Talk to me about compression checks

labbadabba

labbadabba

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What are they really?
What do they tell us or don't tell us?
Why is it measured on a scale of 80?
When are checks done and by whom?

I have a general idea, but I don't know exactly how or why they function.
 
FWIW, they are not really a compression check. Somebody clueless started using an incorrect name and unfortunately it stuck.
It is merely a leakdown test that tells you that if you try hard enough, you might find ONE spot on the cylinder walls where the rings seal decently enough to hold some air pressure. :)

So if you want to know for real how much air your engine can compress, you need a rotation compression test. But be careful, most aircraft mechanics will try to crucify you if you even mention it to them. ;)
 
There are three places you can be losing compression. Past the rings, out the exhaust valve, or out the intake valve. The differential compression check basically tells you if a cylinder is holding acceptable compression and if not, hjust where it is leaking. If the exhaust valve is leaking, you can here it at the exhaust stack. If the intake valve is leaking, you will here it through the carb air box. If it is going past the rings you hear it through the crank case breather tube.

The compression test really is misnamed. It is really just a diagnostic tool to check integrity of the valves and rings.
 
neilw2 said:
All I know about compression checks is not to stand in front of the propeller when @Tom-D is doing them.


Sorry, totally unwarranted but I couldn't help myself. :D
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Does it make any sense to stand in the prop arc at any time?
 
BigBadLou said:
FWIW, they are not really a compression check. Somebody clueless started using an incorrect name and unfortunately it stuck.
It is merely a leakdown test that tells you that if you try hard enough, you might find ONE spot on the cylinder walls where the rings seal decently enough to hold some air pressure. :)

So if you want to know for real how much air your engine can compress, you need a rotation compression test. But be careful, most aircraft mechanics will try to crucify you if you even mention it to them. ;)
Click to expand...
How would you correlate the readings of one type against the other?
 
Can anyone tell me why we use an industry standard?
 
Greg Bockelman said:
In Tom's defense, his system works just fine for him if no one else is around.
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It actually works then,, as long as you don't grab the prop and move off TDC, with out warning.
 
when is the best time to do a compression check? hot or cold?
 
Compression tests are also helpful in identifying cylinder head cracks and separations. In my own history they've identified leaking exhaust valves. As for the prop moving? Mechanics are expected to rock the prop back and forth to find the sweet spot for maintaining cylinder pressure. Don't stand in the prop arc and don't let go.
 
CAPTAIN I CANNOT DUIT

THA DALITHIUM CRYSTALS, THERRRRE GONNA BLOWWW
 
If prebuy and he is wiggling the prop to get a good reading, it's kinda hinky. Rule 1 on prebuy. Owner is NOT ALLOWED in shop during leakdown (compression) test. Really should bring your own meter and hose and hookups etc. Some mechanics have "prebuy calibrated" meters.
 
BigBadLou said:
FWIW, they are not really a compression check. Somebody clueless started using an incorrect name and unfortunately it stuck.
It is merely a leakdown test that tells you that if you try hard enough, you might find ONE spot on the cylinder walls where the rings seal decently enough to hold some air pressure. :)

So if you want to know for real how much air your engine can compress, you need a rotation compression test. But be careful, most aircraft mechanics will try to crucify you if you even mention it to them. ;)
Click to expand...

A leakdown test is something else entirely. Pressure is put in and the air shut off; the time it takes for the pressure to fall a specified amount is the indication of sealing. More time is better. Not used on engines in aircraft shops.

The compression test done on auto engines involves turning the crankshaft with the starter and measuring the highest pressure obtainable in a given cylinder. Too dangerous for airplanes and not at all necessary.

The test we use is the differential compression test. Air is injected into the cylinder, through a calibrated orifice, and the pressure upstream and downstream of the orifice is measured. 80 psi is used as a standard and the tester is adjusted to get that at the upstream gauge. The downstream gauge gives the health of the rings and/or valves in the cylinder, and a number as close to 80 as possible is desired. The thing works on the principle that moving air has a lower static pressure than stationary air; the orifice, in restricting the flow, causes a pressure drop that increases with the volume flowing through it.

