Questions about the Rotax describing how much 100LL needs to be added to automotive fuel

In an attempt to get an answer to my questions, I contacted Rotax.  

I followed up with Rotax about my questions a week ago and got a response from the RFSC ADMIN that I would get a response “As soon as there is an update”.

Attached are screen shots that are the basis of my questions. 

I will share the response when I receive it.

Brett

I've was always taught in class at least a 50/50 blend when using 87 oct. (US AKI) and 100LL.  Better to have a little more 100LL then too little on the ratio. A 50/50 blend should give you 91 oct. (AKI)

Roger: 

Many thanks for responding to this question and to other questions that are posted on this forum.  

I fully agree that the safe way to always meet the 91 AKI minimum requirement is to exceed it by some margin.  Further, that the easiest way to accomplish this is run 100% 100LL.

But, with the lead being bad for the engine, I think the Rotax document provides a method for determining how much 100LL needs to added to automotive unleaded gasoline to meet the minimum octane requirement and reduce the amount of lead that gets in the engine. 

One of my questions is why is the minimum octane of 100LL as described in the Rotax’s document higher than in the ASTM document that defines the properties of 100LL?  Per the Rotax example, one needs to accurately know the octane of each fuel to determine what ratio is needed to get the desired octane of the mixture.  I will be interested to hear Rotax’s response.

In my case I am trying to increase the octane of 90 AKI non-ethanol unleaded to more than 91 AKI.  So the amount of 100LL that needs to be added should be limited.

Brett

Hi Brett,

I think mixing the auto fuel and 100LL at a 50/50 blend greatly reduces the affect that straight 100LL has. Plus you should use a lead scavenger like Decalin to help even more. 

I think you should save yourself the trouble of blending in any 100LL (and then Decalin) and just use auto gas that has an AKI of at least 91.  It sounds like, in your area, that might mean it includes a bit of ethanol.  The ethanol will not be a problem for your engine.  The lead IS a problem, as you know.  And in addition to saving trouble, you will save $$$.

If you just really hate ethanol, you might even consider mixing your 90E0 with 93E10 on a 3:1 ratio, which would bring your AKI up to spec while bringing the ethanol down to around 2%.