things are gettin dicey below 65/80, and insist the jug come off below 60/80. I use this for any horizontally opposed engine.

If this guy has something solid that says different, well... the only constant is change.

 

[erj-145mech]

Compression checks are not the only indication of a weak cylinder. There are too many variables. If you perform a check and come up with say 71/80, I could come up right behind you, using the same set of gauges, and get a completely different reading.

In the maintenance manuals on my O-2A, there is a hard number of 38/80 where the cylinder must be removed. This is the only place in writing that I have seen where a hard number is noted. You won't find a hard number any where else.

If you take a Cessna 182 that hasn't flown in several months, do a ground run to get the engine to temp, and perform a comp check, you will end up with low readings. Now, go fly that engine for an hour or two, and the readings will be 25# higher. So, if you only run the first set of numbers, are you going to tell the owner that he has to buy six new cylinders? You certainly won't be in business long if you do.

 

[IFollowRoads]

So, if you only run the first set of numbers, are you going to tell the owner that he has to buy six new cylinders? You certainly won't be in business long if you do.

I'll send him to you.

 

[avbug]

Compression tests are one tool for sampling engine condition, but don't tell you anything by themselves. The same engine will give different results using the same compression tester at different temperatures, or with different technicians doing the sampling. Continental wants to see a calibrated orfice for the tester. 80 psi is a reference number that's commonly used, but there's nothing magic about that number.

Compression numbers go up and down with testing. Rocking the propeller can artificially change the numbers, as can a warm engine.

A compression test test doesn't tell you about cylinder wear...it tells you about an artificial condition which may revolve around valve wear or valve cleanliness, valve adjustment, cylinder wear, or a host of other things. It may also tell you nothing. Determining engine health based on a compression test is about as scientifically valid as looking at engine oil color. Compression tests are more of a layman's way of looking at the engine; the numbers are more meaningful to the layman than to the mechanic.

 

[IFollowRoads]

I have yet to see a jug that couldn't hold 60/80 that didn't need some luv.

Talk amongst yourselves.

 

[erj-145mech]

There's nothing unsafe or compromising safety about flying a cylinder that tested to 60/80. I've done it, and will continue to do so. That is providing that the engine does not have excessive oil consumption, oil fouling plugs, or other maladies.

 

[USMCmech]

Where this comes from is that AC43.13 states that a cylinder with compression below 60/80 is not acceptable.

Contenital published a service bulliten specifically stating that low compresion alone was OK as long as none of the leakage was from the valves. This SB superseeds the FAAs recomendation (as does all manufacturers instructions).

Lycoming has not issued a similar SB therfore you must revert to AC43.13, ergo 58/80 is no good.


In reality I'm sure a lycoming would run fine at 58/80, but it wouldn't be leagal. When I build my RV-7 with a Lyc, I'm not going to replace a otherwise healthy jug just because of compression.

 

[erj-145mech]

AC43.13 is an ADVISORY circular. It is not approved data. It is only acceptable data. It is a non binding guide line. That is the official word that I got from the Southwest Regions interpertation guru.

 

[Rally]

AC43.13 is an ADVISORY circular. It is not approved data. It is only acceptable data. It is a non binding guide line. That is the official word that I got from the Southwest Regions interpertation guru.

Not disagreeing with you but A) is he a lawyer B) did he put that in writing and sign it?