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Question about cylinder pressures


BaldEagle

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Bought my '78 J back in February and its first annual since the purchase is currently being finished up.  No nasty surprises (thank goodness), but the compressions are high 60's in all 4 cylinders.  Last year they were all in the high 70's.  During the PPI last January three cylinders were in the high 70's, while one was in the high 60s.  When I questioned the A&P he dismissed it as likely a piece of soot or something on a valve seat.  The engine is at about 1,000 hours since factory overhaul.  Should I be budgeting for a premature major overhaul or top overhaul in the near future?  Should I be worrying?

The airplane did sit for a month in January (while negotiating, PPI, etc.), and a month in July (due to illness), but other than that it hasn't sat for more than a couple of weeks without flying since I've owned it.  It has about 50 hours on it the last 8 months or so.  The previous owner only put 40 hours on it the last year he had it, but prior to that it was more active.  The engine uses very little oil, runs real smooth with uniform CHTs on all cylinders.  Prior to my purchase it was based in the Georgia mountains in the summer and Tampa in the winter, so I knew I was taking a risk with corrosion but this was somewhat mitigated by evidence of meticulous maintenance.

After paying for two major overhauls the same year (two difference airplanes -- I'm not that unlucky -- one had a cracked crankcase, the other eating metal) I've become paranoid about airplane engines.

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Was the engine warm when the last check was done? Cold engines generally test a little lower. Sometimes there can be small differences between tests done by different people. High 60s would not overly worry me. I had one cylinder drop almost 10 psi between annuals, by the next one it was right back up again [with no treatment in between].

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Mike Busch's book on engines covers this pretty well, at least it was understandable for me. He spends a lot of time discussing how you know when you need to overhaul, and what compressions really mean.

https://m.aircraftspruce.com/catalog/bvpages/mikebusch-13-21514.php?gclid=CjwKCAjw54fdBRBbEiwAW28S9rkcWKucFvWWp-MsWGZBjD2PuV2OTgAEAA4k9x_nHTJycxs1IQOmwBoCtbAQAvD_BwE


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Here's Mike Busch's webinar on how unhelpful compression testing is:

My take is that compression testing is only important inasmuch as it is required at 100 hour inspections by the FAA, but it is probably insufficient by itself to make any maintenance decision.

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Except for burnt exhaust valves, cylinder wear problems usually progress very slowly. It usually ends when the oil consumption gets intolerable. I would give a borescope inspection more credence than the compression numbers.

Now if the compression on one cylinder goes into the 30s or so, I would fix something.

Other than catastrophic valve failure, cylinder problems are rarely a safety problem.

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21 minutes ago, N201MKTurbo said:

Except for burnt exhaust valves, cylinder wear problems usually progress very slowly. It usually ends when the oil consumption gets intolerable. I would give a borescope inspection more credence than the compression numbers.

Now if the compression on one cylinder goes into the 30s or so, I would fix something.

Other than catastrophic valve failure, cylinder problems are rarely a safety problem.

My favorite quote from Mike Busch

"poor compression is a safety-of-wallet issue, not a safety-of-flight issue."

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Thanks all for the feedback.  I did recently purchase Mike Busch's book on Engines, and although I'm fairly inept mechanically I did find the book very persuasive, especially in regard to on-condition maintenance philosophy (so much so that I'm currently working my way through his webinars on YouTube).

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Except for burnt exhaust valves, cylinder wear problems usually progress very slowly. It usually ends when the oil consumption gets intolerable. I would give a borescope inspection more credence than the compression numbers.
Now if the compression on one cylinder goes into the 30s or so, I would fix something.


I thought 60 was the limit in the regulations
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4 minutes ago, M20Doc said:

Here is Lycoming SI 1191A concerning compression testing.

https://www.lycoming.com/sites/default/files/Cylinder Compression.pdf

Clarence

This reinforces the conventional wisdom that there are specific assessments of cylinder health based solely on the differential pressure check. It was published in 1998.

I believe Textron Lycoming has come around on lean of peak operations, has there been any other communication from them, more recent than this service instruction, regarding the compression checks?

Thanks for posting the SI Clarence.

Cheers,
Rick

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1 minute ago, Junkman said:

This reinforces the conventional wisdom that there are specific assessments of cylinder health based solely on the differential pressure check. It was published in 1998.

I believe Textron Lycoming has come around on lean of peak operations, has there been any other communication from them, more recent than this service instruction, regarding the compression checks?

Thanks for posting the SI Clarence.

Cheers,
Rick

It’s the most recent that I’m aware of.  I certainly would not be removing a cylinder based on one compression check.  Flying it and rechecking would be the norm, unless inspection reveals something else wrong in the cylinder, like burn valves or excess oil.

Clarence

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