Compression: What's good and borderline?

VWGhiaBob

VWGhiaBob

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VWGhiaBob
I am purchasing a plane. Engine details are:

SMOH 200 Continental IO-0550N

Last Overhaul: October 2007 by Pacific Continental Engines, NEW Cylinders at overhaul, Compressions at last annual: 1)76 2)78 3)77 4)77 5)78 6)77, Engine Notes: No leaks. Very smooth running, especially at idle.

Are these good compression numbers? Whats considered good, borderline, and "it's time for some serious maintenance"?

Thanks, members!
 
Everything in the high 70s is very good.

60 or below is getting borderline.

Continental allows something like 45 if it's not an exhaust valve leak.

A&Ps will certainly chime in with more educated numbers.
 
Get the over haul details and find out what else was replaced or overhauled.

The engine sounds strong to me.
 
How much do compressions vary from annual to annual? I saw one where a cylinder went from 78 to 67 to 74. What's up with that?
 
farmerbrake said:
How much do compressions vary from annual to annual? I saw one where a cylinder went from 78 to 67 to 74. What's up with that?
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A lot of it can be inconsistencies in how the mechanic did it and how accurate their equipment was, etc, etc. Too many variables to say for sure.

It is possible for it to drop then come back but it's equally possible for the mechanic to just have botched the measurement.

Too much focus is put on the exact number of the compressions. What matters is that they're not terrible. I consider it a "good" or "bad" test and nothing more.
 
Looks good to me

the IO550 is a sweet engine, got some time behind one, what plane is it in?

If a plane sits for a while it's not uncommon to see low numbers, fly her for a bit and have them come back up.

Anything consistently below 70/80 gets my personal spidy sense going off.

Below 60/80 is NOT AIRWORTHY.
 
:rofl:

From the Mike Busch article I linked above:

One of the most pervasive OWTs about compression checks goes something like this:

* High 70s are excellent
* Low 70s are good
* High 60s are marginal
* Low 60s are bad
* Below 60/80 is unairworthy

Now, perhaps this had some validity back in the days when radial engines were king. But for modern, horizontally-opposed engines, it's simply wrong -- certainly wrong for Teledyne Continental Motors (TCM) engines, where the manufacturer has set forth very specific procedures for doing compression tests and evaluating the results. According to TCM, a cylinder with a compression reading of 50/80 might well be absolutely airworthy, provided the cylinder meets certain other requirements.
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farmerbrake said:
How much do compressions vary from annual to annual? I saw one where a cylinder went from 78 to 67 to 74. What's up with that?
Click to expand...

With an engine such as the 550 you are looking at some very big pistons. In normal operation there are tremendous pressures and extremely high temperatures. During a compression test you are only putting in 80 psi on a relatively cold cylinder (despite having run it prior) Sometimes you can rock it back and forth and you will hear the ring snap into a good seat and the reading will jump 10 or 20 points.

So it's an imperfect test, especially with the bigger engines but it's valuable and can indicate if a real problem exists. Think of it more as a canary in the coal mine rather than a definitive analysis. You can record the numbers for historic reference but you're not going to get a "AHA!" or meaningful picture from them based on slight variations. I think that overall there is a misconception about the test and people tend to put way too much emphasis on the numbers obtained.
 
VWGhiaBob said:
I am purchasing a plane. Engine details are:

SMOH 200 Continental IO-0550N

Last Overhaul: October 2007 by Pacific Continental Engines, NEW Cylinders at overhaul, Compressions at last annual: 1)76 2)78 3)77 4)77 5)78 6)77, Engine Notes: No leaks. Very smooth running, especially at idle.

Are these good compression numbers? Whats considered good, borderline, and "it's time for some serious maintenance"?

Thanks, members!
Click to expand...

Perfect, no issues at all with those numbers. Read Continental's stuff on compression testing/reading. The numbers aren't everything, they ran an engine with compressions in the 40s on a dyno and found no loss of power. You have to figure out where the air is going to understand if you have a problem or not. If you hear the leak in the breather pipe, and you aren't consuming a bunch of oil, you don't have a big problem. If you hear the air coming out the exhaust pipe, you have a problem that needs fixing, the sooner the better.

The numbers you posted give no indication of any issue, they are perfectly good number. Doesn't mean there is no problem, just not one indicated there. Don't forget to pull an oil analysis.
 
Last edited:
8-14. COMPRESSION TESTING OF AIRCRAFT ENGINE CYLINDERS. A test to determine the internal condition of the com- bustion chamber cylinder assembly by ascer- taining if any appreciable internal leakage is occurring is compression testing of aircraft en- gine cylinders. If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in para- graphs 8-15b(5)(i) and (j) fail to raise the com- pression reading, the cylinder must be removed and inspected.
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http://www.faa.gov/documentLibrary/media/Advisory_Circular/Chapter_08.pdf

I recalled it saying it wasn't airworthy below 60/80.

Ether way, I sure as chit wouldn't be buying the plane if it was giving readings below 60, or even near 60 for that matter.

