Compression too high?

Chrisgoesflying

Chrisgoesflying

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Chrisgoesflying
My mechanic and I just completed the second annual on my Piper Comanche. All seems good and happy which is always a huge relief. However, since I moved I flew in my mechanic from out of town (far out of town) and since he flew commercial, some of the more bulky tools, he couldn't bring. We managed to borrow everything we needed and he couldn't bring from a local shop. Today, when I returned the tools to the local shop, the mechanic asked me how the annual went and I said all good. He cut open the oil filter and nothing worrisome was found, which was the last item we had to do. He then asked me how the compression check went. I told him that the lowest cylinder was at 70 and the highest at 78 with the other four somewhere in between those two numbers, which is consistent with the compressions of the last few years. He then said "Oh, the lowest is a bit too low and the highest is a bit too high. You want them all to be somewhere around the 75 mark." I never heard anything like that. Is there such thing as "too high" compressions? And since when is 70 too low on an O-540 especially if it's not trending downwards (cylinder 1 always comes in at around 70 for the last few years). When we did the compression check, my mechanic said all is fine compression wise and raised absolutely no concerns about cylinder 3 being at 78 and cylinder 1 being at 70. What are your thoughts?
 
What’s the difference between 77 and 78? Or 76? Or between 70 and 72? Jeez, it’s not that accurate.

Keep doing what you’re doing. Sounds like flying your own mechanic in was a great move.

Local mechanic is a dork. Context: Imagine me in the cheap seats at a hockey game, since you’re Canadian, berating the ref’s ability to rear children through his mom because of a bad call - I’m not a ref, nor a pro-player, so take it for what it’s worth.
 
Convert to mm/Hg and watch the local’s eyes bug out. Sell it with a good Bob’s your uncle and be on your way.
 
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My thought is all’s well.

IIRC Lycoming. Has a Service Instruction (?) relating all cylinders should

be within 10 % of each other. A static compression check with your

number should be insignificant in operation. It may come down to a

communication problem,
 
Chrisgoesflying said:
What are your thoughts?
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As I don't know the tone or context of your discussion with the mechanic, I can't offer a direct response except that is not the first time I've heard that. The key is why he said it. Anytime someone offers a different observation, its always best to ask why and for any references when available. If there is a possibility this mechanic/shop might be part of your future plans to support your aircraft, perhaps revisit the topic with him and ask why as you are curious since 70-78 is well within the OEM minimum requirements.
 
Bell206 said:
As I don't know the tone or context of your discussion with the mechanic, I can't offer a direct response except that is not the first time I've heard that. The key is why he said it. Anytime someone offers a different observation, its always best to ask why and for any references when available. If there is a possibility this mechanic/shop might be part of your future plans to support your aircraft, perhaps revisit the topic with him and ask why as you are curious since 70-78 is well within the OEM minimum requirements.
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He just said it as a comment without a specific tone, just kind of a “that’s what it is, compressions should be around 75, period” type of thing. He doesn’t know my plane at all. Never saw it. Never worked on it. All he knows is the inside of the filter which he said has nothing concerning, just the usual stuff that is expected of a an engine with 1,000+ hours (my mechanic agrees) and the compressions because i told him after he asked.

I kind of think he may be slightly irritated about the fact that I flew in my mechanic from out of town instead of hiring a local mechanic for the annual as he made some comments alluring to that.
 
Chrisgoesflying said:
I kind of think he may be slightly irritated about the fact that I flew in my mechanic from out of town instead of hiring a local mechanic for the annual as he made some comments alluring to that.
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Probably the source of his comments
 
Chrisgoesflying said:
I kind of think he may be slightly irritated about the fact that I flew in my mechanic from out of town instead of hiring a local mechanic for the annual as he made some comments alluring to that.
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You've already switched the aircraft to N-reg?
 
Chrisgoesflying said:
I kind of think he may be slightly irritated about the fact that I flew in my mechanic from out of town instead of hiring a local mechanic for the annual as he made some comments alluring to that.
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Just a comment from an A&P/IA and former shop manager. I wouldn't be irritated if an aircraft owner brought in their regular mechanic to do their annual instead of bringing the plane to my shop, especially if they just moved to the area. I understand aircraft owners' reluctance to try a new or unknown shop or mechanic. I would be greatly irritated, however, if an aircraft owner brought in their own mechanic and then asked if they could borrow my tools or equipment. That's some real BS.
Your compression numbers are fine, though. I've never heard of "all around the 75 mark" before.
 
