Cylinder compressions

[sternone]

Hello, can somebody explain me please what to look for when they give data about cylinder compressions ?

For example i have following test report, is that good/bad ?

Cylinder Compressions: Cyl. #1: 68/80, #2: 68/80, #3: 60/80, # 4: 78/80, #5: 80/80, #6: 74/80

Thanks

[SkyHawk-N]

Those figures are from a differential compression check. The first figure is Out, the second figure is In, the difference between the numbers is the amount of 'leakage' between the piston, rings, cylinder, valves, etc.

60/80 seems quite low. Is it a Continental?

Here is an informative link...
http://www.sacskyranch.com/eng91.htm

[sternone]

Thanks for the informative link, it's a TCM IO-520-BB

[Zulu Alpha]

Cylinder 4 and 5 look OK, the others look like there is a lot of leakage, either valves and/or rings.

Does it use oil? If so its probably rings. If it hisses from the exhaust/inlet when doing the compression test it is probably valves.

One thing to check is whether it was a cold or warm engine. A cold dry engine can sometimes give low numbers.

[javelin]

Use compression tests with great care. As mentioned above there can be large variations between readings. A much better indication is oil useage, 'eagerness' of the engine, any changes to normal operations.

I could probably change a set of readings with about 30 minutes work on an engine................. not convinced :sad:

[JinjaNinja]

And 80/80 is a touch enthusiastic! It will leak past the rings given time. My rule of thumb is anything over 70 is fine when cold! Obviously when hot they should be "tighter".

Hissing out of the airbox or exhaust stack tends to tell you where to look!


JN

[SNS3Guppy]

Cylinder tests are a diagnostic tool, but the average layman puts far too much stock in the results. The same engine given three different differential compression tests, will produce three different results. Change mechanics performing the tests, you'll get different results. Change the temperature, you'll get different results. Change the test set, you'll get different results. The specific numbers aren't particularly meaningful, though typically you should see 75% of the input pressure as a general guideline.

When I perform a differential compression test, I'm more interested in hearing where air is leaking, than what the actual numbers are, and only remotely interested in the number trends. It's not uncommon for the compression numbers to increase with subsequent tests later on.

Ring position has no real significance, though some seem to think so.

Compression testing can revel ring blowby and cylidner wear, but is a useful tool for targeting valve seating issues.

You don't need to interpret numbers you're seeing. What you've posted isn't bad, but to make any kind of judgement on those numbers alone would be pointless. What you have there is a very small piece of a much larger puzzle.

Hopefully you're doing spectrometric oil analysis with every change, and doing regular oil changes, checking mag timing and internal timing, looking for induction leaks, boroscoping the engine, flowing injectors, etc.

Oil color and consumption is a very poor indication of engine health. A healthy engine can be consuming a considerable amount of oil.

[Them thar hills]

Re the sagging compression scenario, if this particular engine happens to be driving a fixed pitch prop, it's worth comparing present static rpm with the "just run in" static rpm assuming the prop is the same one. 100 rpm is a lot to lose, and the engine power curve will tell how much. :)

[411A]

it's a TCM IO-520-BB
An extremely reliable engine.
How so?
Well lets see, these engines were used on re-engined deHavilland Herons operated by several small airlines in the past, and the maximum FAA allowed TBO was...yes it is true...4000 hours.
I kid you not.
Now then, as regards compression testing with these engines, it is vital that the required master oriface tool be used, as if it is not, the compressions found have absolutely no reliability whatsoever.
Further, TCM now recommends boroscope inspection, and this is a highly effective tool to determine the condition of your engine's cylinders.
Note: I have used bold type for a reason, as it is imperative that the proper procedures/tools are used.
And yes, I personally own an aeroplane with TCM engines...they are very reliable, if looked after properly.

[ericferret]

A few years ago I carried out a compression test on an Lycoming 0-320 fitted to an R22 which had just flown in with no reported defects.

0/80, 0/80 0/80, 0/80

Obviously I was misusing the equipment so I tried again, more than once, same result.

So I got someone else to do the test, same result.

Then we tried another compression tester, same result.

We removed the exhaust and watched the valves moving through the ports.

The guides were so badly worn you could watch the valves jumping up onto their seats.

The only conclusion we could draw was that under power the inertia of the valve was keeping it where it should be, but on a static test the wear was obvious.

The best thing was the irate owner who could see no reason to work on the engine as it still had a 100 hours to go to overhaul!!!!!!!!!!!

[SNS3Guppy]

Re the sagging compression scenario, if this particular engine happens to be driving a fixed pitch prop, it's worth comparing present static rpm with the "just run in" static rpm assuming the prop is the same one. 100 rpm is a lot to lose, and the engine power curve will tell how much.


Part of every post-work inspection engine run by a mechanic includes checking the RPM limits prescribed in the type certificate data sheets (certification paperwork) for the airplane. However, RPM gauges in aircraft are notoriously inaccurate, and you can only truly check this is you use a separate meter to verify the actual RPM. Typically today a photometer or stroboscopic RPM meter is used. If you don't have one, you can't truly tell the RPM being produced by the engine under given operating conditions.

Now then, as regards compression testing with these engines, it is vital that the required master oriface tool be used, as if it is not, the compressions found have absolutely no reliability whatsoever.


TCM prescribes a calibrated orfice, but one must use the specific orfice. Broadly speaking, differential compression testers are produced for large bore engines, and small engines. Some manufacturers also produce interchangable orfices appropriate to the leakdown of the particular size engine and specifically one for TCM engines.

Not using the calibrated orfice does not mean the test does not have validity, and certainly doesn't mean that it's not reliable; it simply means one isn't using the tools called out in the manufacturer documentation...which epending on the inspection bing performed may negate the use of the test to fulfill inspection requirements.