Also wouldn't be strapping myself to a plane with weak compression, my life is worth more than a couple cylinders.
 
Silvaire said:
With an engine such as the 550 you are looking at some very big pistons. In normal operation there are tremendous pressures and extremely high temperatures. During a compression test you are only putting in 80 psi on a relatively cold cylinder (despite having run it prior) Sometimes you can rock it back and forth and you will hear the ring snap into a good seat and the reading will jump 10 or 20 points.

So it's an imperfect test, especially with the bigger engines but it's valuable and can indicate if a real problem exists. Think of it more as a canary in the coal mine rather than a definitive analysis. You can record the numbers for historic reference but you're not going to get a "AHA!" or meaningful picture from them based on slight variations. I think that overall there is a misconception about the test and people tend to put way too much emphasis on the numbers obtained.
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Same sized pistons in a "tiny" 0-320....;)
 
Heck, anything over 50 on a TCM jug is just fine. I've seen them vary from mid 70s down to the low 50s. If the oil consumption isn't a problem, and it doesn't make metal - run it.
 
that engine has averaged 28 hours per year since 2007. Wouldn't you be more concerned with other issues? I would take good compressions as promising, but shouldn't more emphasis be placed on the condition of the camshaft for instance i.e. corrosion? :dunno:
 
rbridges said:
that engine has averaged 28 hours per year since 2007. Wouldn't you be more concerned with other issues? I would take good compressions as promising, but shouldn't more emphasis be placed on the condition of the camshaft for instance i.e. corrosion? :dunno:
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Continental, not Lycoming, cams aren't a particularly known problem on Continentals.
 
N801BH said:
Same sized pistons in a "tiny" 0-320....;)
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You mean 360, because Continental doesn't make a 320 and he wasn't asking about a 320, he was asking about a 550 :dunno:
 
Silvaire said:
You mean 360, because Continental doesn't make a 320 and he wasn't asking about a 320, he was asking about a 550 :dunno:
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Actually, IIRC, the Lyc 320 bore is 5.125" and Cont 550 is 5.25, so pretty close, 1/8 of an inch. The bore size will be rather close in any well designed engine meant to make peak torque in the same RPM band since it's mostly determined by an RPM-Burn/Expansion Speed calculation to maximize the ICP right after the piston hits TDC.
 
Silvaire said:
You mean 360, because Continental doesn't make a 320 and he wasn't asking about a 320, he was asking about a 550 :dunno:
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Geez........................

A piston the size of a syrup bucket + or- .125 " was my point....:mad2:...;)
 
Id be more interested in a good boroscope eval and do a cut filter looking for metal and oil analysis. Also oil burn rate if that could be figured would be good, maybe a 3 hour test flight and see if/how much the dipstick moves. If they have an engine monitor see if you can get a dump of the data to look at.


Sent from my iPad using Tapatalk
 
trepine said:
Id be more interested in a good boroscope eval...
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Interesting, on an engine with compression readings in the high 70's across the board what do you think you're going to see with a borescope? :dunno:
 
Silvaire said:
Interesting, on an engine with compression readings in the high 70's across the board what do you think you're going to see with a borescope? :dunno:
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He is gonna see 6 good looking cylinders...:yes:......;)
 
I'm the new guy here but I'll opine on this.....
I have owned 3 airplanes over the past 15 years. All have had Continentals hanging on the front.
The first was a Taylorcraft L-2M with an A-65 (or O-170 as the USAAF called it).
It had less than 300 hours SMOH when I bought it. What I failed to consider was that 300 hours was accumulated over 30+ years. Yes I was young and it was my first airplane.:redface:
The compressions on that engine were never less than 68. However, after I started flying it on a regular basis, that little unit tried to kill me no less than 3 times! The final straw came after a forced landing in a 900' field due to a cylinder separating from the case!:yikes:
Several thousand dollars and a major overhaul later I had a new engine with compressions the same as before. But my maintenance costs shrunk dramatically.:wink2:
Number 2 was a C-172 with a Continental O-300D. 1200 SMOH on an overhaul done 6 years prior. Compressions always between 68-72. I flew that plane for over 600 relatively trouble free hours. I did have to replace one cylinder the last year I owned it due to the jug leaking around the exhaust valve. It would use a quart of oil about every 8 hours or so. Smoothest engine I have ever sat behind.
It is still flying on that same overhaul to my knowledge. Not bad for an engine that "can't make halfway to TBO before them jugs need to be replaced" according to some hangar buzzards.
Number 3 (and my current obsession) is a C-182 with the Continental O-470R.
I purchased this ship over 5 years ago at 870 SMOH with a plan to do a top overhaul. At the time I bought it the compressions were 58-64. This is a Western Skyways engine BTW. I was able to negotiate the purchase price to take into account a top overhaul so no surprises there.
After much discussion with my A/I he convinced me to fly it a while and see what happens as the plane had flown only 16 hours the previous 2 years.
I was leery of this approach due to my experience with the A-65.
He explained to me how it is not at all unusual for big bore Continental engines to have low compression numbers. He said if a large spread in values between cylinders develops or if they get down in the 40's it is time to get concerned.
Well, I guess he knows what he is talking about. I have put around 340 hours on it without a single hiccup (knock on wood) and I honestly believe if the doors will shut on the plane it will haul it! Last inspection compression values were 62-68. Very little oil consumption if kept at 10 quarts. It always pukes out the first two.:dunno:
I can't speak for Lycomings but lower compressions on a large Continental would not alarm me provided the overhaul is fairly recent.
BTW, to the guy who "wouldn't strap himself behind cylinders less than 60 lbs." - the ONLY engine that ever let me down had a lowest compression value of 68. I would not stake my life on compression values alone!
 