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If he's gonna be fussy about you using a different mechanic, that doesnt exactly encourage you to hire him. Trying to confuse you about the compression numbers seems like a bad move.
 
Again; numbers are fine ; BUT:

My guess is that many folks throwing out opinions actually have no basis in

fact. Before throwing stones at the Tech my suggest is to read AC43.13-1

or other sources. Lycoming SI 1191A says “ A difference of 5 psi is

satisfactory”.

I absolutely agree with Bell’s comment on discussing the matter with the

local. The door swings both ways and he has no obligation to do anything

for you. In areas where there are few Techs this can be critical. Are you

willing to fly your Tech in for a task of less than 1 hour. Good

communication is really important!
 
Bell206 said:
You've already switched the aircraft to N-reg?
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No and i won’t until i have a more permanent status in the US, if that ever be the case. Right now, my status is only good for three years, with indefinite renewals, but still, not permanent.

Magman said:
Are you willing to fly your Tech in for a task of less than 1 hour.
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No, certainly not but i’m also moving away from where i live now in about two weeks. I also have a mechanic here locally who has done some work on my plane but i really wanted my old mechanic do the annual as he will continue doing them at least until the plane gets registered in the US. I thought it would be stupid to have another mechanic do the annual just this year fully knowing my regular mechanic will do future annuals.

I hope i will find a good mechanic who knows Comanches well down in Dallas when i move there next month but even then, my old Canadian mechanic will do the annuals because a US mechanic isn’t allowed to sign off an annual on a Canadian, privately registered aircraft.
 
Chrisgoesflying said:
No and i won’t until i have a more permanent status in the US, if that ever be the case. Right now, my status is only good for three years, with indefinite renewals, but still, not permanent.
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Smart move. But I don't think the other mechanic was "irritated" as he couldn't sign off your annual even if you wanted him to do it. Plus he did lend you the tools. I'd still be interested to know why 75 was his go to number. Knew several mechanics who believed the same way and were able to get big bore engines to go past OH time with regularity. Good luck with the move.
 
Bell206 said:
But I don't think the other mechanic was "irritated" as he couldn't sign off your annual even if you wanted him to do it.
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No, i’m still in Canada, just not where i used to live. He could have done the annual. I’m moving to Texas next month.
 
Chrisgoesflying said:
No, i’m still in Canada, just not where i used to live. He could have done the annual. I’m moving to Texas next month.
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My bad. I took it you had moved south already. Divergent opinions between two AMEs is an entirely different level in my book and one I prefer to stay out of at all costs. ;)
 
I'm glad I read the answer before pulling my heads as my last inspection had a range of 72-78/80 on a cold engine ... :eek:
 
The compression test, oh boy, there are so many misconceptions. The reason you put 80 psi in the cylinders is so that if the prop gets away from you it might knock you out but it won't chop your arm off or kill you. The purpose is to detect if or where any leakage is. It's not a conclusive diagnosis, it's just the first step. Kind of like the canary in a coal mine thing. Aside from that you are going to have to expect a certain degree of sniping and trash talking between mechanics, that's just human nature I guess and that's why "second opinion" became a thing.
 
I have yet to see a mechanic use a digital compression gauge so reading the needle on a analog gauge is going to give you some inconsistent readings depending on how you look at the gauge. Further these gauges are usually in 2 psi increments so the 75 mark is an eyeball.

2307p_rw_savvy1_16x9.jpg
 
Skyrys62 said:
What's the spread between hot vs. cold test typically?

My take is there is no “ typical” but hot generally better.

It seems the worst are after a brief run.

Possibly uneven heating?

Best after flight.

Differences can be dramatic particularly if an engine has not run in awhile.
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Chrisgoesflying said:
My mechanic and I just completed the second annual on my Piper Comanche. All seems good and happy which is always a huge relief. However, since I moved I flew in my mechanic from out of town (far out of town) and since he flew commercial, some of the more bulky tools, he couldn't bring. We managed to borrow everything we needed and he couldn't bring from a local shop. Today, when I returned the tools to the local shop, the mechanic asked me how the annual went and I said all good. He cut open the oil filter and nothing worrisome was found, which was the last item we had to do. He then asked me how the compression check went. I told him that the lowest cylinder was at 70 and the highest at 78 with the other four somewhere in between those two numbers, which is consistent with the compressions of the last few years. He then said "Oh, the lowest is a bit too low and the highest is a bit too high. You want them all to be somewhere around the 75 mark." I never heard anything like that. Is there such thing as "too high" compressions? And since when is 70 too low on an O-540 especially if it's not trending downwards (cylinder 1 always comes in at around 70 for the last few years). When we did the compression check, my mechanic said all is fine compression wise and raised absolutely no concerns about cylinder 3 being at 78 and cylinder 1 being at 70. What are your thoughts?
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You're good to go. The (leak-down) test we call compression test is generally looking for a leaking valve seat or excessive piston ring leakage. After a short run and depending on how much time the cylinders have to cool down changes the leakage also.
 
