Mixing Mogas with 100ll

Since there are 10 octane points between the two fuels, every 10% of the mixture that's 100 octane will raise the final octane by one point.  Thus, to get 91 octane you need 100LL to be 10% of the mixture.  Proof below.

Formula: (Fuel A Octane x Fuel A %) + (Fuel B Octane x Fuel B %) = Desired Octane

If n represents the percentage of fuel that is 100 octane, then the percentage of 90 octane is (1 - n).

90 x (1 - n) + (100 x n) = 91  [re-write formula]

90 - 90n + 100n = 91  [expand]

90 + 10n = 91  [simplify]

10n = 1  [subtract 90 from both sides]

n = 0.1  [divide both sides by 10]

∴ 100 octane = 10% of mixture

Here's the other thing. The Rotax suitable fluids document does say the US min standard is AKI 91. But it also says European EN 228 Super is approved. The minimum standard for EN 228 Super is a MON of 95 and a RON of 85. AKI = (MON + RON)/2.

So, EN 228 Super has an AKI of 90, and Rotax says that's an approved fuel, so draw your own conclusions. 

https://www.oiltanking.com/en/news-info/glossary/octane-number-ron-mon-knock-resistance.html

https://dt176nijwh14e.cloudfront.net/file/275/What%20You%20See%20at%20the%20Pump.pdf

Hi all

As pointed out AKI is an average of RON and MON.  100LL is not tested with 2 values and is more correctly has an MON value of 100.  

The major difference in the RON test is the value will always be much higher than an MON number.  Therefore to average an indicated AKI and pure MON value is not correct.  Personally mixing to an odd number such as a 90% + a 10% is difficult.  The most common and easy way is to just mix it 50/50 to ensure you have enough octane.  The Avgas lead content is cut in have in a resultant mixture.  One of the great advantages of Avgas is it is a more consistent RVP value and this really helps when you travel to avoid fuels with winter blends.  The RVP value of Avgas prevents most of your vapor locking issues, Avgas being far more consistent with no winter blend fuels.  

Just my opinion.

Cheers

I'm with RW. 50/50 is what I was always taught. It's far easier to calculate and you can't go wrong.

This is a question more than a statement: 100LL is actually 100/130LL, meaning it has a lean octane rating of 100 and a rich octane rating of 130. I have always guessed that since the Rotax engines don't have mixture controls, they are designed to run slightly rich. If that is the case, then 100LL would have a higher octane rating than 100, and would require a lower mixture ratio. Am I right about Rotax engines running rich? More of a curiosity question, since I often do the same as Rotax Wizard and Roger Lee. 

Mike