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Rotax 912UL - compression test

in 912 / 914 Technical Questions 10 years ago
Hi. Today I made compression test by single gauge on my 912UL (80hp).
And my reading's was 120/125/120/135 PSI.
In MML 12-20-00 page 15 i can read "The compression should be between 9 and 12 bar (130 and 174 psi)."

so my is under this range but later I can read :
"Individual readings for the cylinder must not differ by more than 2 bar (29 psi).
If the value is below 6 bar (87 psi), inspection, repair or overhaul must be carried out in accordance with the BRP-Powertrain instructions for contin- ued airworthiness."

So my (120psi) is over this limit.

Do I have a problem? My engine has 800hrs and I'm not sure it is normal wear or not. Engine work's fine and when I turning prop by hand I can feel better compression than in many other planes.

I'm not sure if its important but during test throttle was closed.

  • Re: Rotax 912UL - compression test

    by » 10 years ago

    Hi Przemysław,

    Your numbers may be okay. It may be a gauge inaccuracy. Was the engine hot or cold? Sometimes you can change the numbers by rotating the prop a few times.

    I rarely use a static pressure test. I prefer a differential leak down test. I think you will find this more useful and easier to interpret the results. Give it a shot and see what you come up with. My money says you'll most likely be okay.
    Try a differential test. You can use any where from 80-87 psi as the test pressure. If you use 80 psi most of the time the differential is around 76-78 psi and if you use 87 psi for a test pressure then most results are around 82-85 psi.

    I have seen in some logbooks a test pressure of 80-87 psi and the results are the same as the test pressure. The system is not 100% air tight so that can't be so the inspector probably needs some new gauges.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Rotax 912UL - compression test

    by » 10 years ago

    Uhm. I haven't differential test set... It was tested as in manual (engine warmed up to min. 50*C).
    I'll try to build diff test set...

    I made this test before cylinder head 3 removal. I sent it to rotax for replacement of pushroad covers (I had small leakage where tubes are glued to head).

    I make next test when it will come back.


    What does mean these numbers from MML. If I have less than 130 but more than 87 engine is broken or not? What about range between 87 and 130 PSI. It wasn't described.
      
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  • Re: Rotax 912UL - compression test

    by » 10 years ago

    I'll try to build this equipment:
    Zrzutekranu2014-09-15o08.33.46.png (You do not have access to download this file.)
      
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  • Re: Rotax 912UL - compression test

    by » 10 years ago

    Its not so simple Przemyslaw, You need the correct angle (60 degree) in the restrictor: AC 41.13-1B states that the restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:
    Engines with less than a 5” bore: 0.040-inch/1mm orifice diameter, 0.250”/3mm long, 60-degree approach angle.
    You will spend a lot of time finding the parts and making the tester; it is much easier to buy from Aircraft spruce; Only $88. Part # 12-00693
    Normally the differential compression is taken at 80psi and the drop is only 2 psi (78/80). I start to worry and look for problems if it goes below 70/80.
    Rob
      
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  • Re: Rotax 912UL - compression test

    by » 10 years ago

    Hi, you mean I need to make 60 degrees as on picture?


    I'll think to do this by myself because in Europe test set is more than 100EUR and I'll need this probably one time.
    Good clocks and turning service is cheap.
    Untitled-1.jpg (You do not have access to download this file.)
      
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