A cylinder separating from the case had nothing to do with low compressions.

What were the other two issues leading up to a failed cylinder?
 
BillTIZ said:
A cylinder separating from the case had nothing to do with low compressions.

What were the other two issues leading up to a failed cylinder?
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I never said it did. Please read my post more carefully. The cylinders on that engine always had great compressions.

Totally unrelated to cylinder failure...lost power climbing out on two different occasions. As it turns out the needle in the carb was sticking in climb attitude due to previous owner using old car gas and shellacking of the needle and float.
I could go on and on but I don't feel like typing that much........
 
James331 said:
Anything consistently below 70/80 gets my personal spidy sense going off.

Below 60/80 is NOT AIRWORTHY.
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I couldn't disagree more.

Continental service bulletins allow much lower compressions than that, and planes are perfectly airworthy for years. My 0-300 in my 172 square tail had compressions in the 60's consistently. Best engine I ever flew behind, and I'd have headed for Alaska without a second thought. (I'm in Houston, btw).

I've seen compressions in the 40's that became compressions in the 60's later. Blanket statements just don't apply. My I/A had a cylinder with compressions in the 40's. Decided to disassemble to find out why. Took it all apart, examined each piece carefully, and couldn't find anything wrong... Put it back together will all the same parts, changed nothing. Put back on the plane. 1st compression check was in the high 60's. Anecdotal? Sure. True? Yes. It doesn't happen all the time, but it happens. You just have to slow down, think, test, analyze, and repeat.

A good diagnostic mechanic is worth his weight in gold.
 
Learning about compression tests on aircraft was where I first heard the term "master orifice", which seems to be exactly where most opinions about compression numbers come from. ;)
 
Like I said earlier, owners seem to get hung up on the numbers. For an experienced A&P it's an informative and valuable diagnostic procedure and unless the owner insists on it I don't log those numbers because they aren't the whole story.
 
kgruber said:
Primarily to evade warranty claims.
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:rofl: Warranty claims, on an aircraft engine? That right there is funny, I don't care who you are.:lol: Continental would be much better off to say "Anything below 70 requires replacement." They'd sell a lot more cylinders that way, and since you aren't going to see that number until long after any warranty runs out, it would be a profitable transaction.

Continental actually did quite a bit of testing on the subject, I attended one of their seminars on it where they displayed the evidence of the dyno runs comparing an engine in the 40s with one in the 70s, the traces were the same. Rings are made to seal under much higher pressure loads than a compression test. The conclusion was, "If the air isn't going out the exhaust, and you aren't using excessive amounts of oil, don't get too fussed with numbers down to 40."
 
With cars and motorcycles we use dynamometer test and measure actual horsepower. Its by far the best way Ive ever seen to determine engine condition. Ive never heard of a setup for a plane that can do that.

Ive seen mechanics influence aircraft compression tests by moving the prop (and piston) slightly. Doesnt seem like a very good test if you can make signifigant difference with just a wiggle.

Ive often wondered why (other than tradition and the fact that you would have to spin the prop with the starter, and they dont like to do that) aircraft mechanics dont do an auto style compression test, maybe in ADDITION to the leakdown one. Its dynamic and cant be screwed with so easily.

I'll admit Im no real expert.
 
coloradobluesky said:
With cars and motorcycles we use dynamometer test and measure actual horsepower. Its by far the best way Ive ever seen to determine engine condition. Ive never heard of a setup for a plane that can do that.

Ive seen mechanics influence aircraft compression tests by moving the prop (and piston) slightly. Doesnt seem like a very good test if you can make signifigant difference with just a wiggle.

Ive often wondered why (other than tradition and the fact that you would have to spin the prop with the starter, and they dont like to do that) aircraft mechanics dont do an auto style compression test, maybe in ADDITION to the leakdown one. Its dynamic and cant be screwed with so easily.

I'll admit Im no real expert.
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:confused: There is no problem hooking up a dyno on an aircraft engine, just bolt the pump drive to the crank and hold the case in reactionary. You can use the same one we used to use on outboard motors still on the boat.

I have done regular compression tests on my aircraft, and have used the leak down test on cars and other applications. They tell you different things, and for diagnostics, the leak down gives you more information as you can identify where the leak comes from.
 
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