compression check in the 60's

[tazin river]

First compression check after the plane sat all winter (7 months) without running and looks like a couple cylinders read in the mid-60's. Cause for concern? Friend of mine engineer said it is not unusual after sitting so long and said to take the plane up and run it hard for a while and do another compression check after landing.

Cessna 180, O-470 Continental

Makes sense?

[PilotDAR]

I agree. The compression check number are important if you're buying or selling the plane. If you're running it, and it runs smoothly, and develops the expected power, I'd leave it be - particularly Continentals. Running it more will help. Changing out a cylinder has cost and risk, so I have learned over the years to get to know my Continental, note any changes or degradation, and otherwise run it normally.

High compression numbers for an engine which is not running are very nice. A running engine doesn't have time for a compression leak down as it does when it's stopped, so I worry less about compression of the running engine. I like engine scanners, so you can learn what normal is for your engine for more parameters, and then see abnormal should it happen...

[photofly]

Limits for that engine are 40/80, I think. At that point TCM recommends running it again, and checking once more. Mid 60's is pretty good.

[boeingboy]

As the others have said....completely normal for a Continental.

[Heliian]

I would just run it up on the ground first, let cool, then check again.

As long as it's within limits you can let it go.

[ahramin]

What did the boroscope show? Is this a commercial aircraft or private? Commercial you need to follow Continental's instructions, private you should follow them.

[hangarline]

As per the Service Bulletin that "ahramin" attached is the correct method to perform the test. Anything else is not going to give you accurate results. Too many people get hung up on the compression readings of their engine. Remember, it is a number that changes every time you perform it. The readings are only a "snapshot" in time. What you should really look for is a trend of successive readings taken over the course of 25-50 hours of operation if you suspect a problem. The exception would be a static leak (valves). A static leak should be investigated further with a boroscope inspection before returning to service. But again, to repeat the comments of others, a reading in the 60's is quite normal on the 0-470. Fly the aircraft and check again in 10 hours if you are concerned. There is never a limit on how many times you want to perform the compression test.

[tazin river]

Thanks folks

Very helpful as usual.

[2R]

Might be the oil . Change the oil . Run it up .
If you make static RPM . You have enough power for some circuits .
Check it again after a few hours , if they are all about the same gtg . If you get one low and the rest high . Try break in oil for an hour ground run . It is more important to have a close match for long term balance and wear.
Oil degrades . Some large engines require careful viscosity checks and oil cleaning .

[Its What I do]

i bet multi oil is being used .....

i would change oil to straight weight and also de - sludge the cylinders as the leakage is most likely the rings .


good luck

[Its What I do]

i bet multi oil is being used .....

i would change oil to straight weight and also de - sludge the cylinders as the leakage is most likely the rings .


good luck

[PilotDAR]

I only use straight grade oil in Continentals.

[2R]

If you group buy the oil by the case . It is cheap .
Do not be like those guys who buy one quart at a time and change oil once a year because they overpay for oil so it must be good , cause they bought the best .

Buy it by the case and change the oil like you change your socks or underwear . It it changes colour or starts to smell bad time to change them !!!!

Oil is cheap bought by the case

[Big Pistons Forever]

As hangarline noted the absolute value of a compression check is almost useless unless it is extremely low. The trend in compression checks is what you want to know.

Also where is the air going. If you are getting significant air through the exhaust valve ( ie you can hear air coming out of the exhaust pipe when the cylinder is pressurized) you should get a boroscope and have a look at the exhaust valve. If it is going by the rings ( ie coming out the breather) then no immediate cause for concern. Run it for 25 hrs or so and try another check

Continental once set up an engine with the rings ground undersized so that all cylinders had effectively zero over eighty on a DP check. The engine still made 95 % of full power, although with very high oil consumption.

As a general comment my experience with the 0 470 Continentals is the cylinders often seem to quickly go to the mid 60's and then stay there, I have no idea why.

On a related note; I carefully log oil consumption. A sudden increase in oil consumption is almost always an indication something bad has happened to your engine, and needs immediate investigation

[photofly]

Continental once set up an engine with the rings ground undersized so that all cylinders had effectively zero over eighty on a DP check. The engine still made 95 % of full power, although with very high oil consumption.

I have heard this many times; and while I have no reason to doubt it's true, does anyone have a reliable reference for it? I would very much like to read the source material.

[iflyforpie]

Yep.

Nothing to worry about and pretty typical for a big bore continental.

Pulling a jug to fix low compression IMHO introduces more risk than just living with the low numbers, which are at much lower pressures and speeds than an actual compression and combustion event.

The only time it gets serious is if its excessively low (below 40/80) or is associated with high oil consumption or an exhaust leak (borescope the exhaust valve for telltale signs of discoloration and imminent failure).

[boeingboy]

I have heard this many times; and while I have no reason to doubt it's true, does anyone have a reliable reference for it? I would very much like to read the source material.

I've herd this as well....and while I can't find a specific article on that test - this is mentioned in official continental information....

Engine testing has shown TCM engines continue to meet the certified
horsepower ratings even when all cylinders are at or below the minimum
allowable calibrated cylinder differential pressure reading established by the
master orifice tool.

Essentially 0 compression.

From page 58 TCM tips on engine care. A very good read.
http://www.insightavionics.com/pdf%20fi ... nental.pdf

[tazin river]

Thanks again folks

What type of straight oil would you recommend?

[photofly]

I have heard this many times; and while I have no reason to doubt it's true, does anyone have a reliable reference for it? I would very much like to read the source material.

I've herd this as well....and while I can't find a specific article on that test - this is mentioned in official continental information....

Engine testing has shown TCM engines continue to meet the certified
horsepower ratings even when all cylinders are at or below the minimum
allowable calibrated cylinder differential pressure reading established by the
master orifice tool.

Essentially 0 compression.

Minimum allowable for TCM engine is 40/80, no?

[boeingboy]

Minimum allowable for TCM engine is 40/80, no?

That's a flat cylinder by far. Most days around 47 is 0 compression using the master orifice tool.