Thoughts on compression

[Joe Larussa]

So plane is in for second annual after engine overhaul. Compressions last year 77,77,77,79.

This year 70,73,73,75. I have put about 240 hours since overhaul. I'm thinking what the hell.

Did a boroscope and found a little bit of oil. Valves were perfect. Oil comsumption totally normal.  Never a fouled plug. Thoughts?

[N201MKTurbo]

Let it ride...

[carusoam]

1) How many hours this year?

Asked As if you stopped flying and had rust form on the cylinder walls...  probably unlikely.

2) It is possible to get rings to align, making it easier for air to escape...  unlikely to happen, much less on all four cylinders at the same time.

3) Was the compression test performed the same way by the same shop?  Using the proper orifice might be a challenge... Can you ask your mechanic?

4) Since they all behaved the same, there isn't something standing out as needing service...(?)

5) oil isn't being used up at a high rate or soaking the lower plugs. Typical of rusty cylinders and worn rings.

6) Lycoming compression tests are usually very consistent and reliable.  Continental compression tests are hard to get good results from in comparison...

7) What is the lower acceptable limit for compression for your engine? Above the limit, good to fly(?)

8) Is the oil dark colored, and smell like exhaust? Other signs of broken or worn rings...

PP thoughts only, outlining questions you may want to ask your mechanic... Did you have a discussion with the mechanic?

Best regards,

-a-

[marky_24]

Hot vs cold engine when the compression check was done? 

[StevenL757]

How many hours between oil changes?  LOP or ROP operations?  How long are most of your flights?  With those compression numbers alone, I'm not seeing where you think you have a problem on a Lycoming with ~120 hours per year since overhaul 2 years ago.  Agree with Rich...keep flying the airplane.  It's a non-issue.

Steve

[jetdriven]

Dont pull any cylinders off. Look at the valves with the dental camera, if no asymmetrical deposits, fly it. If low compression go do a high power runup or even better fly it really hard for an hour, then recheck.

[Mooneymite]

Where is the air going?

Past the rings, or valves?

[ArtVandelay]

Where is the air going?
Past the rings, or valves?

If past the valves, it will come out the exhaust or intake, if past the rings it should come out the crankcase breather hose, your mechanic should be able to tell you. Frankly unless it gets close to 60 I would not worry about it. Did you break it in as recommended?

[Joe Larussa]

11 hours ago, carusoam said:

1) How many hours this year?

Asked As if you stopped flying and had rust form on the cylinder walls...  probably unlikely.

2) It is possible to get rings to align, making it easier for air to escape...  unlikely to happen, much less on all four cylinders at the same time.

3) Was the compression test performed the same way by the same shop?  Using the proper orifice might be a challenge... Can you ask your mechanic?

4) Since they all behaved the same, there isn't something standing out as needing service...(?)

5) oil isn't being used up at a high rate or soaking the lower plugs. Typical of rusty cylinders and worn rings.

6) Lycoming compression tests are usually very consistent and reliable.  Continental compression tests are hard to get good results from in comparison...

7) What is the lower acceptable limit for compression for your engine? Above the limit, good to fly(?)

8) Is the oil dark colored, and smell like exhaust? Other signs of broken or worn rings...

PP thoughts only, outlining questions you may want to ask your mechanic... Did you have a discussion with the mechanic?

Best regards,

-a-

Put about 110 hours on it this year. Same shop doing test but different guy. He told me about the way he does it

which seems a little different than Iv'e seen in the past. This plane never sits for more than a week. Average flights

from one to three hours at a time.

[Joe Larussa]

11 hours ago, marky_24 said:

Hot vs cold engine when the compression check was done? 

Engine was hot

[Joe Larussa]

11 hours ago, jetdriven said:

Dont pull any cylinders off. Look at the valves with the dental camera, if no asymmetrical deposits, fly it. If low compression go do a high power runup or even better fly it really hard for an hour, then recheck.

Valves were in great shape. My mechanic boroscoped and mentioned seeing a little oil. I'm used to seeing higher

numbers in previous engines like my old 320 E2D which after overhaul were always around 78-79

[Joe Larussa]

7 hours ago, teejayevans said:

If past the valves, it will come out the exhaust or intake, if past the rings it should come out the crankcase breather hose, your mechanic should be able to tell you. Frankly unless it gets close to 60 I would not worry about it. Did you break it in as recommended?

Yes did do the recommended break in. Certainly always finding a little oil on the floor

coming out the breather but thought that was normal.

[Joe Larussa]

7 hours ago, teejayevans said:

If past the valves, it will come out the exhaust or intake, if past the rings it should come out the crankcase breather hose, your mechanic should be able to tell you. Frankly unless it gets close to 60 I would not worry about it. Did you break it in as recommended?

Not that I'm worried about it but after spending 35,000 on this two years ago I'm concerned.

[orionflt]

doesn't sound like anything to worry about, retest it later. technique and equipment calibration can give differnt readings

Brian

 

[Mooneymite]

Compression tests can be very fickle. How the compression test is done, the equipment used, how hot/cold and the alignment of the rings at any particular time can all affect the readings. 

Since yours are all still pretty good and generally uniform readings, there's nothing to do but note them and consider the trend in susequent compression checks. 

[Joe Larussa]

Yeah I requested a re-test after annual is done and to be by a different tech just to see.

[Mooneymite]

In our community we have several "loaner compression gauges".  We joke about "buyer" and "seller" gauges.  The buyer gauges supposedly read ridiculously low; the seller gauges, high.

Ask your mechanic which one he's using.  

[Joe Larussa]

2 minutes ago, Mooneymite said:

In our community we have several "loaner compression gauges".  We joke about "buyer" and "seller" gauges.  The buyer gauges supposedly read ridiculously low; the seller gauges, high.

Ask your mechanic which one he's using.  

Hahahahah

[Oldguy]

Have had various compression tests done with anywhere from 5-10 different depending on hot/cold/phase of the moon or whatever. Until they get to be consistently in an area my mechanic considers unhealthy, valves do not look good, and/or some other condition occurs leaving me with his recommendation to IRAN it, I run it. It makes good power, no significant oil leaks/usage change and currently with 1600+ hours on it.

[Joe Larussa]

1 minute ago, Oldguy said:

Have had various compression tests done with anywhere from 5-10 different depending on hot/cold/phase of the moon or whatever. Until they get to be consistently in an area my mechanic considers unhealthy, valves do not look good, and/or some other condition occurs leaving me with his recommendation to IRAN it, I run it. It makes good power, no significant oil leaks/usage change and currently with 1600+ hours on it.

I agree with you there. I'm just thinking my engine was overhauled 2 years ago and has somewhat of a warranty on it.

Hate to wait and see and have repairs done on my dime. Love being an airplane owner!

[peevee]

compression good, no compression bad. That's all I know

 

I wouldn't sweat in the 70s at all.

[Hank]

1 hour ago, peevee said:

compression good, no compression bad. That's all I know

 

I wouldn't sweat in the 70s at all.

Yep!

I've been using a "smaller" mechanic the last couple of years. This year, checking compression lifted his feet off the ground . . . So I'm happy with it.  

[Joe Larussa]

Just a follow up. Annual completed and I asked for the compression check to be done a second time by different mechanic. 75,77,77,78

What do you know about that!

[Mooneymite]

Some mechanics find TDC, apply air pressure and write down the reading.

Other mechanics "rock the prop" to get the highest possible reading.

The different techniques can yield significantly different readings.

[Tommy]

The compression test is very subjective. 

There are times when I swore that it reads 77 but my mechanics said it's 76.