STC for Rotax Engine to use Mix fuel to meet AKI 91

Remember the manuals are written for the world not just the US. So knowing this we need to look at things like the metric system and AKI and Ron.

Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

Another type of octane rating, called Motor Octane Number (MON), is determined at 900 rpm engine speed instead of the 600 rpm for RON.1 MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern pump gasoline will be about 8 to 12 octane lower than the RON, but there is no direct link between RON and MON. Pump gasoline specifications typically require both a minimum RON and a minimum MON.[citation needed]

in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2). It may also sometimes be called the Posted Octane Number (PON).

Difference between RON, MON, and AKI

Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's Sensitivity,[4] and is not typically published for those countries that use the Anti-Knock Index labelling system.

The Wikipedia article also contains a conversion table.

Because there is no direct link between RON and MON (they're based on different measurements), there's no formula to calculate one from the other.

I wouldn't loose a lot of sleep over trying to be dead on the octane mixture. Whether you use 87 or 89 oct. and then mix at least 50% 100LL you'll never have to worry or try and rack your brain to make it perfect. The engine doesn't care if it's perfect. You just don't want to have a too low.  Octane.

Roger:

Thanks for your response. 

I agree that MON and RON are independent measures of octane.  But, the AKI is the average of MON and RON. Namely, AKI = (MON + RON) / 2  So, by knowing two of the values one can find the third.

In my case I am trying to increase the octane of REC-90 (90 AKI non-ethanol unleaded gasoline) to meet the minimum 91 AKI octane requirement.  Based on adding one gallon of 100LL to five gallons of REC-90 this results in an AKI index of 92.3 assuming the 100LL has a minimum AKI of 104.  

I’m attempting to allow for some margin at meeting the 91 AKI minimum and limit the amount of lead in the fuel.

I think correctly knowing the octane of the 100LL is important so that the minimum octane is achieved AND that the amount of lead is also minimized.  A 50 / 50 mix of 87 AKI and 100LL would be 95.5 AKI if I am understanding correctly.

My apologies if I am misunderstanding.

Brett