Engine compression?

Chesterspal

Chesterspal

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Chesterspal
If I'm inspecting the logs for a plane I'm interested in purchasing and I find, in the annuals, the engine compression is dropping a few points each year.... how much of a concern is that?

For instance: in 2017 it's 76/80 for #1, in 2018 it's 73/80 for #1

Also, how far apart can they be from each other before I need to worry?

Some articles I've read state that 60 in any cylinder is the minimum before overhaul while others state that is not necessarily the case and it can go lower. Which is correct?
 
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How much has the plane been flown between annuals? If it is not flown regularly the compression test numbers can be all over the map, and the variations may not mean much.

The absolute numbers are just one aspect of the compression test to consider. An experienced mechanic can tell whether any loss of pressure is due to wear in the rings versus issues with the valve train.

Think of a compression test as analogous to a doctor using a stethoscope to listen to your heart. Even if something is detected, its not likely jumping immediately to open heart surgical "overhaul" is necessarily the wisest course of next action. ;)
 
They can be much lower than that and still be okay. They can also get better with regular use instead of worse.
 
Those are leakdown [differential pressure] numbers, not dynamic compression numbers. Both tests can be done, but those numbers are swell as they are.
 
the numbers will go up and down....what's most important are the visuals....get a look inside the cylinder with a camera and "see" the valve faces. Hearing where the leakage is ...is also important.
 
Ravioli said:
Calling @Tom-D!

My opinion is 3PSI doesn't matter. That's more or less testing error.
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My impression of Compression tests.
90% of how it was done, and 10% of what the owner thinks.
 
Kenny Phillips said:
Those are leakdown [differential pressure] numbers, not dynamic compression numbers. Both tests can be done, but those numbers are swell as they are.
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Tell us how to do a dynamic compression test?
 
so rocking the prop and taking the highest pressure doesn't count? :D
 
I attended the Continental Engine Maintenance class earlier this year and this was discussed a lot. Unofficially they wish that the FAA would allow them to do away with it. Borescope tells so much more. The air pressure is helpful for listening for air escaping past valves, induction leaks etc. until then, people will fret over the “numbers” - insert dramatic music here...

I just attended a Bonanza Service Clinic and the Continental guy said “your compressions are a little low and you have some rust areas in every cylinder - I had to stay on the ground almost three months, he gave me his write up and looked at me and said “truthfully you have nothing to worry about and go fly the snot out of it.”

I can do a compression check on my plane three days in a row (I have) and get three different readings. Those numbers are fine. Go forth and fly.

PS....continental and the FAA have a service bulletin that describes the compression tests in detail and one key factor that most do not know about is the Master Orifice. On our engines a Master Orifice of .40 is used. Air is applied and the test lever is rotated and the “test” pressure is read....for that day. Air density and pressure changes. My unit usually varies between 41 and 43 psi. According to the FAA and Continental, as long as your compression is above those numbers then you “pass”. 60 psi was an arbitrary number thought up because we need a number.

Leaving now but will try to find that bulletin and post here.
 
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Actually, I did find and read those articles... but thank you. Also, read about this special orifice to calibrate/test the compression gauge.

Just another issue that gets tossed around. More to confuse the buyer and perhaps also the seller and hold up the purchase for no good reason.
 
Fantastic! You are ahead of most by a long shot. It’s becomes a subjective subject but when you understand that compression check’s are just a part of the decision to buy or perform more maintenance, then YOU can and will make the right decision.

Question the old wife’s tales and the airport expert.

Good luck
 
Checkout_my_Six said:
so rocking the prop and taking the highest pressure doesn't count? :D
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Rocking the prop is the recommended procedure by Lycoming. Probably the biggest difference is if the check is done on a hot or cold engine. In addition it’s possible for the rings to line up and lower compression or a valve rotate to a position where it does not seat quite right. That is why if you do have a unusual drop in compression the recommendation is normally to fly it for 10 hours and recheck.
A compression check combined with a borescope is the best way to go.
 
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From what I've read, and what I've experienced with my own little VW conversion, the numbers, by themselves, don't mean much. There are so many variables. They're a great way to track how the engine is wearing as you put hours on it. The numbers are great. But, what's more important IS how the engine is wearing.

With compression tester that meets AC43.13 standards, and using a master orifice, you could have a compression of 80/40 and still be ok. But, if you have any leak-down through the exhaust valve, with that piston at top dead center, you have to do something about it. And, there are only so many things you can do before you just have to pull the cylinder (or just the head in the case of my VW conversion).
 
