Two out of four.

[Mcstealth]

The Darter I am training in went in for the 100hr inspection. Two cylinders have compression in the 40's. 

My questions are; how could I not feel something, anything, that would clue me off that 50% of my engine is not pumping right?

Shouldn't I have noted increased oil consumption, or something to do with the oil?

Grrrrr

 

[N201MKTurbo]

Engines with poor compressions often make full power, or close to it. 

[Niko182]

Last annual i had a cylinder that came back with 0/80. They tested it 4 times and each time it came back with the same result. If i hadn't brought it in, i wouldve just kept flying it, and wouldve never known.

[0TreeLemur]

58 minutes ago, Mcstealth said:

The Darter I am training in went in for the 100hr inspection. Two cylinders have compression in the 40's. 

My questions are; how could I not feel something, anything, that would clue me off that 50% of my engine is not pumping right?

Shouldn't I have noted increased oil consumption, or something to do with the oil?

Grrrrr

 

Did you re-run the engine and re-test?  If not, do it.   Sometimes ring gaps align, resulting in temporary low compression.   If you did re-run and you still have low compression, {m sorry for your loss.

[LANCECASPER]

1 hour ago, Mcstealth said:

The Darter I am training in went in for the 100hr inspection. Two cylinders have compression in the 40's. 

My questions are; how could I not feel something, anything, that would clue me off that 50% of my engine is not pumping right?

Shouldn't I have noted increased oil consumption, or something to do with the oil?

Grrrrr

 

I know this is going to be controversial on here, but it might be worth a try: 

 

[EricJ]

2 hours ago, 0TreeLemur said:

Did you re-run the engine and re-test?  If not, do it.   Sometimes ring gaps align, resulting in temporary low compression.   If you did re-run and you still have low compression, {m sorry for your loss.

This works sometimes.    And sometimes if you just fiddle with it a little bit whatever is leaking will seat and suddenly make compression.    e.g., sometimes just wiggling the prop around, especially pulling it to where the rings are sitting on an upstroke, so they seat against the bottoms of the lands, makes the difference.    This has to be done carefully letting the compression off and retesting, or very small movements at TDC, to keep the prop path safe.

Leak-down compression tests are highly subject to differences due to operators, etc.   It's good to not make a big decision based on a single test.

[0TreeLemur]

14 hours ago, EricJ said:

This works sometimes.    And sometimes if you just fiddle with it a little bit whatever is leaking will seat and suddenly make compression.    e.g., sometimes just wiggling the prop around, especially pulling it to where the rings are sitting on an upstroke, so they seat against the bottoms of the lands, makes the difference.    This has to be done carefully letting the compression off and retesting, or very small movements at TDC, to keep the prop path safe.

Leak-down compression tests are highly subject to differences due to operators, etc.   It's good to not make a big decision based on a single test.

Great points @EricJ.  During the tests with low compression, can you hear anything in the exhaust?   Have they scoped the cyls to inspect the condition of the valves?

[Mcstealth]

Good question. I will ask. Thanks. 

[GeeBee]

Continental did a test with the ring gaps all filed out so there was zero compression on all six cylinders. The engine turned up full rated horsepower. As long as it turns up full power, you won't notice anything wrong with a low cylinder.

 

[carusoam]

Which engine does the Darter have in it?

Compression tests are different for each manufacturer…

Proper orifice is required for accuracy…

 

So…. If the compression tests are correct, and you have bad compressions….

This is where the dental camera comes into play….

Looking for things like cross hatching on the cylinder walls and valve pizzas….

If the rings are failing…. Be on the look out for signs of blow by… dirty bubbly smelly oil….

PP thoughts as usual…

-a-

[Mcstealth]

On 5/26/2022 at 3:23 PM, carusoam said:

Which engine does the Darter have

I guess I should know exactly,  which I don't, but it is the 150hp Lycoming. 

[carusoam]

11 hours ago, Mcstealth said:

I guess I should know exactly,  which I don't, but it is the 150hp Lycoming. 

That’s the important part…

the Lycoming cylinder compressions are more meaningful than Continental… Continental can more easily give low numbers… randomly.

Time to find out what isn’t right….

unless re-taking the compression tests to make sure they are correctly reporting a problem…?

An example… compressions on a cold cylinder can give funky results…. Once around the pattern is the normal step before the compression test…

PP thoughts only…

Best regards,

-a-

 

[A64Pilot]

To test compression we pressurize the cylinder to 80 PSI, that’s not much to an engine, higher pressures will force the rings to the cylinder and help the seal and then there is the fact that at 2400 RPM it’s going through 20 compression strokes per sec 1/20 of a second is a real short interval for something to leak out. real low compression usually shows up as starting difficulties.

But I bet with the new jugs you will notice some differences

2400 Div by 2 to get 1200 compression strokes per min, div by 60 to get per second. I spelled it out as I often make math errors. 20 per sec seemed high

Edited June 2, 2022 by A64Pilot