Compression readings...what do they really mean?

hish747

hish747

Pre-takeoff checklist
Joined
Mar 31, 2015
Messages
247
Location
Skylark Airpark
Display Name
Display name:
Hish747
How do engine/cylinder compression readings relate to engine performance and longevity?

Let's say that 4 of your cylinders are 60/80 psi. Does that mean that you only have 3/4 of the horsepower of new cylinders available to you?

Also if your Lycoming (IO360 for example) compression readings are in the low 60s does that indicate a need to overhaul the engine soon? Replace cylinders?
H
 
A compression tells us nothing about the engines condition other than the combustion chamber does not leak or how much it does.
 
There are other factors that contribute to HP, such as RPM. Reduced compression will cause the engine to not be as efficient. That will probably reduce max RPM and HP. At some point, reduced compression will cause the engine to not run. I don't know what that number is, it may vary. But, let's assume it's 20 psi. You surely wouldn't see 1/4 hp then.
 
Last edited:
Tom-D said:
A compression tells us nothing about the engines condition other than the combustion chamber does not leak or how much it does.
Click to expand...
To reply to my statement, we make repairs to engines, I can legally remove the cylinders, completely rebuild the lower end, and replace the old cylinders, It is legal to remove and replace the cylinders with new, and leave the lower end as it was.
As I've said here many times, you never know what is any engine until you tear it down and look.
So,, you must treat each engine, as pandora's box.
 
GlennAB1 said:
There are other factors that contribute to HP, such as RPM. Reduced compression will cause the engine to not be as efficient. That will probably reduce max RPM and HP. At some point, reduced compression will cause the engine to not run. I don't know what that number is, it may vary. But, let's assume it's 20 psi. You surely wouldn't see 1/4 hp then.
Click to expand...
TCM ran a test as to how much a valve could leak and still meet certification requirements. So, they cut away some of the valve, tested it for leak down, each time they cut away more of the valve, until they had 20/80 and it still made the horse power of certification.

That's why we believe in dynamic compression.
 
GlennAB1 said:
At some point the engine won't start. Did they make a determination what psi that would be?
Click to expand...
You must remember that a 60/80 engine is the easiest starting, smoothest running engine out there.

At what point won't the engine start? I've never seen one that would't
 
It's a simple leak down test to check the sealing integrity of the valves and piston rings. A leaking valve is not going to fix itself or get better, the leak will continue to erode the seat and valve face. Leaking compression rings will result in blow-by, increased oil consumption and crankcase pressure. The test is performed at a low pressure of 80 psi for safety reasons. This is nowhere near the pressure these components experience in operation and it is unlikely that you would be capable of noticing any performance detriments with even a very low differential compression test result. It's also very unlikely that you would be able to visually detect any internal anomaly with a borescope on a cylinder that had anything over a 0/80 result.
 
There was on infamous owner at a now gone airport who all the old timers prattled on about how his old engine always got 40's on compression tests but he flew it for years like that anyway. His problem wasn't valves, it was rings and he changed the oil constantly and it came out looking like crap, not to mention burning a lot of it. But it ran and his airplane flew and never let him down.

I wouldn't have put him in the "conscientious pilot/owner" labeled box on the brain shelf, but he knew he was flying a claptrap old Cessna and never took passengers, so he took his own risks and managed them as he saw fit I guess. Dude was more than a little "eccentric" shall we say.
 
Excerpted from a TCM publication I have discussing the TopCare HealthCheck Inspection procedures [The Capitals are TCM's emphasis]:

"It is important to note that differential compression checks are used to identify cylinder leakage rates and the source of the leakage. This check cannot be directly related to engine horsepower. ENGINE TESTING HAS SHOWN THAT CERTIFICATION HORSEPOWER RATINGS WILL CONTINUE TO BE DELIVERED EVEN WHEN ALL CYLINDERS ARE AT OR BELOW THE MINIMUM ALLOWABLE CALIBRATED COMPRESSION READING AS ESTABLISHED BY THE MASTER ORIFICE TOOL."​
 
Especially true when you have 6 cylinders :)

Last year at this time, I pulled a cylinder for a leaking/burned exhaust valve (indicated barely 5/80 on the diff comp check). I had just flown it 3.5 hours the day before, it made rated static rpm, wasn't using excessive oil, etc. Started and ran beautifully, just wasn't serviceable anymore!