sansoneservices said:
You're good to go. The (leak-down) test we call compression test is generally looking for a leaking valve seat or excessive piston ring leakage. After a short run and depending on how much time the cylinders have to cool down changes the leakage also.
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We always test them cold. I heard they usually tend to read a little higher when tested after flight. Since they’ve always been tested cold even with the previous owner, i keep it that way to establish trends and compare apples to apples.
 
sansoneservices said:
The (leak-down) test we call compression test is generally looking for a leaking valve seat or excessive piston ring leakage
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It's not a leakdown. That's a totally different technique. We use the differential compression test, with the difference between the gauge readings being the test result. And we are looking for leaky rings and valves, too. Listening at the oil filler for noise detects ring leakage. Noise at the exhaust or air intake tells you which valve might be leaking.
 
Dan Thomas said:
It's not a leakdown. That's a totally different technique. We use the differential compression test, with the difference between the gauge readings being the test result. And we are looking for leaky rings and valves, too. Listening at the oil filler for noise detects ring leakage. Noise at the exhaust or air intake tells you which valve might be leaking.
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Yeppers, differential pressure test with one gauge showing combustion chamber leakage. Followed by a dual camera bore scope is routine here
 
Dan Thomas said:
It's not a leakdown. That's a totally different technique. We use the differential compression test, with the difference between the gauge readings being the test result.
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The test typically performed on aircraft with reciprocating engines is called "differential compression test" by the FAA:

It is very similar to what the automotive world calls a "cylinder leakdown test":

Please describe the "totally different technique" that you are calling a leakdown test. Inquiring minds want to know!
 
I don't want to derail us too much but rather to get some clarity on a few points ...

The FAA says a compression test must be done but as I understand it they do not specify what manner must be used (dynamic, differential) so I guess it's left to the engine manufacturer to give guidance. If that's true does the FAA mandate the following of manufacturer guidance? I know we hear, "below 60/80 pull the cylinder" but I don't believe this is mandatory. I'm asking the question because I might be incorrect for the second time in my life ...
 
Daleandee said:
If that's true does the FAA mandate the following of manufacturer guidance?
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Not directly. The regulatory requirement is Part 43 Appx D(d)(3) below. How you perform that (d)(3) check is in Part 43.13(a) below. So your performance option is the MM, ICA, or acceptable to the Admin. For example, in general, the MM/ICA route would give you the 60/80 value per Lycoming, or calibrated Master Orifice value (which can be as low as the 40s/80), or for the acceptable to the Admin version the 60/80 value found in AC 43.13-1B. Or you could use any other method provided it fits into one of those 3 performance standards.

So lots of guidance with no one method "mandatory" unless it was required by a rule like an AD. Ironically, one could also use the TCM Master Orifice value on a Lycoming as the process is acceptable to the Admin. So lots of flexibility and still be good.

2nd time a charm?:)

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1727110503898.png
 
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And the manufacturers generally say the first step after failing a compression test is to go fly it for a couple of hours and try again.
 
Daleandee said:
I don't want to derail us too much but rather to get some clarity on a few points ...

The FAA says a compression test must be done but as I understand it they do not specify what manner must be used (dynamic, differential) so I guess it's left to the engine manufacturer to give guidance. If that's true does the FAA mandate the following of manufacturer guidance? I know we hear, "below 60/80 pull the cylinder" but I don't believe this is mandatory. I'm asking the question because I might be incorrect for the second time in my life ...
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Continental's take of differential compression and top-end care:
https://www.dropbox.com/scl/fi/bdzq...cope.pdf?rlkey=pfucdu0x839fkfghq1wag6fei&dl=0
https://www.dropbox.com/scl/fi/di33...7-2B.pdf?rlkey=bh5ile1np25rq0ouq4w7ou86y&dl=0
 
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