The compression test, which is a differential pressure leak down, is a very valuable examination of the sealing surfaces and integrity of the cylinder. It will reveal leaks past rings or valves and also a cracked head. It's not about writing down a number and making a decision based just on that. I'm going to disagree with the popular myth that looking inside the cylinder with a video-scope is a far more valuable diagnostic tool. I can say from experience if you look in a cylinder and see a burnt valve your compression reading is very likely to be 0/80. I've seen cylinders with readings as low as 10/80 with leaking valves that show no visible anomalies. Furthermore, if you have a compression reading of 79/80 and someone pokes a $100 video-scope in there and says he sees "discoloration" on a valve are you gonna let him pull that jug? Nope, that's the answer. I think the whole borescope thing was so people would look in there, see nothing and stop pulling cylinders just because the compression number was a little low. It's a simple diagnostic test that requires further investigation and ultimately a decision as to whether or not something serious is going on. You can't simply look at a sheet of paper with numbers written on it and decide that a cylinder needs replacing.
 
Another thread remind3d me....SB 1006. Not an AD but should be a required action. Found out after the fact that the owner who,sold me the cherokee did so to avoid even checking for SB 1006 issues.
 
Silvaire said:
The compression test, which is a differential pressure leak down, is a very valuable examination of the sealing surfaces and integrity of the cylinder. It will reveal leaks past rings or valves and also a cracked head. It's not about writing down a number and making a decision based just on that. I'm going to disagree with the popular myth that looking inside the cylinder with a video-scope is a far more valuable diagnostic tool. I can say from experience if you look in a cylinder and see a burnt valve your compression reading is very likely to be 0/80. I've seen cylinders with readings as low as 10/80 with leaking valves that show no visible anomalies. Furthermore, if you have a compression reading of 79/80 and someone pokes a $100 video-scope in there and says he sees "discoloration" on a valve are you gonna let him pull that jug? Nope, that's the answer. I think the whole borescope thing was so people would look in there, see nothing and stop pulling cylinders just because the compression number was a little low. It's a simple diagnostic test that requires further investigation and ultimately a decision as to whether or not something serious is going on. You can't simply look at a sheet of paper with numbers written on it and decide that a cylinder needs replacing.
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1.) What would you call this?
47754543352_6d7431e966_c.jpg



2.) And this?
https://www.aopa.org/news-and-media/all-news/2017/may/pilot/savvy-maintenance-borescope ---"Over the years, the compression test has proved untrustworthy and prone to false positives, resulting in tens of thousands of cylinders being removed unnecessarily (including a few of mine). That’s why the SB03-3 guidance calls for any disqualifying compression test that is not corroborated by borescope evidence be retested after flying for at least 45 minutes."
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wilkersk said:
1.) What would you call this?
47754543352_6d7431e966_c.jpg



2.) And this?
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The term "compression check" is an inaccurate description. But so is the term "leak down check". Personally, I think the best description would be "cylinder leakage check" since the air pressure remains applied throughout the test.
 
Silvaire said:
The compression test, which is a differential pressure leak down, is a very valuable examination of the sealing surfaces and integrity of the cylinder. It will reveal leaks past rings or valves and also a cracked head. It's not about writing down a number and making a decision based just on that. I'm going to disagree with the popular myth that looking inside the cylinder with a video-scope is a far more valuable diagnostic tool. I can say from experience if you look in a cylinder and see a burnt valve your compression reading is very likely to be 0/80. I've seen cylinders with readings as low as 10/80 with leaking valves that show no visible anomalies. Furthermore, if you have a compression reading of 79/80 and someone pokes a $100 video-scope in there and says he sees "discoloration" on a valve are you gonna let him pull that jug? Nope, that's the answer. I think the whole borescope thing was so people would look in there, see nothing and stop pulling cylinders just because the compression number was a little low. It's a simple diagnostic test that requires further investigation and ultimately a decision as to whether or not something serious is going on. You can't simply look at a sheet of paper with numbers written on it and decide that a cylinder needs replacing.
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I'm glad you wrote all that so I didn't have to type something similar.

To me, this recent obsession with cameras and looking in the cylinder is just as worthless on it's own as the leak testing is. The engine and its condition need to be evaluated as a whole, using all available resources.
 
I've been doing this for over 40 years. The term is "compression check" I say that and everybody knows exactly what I'm talking about.
 
Silvaire said:
I've been doing this for over 40 years. The term is "compression check" I say that and everybody knows exactly what I'm talking about.
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Common terms are common terms, that have different meaning to different people.
 