Incidentally, I sent it out to a well known shop to be overhauled and it was rejected for cracks in the exhaust port. Like I said... It ran fine...

V/r,

-Dana
 
I have a different opinion. When I've found cylinder compression sagging in a single cylinder of my big bore TCMs it's because I took it to my mechanic complaining that it wasn't running right. The last time I had 5 at 77-78/80 and one at 64/80. TCM can tell you it makes rated power using whatever tolerances they use but that doesn't mean it's running to it's potential and it almost certainly isn't as smoothly as it could be. If guys don't recognize it? I don't know what to say about that. I need 100% from my engine. When it dips below that I fix it. If 60/80 compression as ideal, as has been implied, the factory would change the cylinder tolerances to achieve it. In my world 80/80 is ideal.

Sagging compression discussions almost always center on TCM engines. Does Lycoming still have a 10% compression variation tolerance?
 
A worn cam will reduce horsepower before worn rings.

Some data points - I had a 1964 Sunbeam Alpine that burned oil like it was going out of style - you had to roll the windows down when you stopped at a light because the interior would fill up with smoke from the blow by and you couldn't see. When I tore it down there were several broken rings and the cylinders were not in the best condition either. But it was starting and running just fine. (Don't recall compression numbers or if we even bothered to check.)

I've known an engine to go thousands of miles with the rods clattering so loud you could hear them a block away.

A buddy bought an engine from a drag racer who missed a shift, floated the valves and thought he had destroyed the engine. Turns out that the valves had just kissed the pistons hard enough to make little smiles (and a LOT of noise) but not bend. Lap the valves, re-assemble, and my buddy had a good engine that he had bought at scrap prices.

I can also tell you that an engine may run for quite a while even after it runs out of oil. Hydraulic lifters collapse and make a lot of noise and you loose a lot of power, but it may keep going.

But, I've also seen an engine stop cold at 200 feet AGL over Alaskan tundra. The suspicion was that it swallowed a valve, but I don't know if the engine was ever salvaged or not.

Low compression is an indication of internal wear or degradation. The exact nature of that is not clear from just the compression numbers - could be rings, could be valves. Might be worn, or just rings coked up and not seating properly. The key is that the engine condition is deteriorating and one might consider fixing it now rather than wait for a real problem.
 
Few mechanics want to do a dynamic compression test (you have to adjust for elevation), but it is in a way, a better test of engine condition, though it does not give as good info on whether it is valves or rings that is causing it. For that you need differential test. One reason they dont is they have to spin the prop and they cant do that inside.

The best test would be a dynamometer (standard for cars and motorcycle tube shops) that gives horsepower ( adjusted to sealevel), but Ive never seen one in Aviation.
 
Last edited:
Stewartb said:
I have a different opinion... If guys don't recognize it? I don't know what to say about that. I need 100% from my engine. When it dips below that I fix it. If 60/80 compression as ideal, as has been implied, the factory would change the cylinder tolerances to achieve it. In my world 80/80 is ideal.

Sagging compression discussions almost always center on TCM engines. Does Lycoming still have a 10% compression variation tolerance?
Click to expand...

A differential compression test is just one of many tools that you can use to help determine the condition of the engine. A 60/80 cylinder does not have 25% less power than one that has 80/80 compression. Your readings on a differential compression test depend on lots of different variables:

-Condition of the cylinder walls
-Condition of the rings
-Condition of the intake/exhaust valves
-Condition of the seats
-Atmospheric conditions on the day you took the test
-Temperature of the engine
-Amount of oil film retained on the cylinder walls
-Amount of texture (hone pattern. or chrome channels, etc) that still exist on the cylinder walls.
-And the list goes on, etc.

