100LL-Avgas versus MoGas-Autogas

Hello all. Quite an informative discussion on AVGAS and MoGAs. I have been facing an issue with my Rotax Engine 912 ULS fitted in Pipistrel Virus SW80. Hope you can guide. We had to switch over to AVGAS from MoGAS due to ethanol blending at 20% as per govt rules. Ever since the switch over , the max Rpm on ground has reduced drastically by over 200-250RPM. The Max RPM WOT with AVGAS is around 5150 compared to WOT 5300-5400RPM with MoGas. Also, the climb in CHT during taxing increases so rapidly with AVGAS, during take-off I am very close to the Max CHT limit. This results in further RPM drop and the RPM at take off roll barely reaching 500RPM. This drop in max RPM has happened only after switching from MoGAS to AVGAS.

Any thoughts as to why this is happening. The OAT is usually between 23-29 degree Celsius. 

Hi SSN...good observation. 

Indeed higher octane fuel may react this way depending on  your prop loading. (pitch)  Just unload it a bit and all should be fine.

What is happening is the 100LL in effect burns slower than the auto fuel.  The lower octane is easier to ignite but this also can have a greater tendency toward detonation.  Higher octane fuel is very good at preventing detonation and is much safer for the engine.  So the answer to your problem is yes, it indeed is burning slower and is better protection for the engine.  Just reset the prop to get back to your normal RPM numbers and the performance will feel exactly the same.

If in your region it gets cold at all, winter flying, the 100LL will also be harder to start.  Should that be the case you will need to do some pre-heat of the engine to solve that.  (keep it in a hanger for example if possible) 

Cheers