So, to be clear on this...

If the compression check shows a healty 73 to 76 on all four cylinders, regardless of discoloration seen inside with a camera, leave things as they are for now and keep flying.

Is this correct?
 
Chesterspal said:
So, to be clear on this...

If the compression check shows a healty 73 to 76 on all four cylinders, regardless of discoloration seen inside with a camera, leave things as they are for now and keep flying.

Is this correct?
Click to expand...
yes
 
Chesterspal said:
So, to be clear on this...

If the compression check shows a healty 73 to 76 on all four cylinders, regardless of discoloration seen inside with a camera, leave things as they are for now and keep flying.

Is this correct?
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Do you mean discoloration on the valves?

Tom-D said:
yes
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Assuming the answer to my question above is "yes" , what if the discoloration is indicating the start of a "burnt valve" such as the image below?
Would it not be better to deal with it now?

BurntValve1.jpeg
 
Tom-D said:
Tell us how to do a dynamic compression test?
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Oh, please. Put a pressure gauge in a spark plug hole and spin the engine, of course. The same way it's been done for a century. Two different tests, our guys did both (as I do to race engines).
 
Kenny Phillips said:
Oh, please. Put a pressure gauge in a spark plug hole and spin the engine, of course. The same way it's been done for a century. Two different tests, our guys did both (as I do to race engines).
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Not true, Dynamic compression is a BMEP of a running engine. And you can't do it with out a BMEP gauge in the cockpit.
 
Tom-D said:
Not true, Dynamic compression is a BMEP of a running engine. And you can't do it with out a BMEP gauge in the cockpit.
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We called it "dynamic compression test" fifty years ago, and it meets that definition. It's not a differential pressure test, which most folks, including some A&Ps, mistakenly call a "compression test". My Snap-On tester says "differential".
There's curmudgeon and there's pedant, and sometimes a combination, in many of these threads. But the bottom line is that they are two different tests, one of them is not a compression test.
 
Kenny Phillips said:
We called it "dynamic compression test" fifty years ago, and it meets that definition. It's not a differential pressure test, which most folks, including some A&Ps, mistakenly call a "compression test". My Snap-On tester says "differential".
There's curmudgeon and there's pedant, and sometimes a combination, in many of these threads. But the bottom line is that they are two different tests, one of them is not a compression test.
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What you describe is known as a "bang compression" , dynamic compression is the actual compression the cylinder makes during operation.
 
See what I mean when I say standard terms mean different things to different people.
 
Kenny Phillips said:
Oh, please. Put a pressure gauge in a spark plug hole and spin the engine, of course...
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Why would you do that? If you get a low reading you're still going to have to do a leak-down to determine the source. Since an airplane has this big propeller that makes it so easy to position and hold the piston at TDC you may as well just start there. If you get 80/80 there is no reason to do a cranking compression test. It is used on cars because of the difficulty in positioning and holding the engine at the desired point, the crank pulley nut is usually buried down among belts and accessories and hard to get on. Not so with an airplane, this is just plain common sense.
 
Cap'n Jack said:
Do you mean discoloration on the valves?


Assuming the answer to my question above is "yes" , what if the discoloration is indicating the start of a "burnt valve" such as the image below?
Would it not be better to deal with it now?
Click to expand...

There's no such thing as the "start of a burnt valve". If you have an actual visual indication then it's burnt and the compression test is already telling you that without even looking. All of this came from the Continental service bulletin then Mike Busch took those pictures and started the myth that you can just look in there with a borescope and foretell the future. Problem is that all of those pictures are from cylinders that failed compression tests and were removed. Those visual clues don't foretell the future, those valves are already burnt and they already knew it before taking a picture of it because it was leaking like a sieve.
 
My point is there are two scenarios:
#1 compression is 10/80 leaking from the exhaust valve. You poke your $99 video scope in there and don't see anything. What are you going to do?
#2 compression is 79/80. You poke your $99 video scope in there and see "discoloration" on the exhaust valve. What are you going to do?
In either case, did the video-scope make any difference?
 
Checkout_my_Six said:
the pictures "confirm" the leaks.....scrapes and wear.
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No it doesn't, all it really does is give a bunch of on line viewers some thing to talk about.

because valves rotate. How would you know where the leak was?
 
Tom-D said:
No it doesn't, all it really does is give a bunch of on line viewers some thing to talk about.

because valves rotate. How would you know where the leak was?
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guess you really don't know....now do you? ;)
 
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