I've seen shiny cylinder walls that had excessive wear (no longer had surface texture pattern to retain oil) on them that could pass a differential compression test with flying colors, but really were on their last legs. I've also seen good running cylinders that passed a differential compression test as low as 48/80 on that particular day.

Lycoming/Continental/Franklin/Jabiru/Jacobs/Wright Cyclone branding doesn't really matter. If the cylinder is serviceable, and the engine meets static run up RPM requirements, and isn't filling the filter with metal--the engine is probably just fine.

Run to the overhaul shop when your engine reaches TBO (or 12 years) if it suits you. But I've seen untouched Lycomings/Connies each with 3600hrs on them TSMOH. Statistically, your engine is much more likely to crater in the first 400hrs after overhaul than it is when it reaches MFR recommended TBO. (Although, I'd admit its probably going to leak more oil in the latter condition).

V/r,
 
Lowering the bar to reduce problems has become popular. No engine left behind!
 
a borescope of the valves, seats, and cylinder walls is more telling than the compression check. A cheap $50 USB dental camera is worth it's weight in gold. Every aircraft owner who removes a spark plug should own one.
 
Stewartb said:
I have a different opinion. When I've found cylinder compression sagging in a single cylinder of my big bore TCMs it's because I took it to my mechanic complaining that it wasn't running right. The last time I had 5 at 77-78/80 and one at 64/80. TCM can tell you it makes rated power using whatever tolerances they use but that doesn't mean it's running to it's potential and it almost certainly isn't as smoothly as it could be. If guys don't recognize it? I don't know what to say about that.
Click to expand...
Excellent point. Just add more fuel to make power.
 
Just saw a 182 today with one cyl at 60/80, all the others were at 70+. it has about 150 smoh with new cylinders.
can hear it through the oil filler. So indication is problem with rings. That's it. It runs fine, no abnormal oil consumption.
May be a weak, or sticky ring, which under the much higher pressures of running, seals just fine.
 
Silvaire said:
It's a simple leak down test to check the sealing integrity of the valves and piston rings. A leaking valve is not going to fix itself or get better, the leak will continue to erode the seat and valve face. Leaking compression rings will result in blow-by, increased oil consumption and crankcase pressure. The test is performed at a low pressure of 80 psi for safety reasons. This is nowhere near the pressure these components experience in operation and it is unlikely that you would be capable of noticing any performance detriments with even a very low differential compression test result. It's also very unlikely that you would be able to visually detect any internal anomaly with a borescope on a cylinder that had anything over a 0/80 result.
Click to expand...
Compression rings have nothing to do with oil consumption.
 
Valve guides are the biggest wear factor we have that require removal of the cylinder to repair. not rings, valves, or cylinder barrels. worn guides allow the valve to seat wrong and leak, you can't see that with a bore scope. Lycoming has a SB on how to check for valve wobble, TCM does not, but suffers the same wear problem. worn guides are 5 to 1 the major cause of leaking valves and poor compression.
 
Checkout_my_Six said:
Oh....do tell Capernious. :D
Click to expand...
Well why do they list it as a "oil Ring" (with a different P/N) not a compression ring? plus why is it shaped different? Why does it have openings to allow oil to escape back into the cylinder? and why is located below the compression rings?
To any one with a tiny bit of mechanical knowledge It's pretty obvious.
 
Worn rings promote burning oil and losing oil through the breather from blowby. There's no question that oil consumption increases with bad rings. There's also no question that oil consumption may be caused by something other than bad rings.

Just keeping it real.
 
Tom-D said:
Well why do they list it as a "oil Ring" (with a different P/N) not a compression ring? plus why is it shaped different? Why does it have openings to allow oil to escape back into the cylinder? and why is located below the compression rings?
To any one with a tiny bit of mechanical knowledge It's pretty obvious.
Click to expand...
Soo many questions....from someone who knows soo much. :D
 
Last edited:
Stewartb said:
In my world 80/80 is ideal.

...
Click to expand...

