Compression Lycoming AEI0-360-A1B6

jantar · #1 05-31-2006, 07:40 PM

72/80, 72/80, 74/80, 75/80

Are these values average/below or above? The engine is installed in aerobatic aircraft, but it did not fly much aerobatics.

All comments welcome...

cjensen · #2 05-31-2006, 09:15 PM

Those are good average numbers for a Lyc. Most 4 cyl. Lycs will remain in the 70's throughout their lifespan. If they drop in to the 60's, something needs to be looked at...rings or exhaust valve leaks probably amongst the most common leaks.

What's the time? Has is been sitting? No worries on the compressions though.

jantar · #3 05-31-2006, 09:32 PM

My RV building is going rather slow and I was considering buying a Zlin 242L to get me through the next few years.

The engine has 1100 hours on it. I looked at the compression numbers and I did not quite understand the notation 72/80. I think, 72 is cold and 80 is warm. For a mid time engine it looked to me like 72/80 is kind of low.

More comments please...

Mark Burns · #4 05-31-2006, 09:57 PM

Quote:

Originally Posted by jantar

I think, 72 is cold and 80 is warm. For a mid time engine it looked to me like 72/80 is kind of low.

More comments please...

The 80 after the "/" is the test pressure. The first number is how much of the 80psi the cylinder could hold. The compression tester has a "precision" orifice between the two gauges. 80/80 would be perfect.

I've never heard of any pressure other than 80 used to test the compression of aircraft engines. Sometimes people will just leave the 80 off. Then the compression would look like 72,72,74,75.

These compression #'s look fine to me. I like that they are fairly even.

Mark

gmcjetpilot · #5 06-01-2006, 02:08 AM

Quote:

Originally Posted by jantar

72/80, 72/80, 74/80, 75/80 Are these values average/below or above? The engine is installed in aerobatic aircraft, but it did not fly much aerobatics. All comments welcome...

These all sound reasonable and well balanced between cylinders, which means they're believable. I get suspicious when the different cylinders have a big spread or all 79 or 80. There's a few tricks in getting comp checks up a few digits, so the absolute number's are not critical unless it's real low. This is not the case here with low/mid 70's, which are fine.

Comp numbers are like your wife or girlfriend, their mood might change week to week. Compression changes month to month, going up, down, up. Usually you only have to worry if they're consistently low. Sometimes the ring ends randomly align from time to time or a valve gets a piece of crud under it. Just running the engine can bring the Comp number up on a subsequent check. That is another factor. Has the engine been run a lot recently.

One Comp check is a good clue of health but not the whole picture. A consistent trend over several checks is better. You have to take the Comp numbers along with how is it running, how the plugs look, oil consumption and oil analysis over several changes to get a bigger clue. How many hours on the engine? The leak down check alone just says you don't have anything obviously wrong with the top end. However it's not a big deal in a certain range, but we all want and get a warm fuzzy when it's higher, even though it's not an absolute number.

robertahegy · #6 06-01-2006, 06:06 AM

Like your wife or girlfriend, George? Please!!!

Roberta

PS: the numbers are good.

pierre smith · #7 06-01-2006, 06:26 AM

How 'bout 'wife AND girlfriend'?

Allen Barrett · #8 06-01-2006, 07:01 AM

Even compression readings in the mid 60's are okay on a mid to high time engine AS LONG AS the leakage is not going past the valves. George is right, that compressions are only one scenario in determining the health of the engine. Study the logs, there SHOULD be valuable information recorded. Oil analisys records are also a good source if available. If the aircraft has sat for a long period inactive, bore scope the jugs looking for corrosion.

Allen

jantar · #9 06-01-2006, 07:20 PM

Thx for the info.

az_gila · #10 06-01-2006, 09:58 PM

You might find out what size orifice was used.
A lot of mechanics are not familiar with the new changes that came out with change 1B of AC43.13 released in late 2001.

The "bible" now calls for a 0.060 orifice for our Lycomings with a 5.125 inch cylinder bore. The old standard was a 0.040 orifice, and a lot of mechanics are still using their old equipment.

With the new, bigger orifice, I have found that readings on my Tiger are now in the 78/80 range.... may be less accurate, but that's the way the FAA wants us to do it. I personally would be very concerned with 60/80 with the larger orifice... even though the FAA says it may be OK

Details on page 8 of this AC section...

http://www.airweb.faa.gov/Regulatory...apter%2008.pdf

Anyone know if Lycoming has a recent Service Bulletin that says anything different?

Note that Continental has published a different procedure and check items for their engines.

gil in Tucson