The big advantage with the differential test, besides being a lot safer than spinning the crankshaft, is that one can listen at the oil filler, exhaust, and intake to see where the leaks are. Can't do that with an automobile-engine type test.
 
Dan Thomas said:
A leakdown test is something else entirely. Pressure is put in and the air shut off; the time it takes for the pressure to fall a specified amount is the indication of sealing. More time is better. Not used on engines in aircraft shops.

The compression test done on auto engines involves turning the crankshaft with the starter and measuring the highest pressure obtainable in a given cylinder. Too dangerous for airplanes and not at all necessary.

The test we use is the differential compression test. Air is injected into the cylinder, through a calibrated orifice, and the pressure upstream and downstream of the orifice is measured. 80 psi is used as a standard and the tester is adjusted to get that at the upstream gauge. The downstream gauge gives the health of the rings and/or valves in the cylinder, and a number as close to 80 as possible is desired. The thing works on the principle that moving air has a lower static pressure than stationary air; the orifice, in restricting the flow, causes a pressure drop that increases with the volume flowing through it.

The big advantage with the differential test, besides being a lot safer than spinning the crankshaft, is that one can listen at the oil filler, exhaust, and intake to see where the leaks are. Can't do that with an automobile-engine type test.
Click to expand...

That's a perfect description of an automotive leakdown test, except the pressure is 100 PSI instead of 80. Presumably the orifice is different, too. Automotive leakdown is usually stated in percent, but if you're using 100 PSI like you should, 1 PSI = 1 %. It's a very sensitive test. And the crank kicking over is irritating ('cause you gotta do it again), but not dangerous unless you left a ratchet on the end of the crank, and some body part is in the way.

You do it for all the same reasons, and you get one more benefit not usually needed for airplanes -- you can see if any of the air makes it into the radiator, to test the integrity of the head gasket and water jacket.
 
Tom-D said:
Can anyone tell me why we use an industry standard?
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Someone has probably posted this already, but here goes:
The 80psi standard came about due to the nessessity of holding the engine at TDC, by holding the prop. Figuring it to be less dangerous than say 120psi. Rocking the prop while cylinder is under compression at or near tdc, can have severe concequenses with pressures much higher than 80.
But 80 will still cause pain, and agony, if you let it get away.
 
ircphoenix said:
7:48am (local) after running the engine for precisely 23 minutes.
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when does the engine exhibit morning sickness? hot or cold ? After the engine is warmed up can you find a stuck valve, that only sticks when it is cold?
When you find a cylinder that is 60/80 when cold but 70/80 when hot is it still sick?
 
I really hate to tell you guys that you really do not have to hold that blade. set it correctly, get away from it, and stay away from it.
 
Trivia question.. who determined 80 was enough to be the standard?
 
Tom-D said:
I really hate to tell you guys that you really do not have to hold that blade. set it correctly, get away from it, and stay away from it.
Click to expand...
yup....keep the customer away from that thingy....:eek:
 
The Compression test that is normally excepted is NOT I repeat NOT a leak down test, It is a differential pressure test, that is why we call it a DIFFERENTAL compression test. Funny how that works.
 
Pumping the blade back and forth to get a better reading is just another way to pass a bad cylinder
 
Way back when we had real engine mechanics, we preformed a compression test by inserting a bang gauge in the front row of plugs and actually ran the engine on the back row of plugs and read the actual BMEP of each cylinder.
 
The reason they use 80 PSI is because it's just enough to crack your head open and not enough to knock it clean off.
 
Except you have to accept the fact that exceptions are commonly acceptable exceptions. However, some exceptions are actually exemptions.
 
retpd2001 said:
Lycoming says hot. Here is a You Tube video my A&P IA made.
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Lycoming would like you to have the highest reading possible. When in reality you could be experiencing morning sickness when the engine is cold.
 
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