Hi all,

Has anyone done any dyno/power tests comparing avgas to mogas in the same engine/application? I have been running my 912S on shell v-power (91 octane and no ethanol), but i have just read a UK gyro article that suggests using 100LL in the warmer months for increased power.

I have never felt my 912 to be underpowered, but of course flying with big friends on warm humid days, a little extra puff would be appreciated.

Just wondered if anyone had any experience with this to help determine whether using 100LL would give enough extra puff to justify the increase in cost and oil changes.


  • Re: avgas v mogas

    by » 11 years ago

    I also burn the shell premium 91 octane unleaded, I don't know about 100ll producing more power but it does have lead build up issues, even when using a lead scavenger additive and changing the oil at 25 hrs. I think you are currently are burning the best of the best for the 912 uls . I only use 100ll if I can't get premium unleaded mogas at the airports I fly to. I also use a semisynthetic- oil aeroshell sport plus4 , just because of the occasional 100ll burn , otherwise I would use a full synthetic if I could guarantee the use of only 91 octane mogas.

  • Re: avgas v mogas

    by » 11 years ago

    HI Kevin,

    I also only use 100LL when away for a top up and thought I was doing ok. But the comment relates to temperature, a factor I would not have considered.

    The info (clip now attached) was contained in a leaflet issued by the CAA and the British Rotorcraft Assn. Both respectable organisations. It was very specific and was clearly inserted for a reason. I would like to think these reasons were well researched.

    I have just heard back from the BRA that this comment/practice is intended to reduce the risk of a vapour lock, caused by winter blend fuel warming up and vaporising in the system. Apparently a number of incidents have been caused by this. If you fly a lot then this is probably not a concern, but valuable to know.

    clip.jpg (You do not have access to download this file.)

  • Re: avgas v mogas

    by » 11 years ago

    Hi Kevin

    I'm no expert what so ever, but I would suggest that if it did produce more power, it would not be noticeable. I can't see it making any change to performance, and yet you would have all the lead contamination issues which can be quite bad.

    Stick to the fuel you are using.


  • Re: avgas v mogas

    by » 11 years ago

    I'm with Mark,

    If there is any change it would be un-noticeable. As far as vapor lock I hear people worry about it all the time, but we fly in the Southwest US with temps over 100F and we don't have issues. Certainly if you get anything hot enough it will produce vapors. This is one very good reason to make sure your aircraft has a re-circulation line. Except that 100LL will resist detonation a little better that 91 Oct. won't vaporize quite as soon there isn't any thing good about 100LL in our 912 engine. To me it just trashes it over time.

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

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