So is Miss January. :D

Serviceable works for me. Ideal is for the boyz with the really expensive aluminum (or maybe the plastic fantastic with the red handle).
 
Stewartb said:
Lowering the bar to reduce problems has become popular. No engine left behind!
Click to expand...

That comment pins the BS meter.

We have far better engine monitors installed on more airplanes than ever before. Far better metallurgy than ever before. Far better lubricant technology than ever before. And given the cost of running an aeroplane & fixing engines AND the cost of having an accident today, compared to "the good old days", far more attentive owners and pilots. Sure there are some owner exceptions, but that's what I see going on around my home base.

We also have a lot more information sharing capability, so maybe it's not surprising there are widely varying views in these vigorous online debates?
 
Last edited:
How many engines have you broken in? Were you careful? Maybe even meticulous? Why? I mean, if 60/80 does it for you, why not shoot for that?

My no engine left behind comment referred to TCM in the 90s when they changed the rules about compression. Handy for them since they couldn't make a cylinder that'd maintain compression. Yes, they've gotten better since although they haven't gotten any more of my money. How their acquisition of ECI changes things will be interesting. I always liked ECI but my next engine, coming soon, will come from Superior.
 
Tom-D said:
Valve guides are the biggest wear factor we have that require removal of the cylinder to repair. not rings, valves, or cylinder barrels. worn guides allow the valve to seat wrong and leak, you can't see that with a bore scope. Lycoming has a SB on how to check for valve wobble, TCM does not, but suffers the same wear problem. worn guides are 5 to 1 the major cause of leaking valves and poor compression.
Click to expand...

Way back in 1999 Lycoming changed to a high-chromium content bronze for their valve guides. The guide wear problem decreased dramatically.
 
The bigger problem with worn rings involves all the junk that gets past them into the crankcase, dirtying the oil much faster and causing more corrosion. Water vapor reacts with oil components in the presence of metals and forms some nasty acids. Carbon and lead compounds from combustion make a mess. Sludges form in the crankcase and lifters. The oil oxidizes faster. Good rings help make an engine last longer.

http://www.le-international.com/pdf/068-motor-oil-degradation.pdf
 
Dan Thomas said:
Way back in 1999 Lycoming changed to a high-chromium content bronze for their valve guides. The guide wear problem decreased dramatically.
Click to expand...
Isn't that about the time they changed to half inch valve stems, to lessen the wear?
But still guides are their weak point.
 
Silvaire said:
It's a simple leak down test to check the sealing integrity of the valves and piston rings.
Click to expand...
Finally somebody named it right, thank you, sir.
Yes, this is NOT a compression test like the one that is done on automotive engines by cranking the engine and compressing air in the cylinders.
This is a bad misnomer for a simple leakdown test to show how much air pressure a cylinder can hold and whether the valve seats and rings are seated well or not.
Generally, mechanics call an engine "bad" if the leakdown is less than 60psi (out of 80) but if any of my cylinders shows below 75, I start investigating because something is definitely not right.
 
Is that like a cabin pressure leak down rate check, where the cabin is pressurized to a specific psi then pressurization stopped and you time how long it takes to leak down? Or is it a differential pressure check?
 
GlennAB1 said:
Is that like a cabin pressure leak down rate check, where the cabin is pressurized to a specific psi then pressurization stopped and you time how long it takes to leak down? Or is it a differential pressure check?
Click to expand...
It's a differential pressure test. Air is run through a calibrated orifice in the tester, with a pressure gauge on each side of the orifice, and into the cylinder. The pressure is raised to 80 psi on the upstream gauge and the leakage is read as a pressure reading on the downstream gauge. More leakage results in a lower pressure reading, since the orifice exerts drag on the air as it accelerates.

The mechanic can wiggle the propeller a bit to see if the reading improves, and it often will as the rings move in the lands and the piston cocks a bit. he can listen at the oil filler or breather to see if the leakage is through the rings, or at the intake or exhaust to see if the leakage is through those respective valves.
 
You must log in or register to reply here.
Back
Top