You guys are mixing unlike octane units without really knowing it. That's an incredibly dangerous thing to do when you are talking about fuels for your aircraft.

There are 3 different ways Octane is calculated, and the resulting numbers are all significantly different. You can't add octane calculated with one type of octane number to a different type of fuel with a different octane number and have a result that is going to make any kind of sense unless you convert them to the same octane units.

Aviation fuel Octane numbers are derived via the Motor Octane Method (MON).
European car gas Octane numbers are derived via the Research Octane Method (RON).
US Car Gas Octane numbers are actually Anti-Knock Index Numbers (AKI) They are derived by using (R + M) / 2, which you will find marked on every Automotive Gas Pump in the USA if you read the small print next to the AKI Number. AKI numbers are also sometimes referred to as Pump Octane Numbers (PON).

So, now you have MON, RON, and PON as three different ways to come up with something that is called an octane number, but are actually significantly different from each other..

RON numbers for the same fuel are usually significantly higher than the MON numbers, so that also skews the AKI numbers significantly upwards.

For example, 100LL which is a MON number calculates out to be roughly the equivalent of 106 AKI if we were talking about US pump gas.  In Europe, that would be 112 Octane RON at the fuel pump.

The old 80/87 Octane Avgas is actually an 87 octane MON fuel. At 87 Octane MON it is actually the equivalent of 91 AKI Pump gas and the equivalent of 95 Octane RON (Eurpoean car gas). Now, many of the small Continentals don't even need that much as they were certified for the old 72 Octane MON fuel. That's where the Mogas STCs got started with 87 or 88 octane Mogas.

I had a friend that was mixing fuels for a high performance avgas engine thinking they were all the same. I had a slow day at work that day, so did some research and put together a spreadsheet to help him figure out roughly equivalent numbers between the various "octane" Methods. 
LEGAL NOTICE: The following numbers are my calculations made bu deriving various octane numbers and calculations found on the internet and are presented to help thosereading this post to understand the huge differences in octane calculations.  Find your own numbers and calculate for yourself or stick to the STCs and engine manufacturers recommendations.
RON - European fuel octanes (Research Method)
MON - Aviation Octanes (Motor Method)
PON - AKI (US Pump Anti-Knock index)

90      83     86.6
92      85     88.5
93.5   86     90
95      87     91
96      88     92
97      89     93
98      90     94
100    91.5  95.8
103    93.3  97.9
105    95     100
110    99     104.5

Of course there is a lot more to the fuels than just the octane numbers.  Vapor pressure varies in Mogas depending on the seasonal blend.  Winter fuels have a higher vapor pressure, which could lead to vapor lock if used in the summertime.  All Mogas has a significantly higher vapor pressure than any of the Aviation gasolines.
The 80/87 octane (red) fuel we used to run on our planes 50 years ago had  .14 grams per liter of Lead added as an antiknock compound.  100LL has a minimum of .56 grams per liter of Lead, which is exactly 4 times the amount of lead desired for the smaller Continentals.  That's why my preference is to run a mixture of roughly 20 - 25% Avgas mixed with Mogas in the KR.  The excess lead tends to build in the valve stems and causes premature wear and failure of the exhaust valve guides.
The lead in the fuel also served a second purpose.  It is a sacraficial metal that builds on the exhaust valves and gets sacrificed as the hot valve closes against the Exhaust valve seat.  Without lead, the exhaust valve seats tended to wear away due to an exchange of metal between the exhaust valve seat and the exhasut valve face.  That's the definition of valve seat recession, which was common with engines with solf valve seats in teh early days of unleaded fuels.  Most engines now have hardened valve seats and hardened valves, so for the most part, valve seat recession with unleaded fuels has become a thing of the past. 
So, thanks to the damage I've seen from the mysterious chemical(s) in the local Mogas, I'll be running straight 100LL from now on, and adding my own chemicals to hopefully reduce the damage done by the large lead content of the fuel.  Additionally, when operating on high lead content fuels, it is important to run the engine as lean as reasonably possible at cruise (less than 75% power settings).
-Jeff Scott
Cherokee Village, AR

> All I've seen around here is 91. That's what I'm gonna use.


At 7:1 compression the 0-200 cut it's teeth and grew in to old age with
80 octane  fuel.  You guys are running higher compression so a higher
octane is needed.  Is there an additive you can add to up the octane? 
I'm guessing Langford has the answer in the laptop he flies with.

Larry Flesner
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