Octane for 912 ULS

Category: General Tech Discussion 5 years ago
While 91 octane is recommended, I have been flying with unleaded, non-ethanol, 89 octane Chevron mogas for the past year and a half. The engine runs smoothly at all rpms and seems to love it, with no noticeable adverse effects, whatsoever--certainly no indications of knocking ( i.e., pre-ignition ) at any power setting. I cruise at 5,200 rpms. The oil seems as clean as a whistle for extended periods, too.

....anyway, does anyone have insight on whether I should not continue to operate with this fuel ??
  • Re: Octane for 912 ULS

    by » 5 years ago


    Hi Scott,

    You could easily get away with that 89 oct., but some day you will experience just the right circumstances that will cause detonation. It will happen so fast that when you figure something isn't right in the cockpit it will be $4500 too late. It will happen fast and you'll never know when that special moment will come. So long as your fuel tanks are good with the ethanol the engine is fine with it. It is better to use 91 with ethanol than using 89 without.

    I would highly recommend going back to 91+ oct. before you have your special moment.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-574-1080 Home (TRY HOME FIRST)
    520-349-7056 Cell

    Thank you said by: Scott Church

  • Re: Octane for 912 ULS

    by » 5 years ago


    I did an octane calculation based on Rotax 912 compression ratio of 9:1 and extrapolated that to 10.5:1 for 912S. The 912 is approved to use 87 Oct mogas at sea level and I concluded that you would develop the same internal pressure in the 912S prior to combustion if the air density was above 3000'. So in theory, if all your flying was always done above 3000', you should be able to use the lower octane. However, should you land at a lower place near sea level and takeoff with the 87 oct in tanks, it could detonate. It's safer to use the higher octane fuel and avoid bad problems.

    Thank you said by: Scott Church

  • Re: Octane for 912 ULS

    by » 5 years ago


    Thanks for the computations for 87 octane. However, I am using 89 octane, which your computations would seem to indicate is fine. My pressure density altitude, by the way, is almost always over 1,000ft.

    ( the same 89 Octane with 10% ethanol is rated at 91 octane, as the ethanol raises the octane 2-3 pts ).

    So,

  • Re: Octane for 912 ULS

    by » 5 years ago


    clean, quality, 91 octane for my 912 uls and me . and preferably no ethanol .

    Thank you said by: Scott Church

  • Re: Octane for 912 ULS

    by » 5 years ago


    The 912ULS has 11:1 compression ratio.Ill let Bob do the math..

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