I've been getting some questions on what octane I should run in the Mercury V6 2-stroke outboard (2.0, 2.4, 2.5 liters) based on heads/compression. First, it's important that our baseline is correct and we understand which octane rating is being used for your specific fuel.
In the U.S., we use the average of RON plus MON, also known as AKI, (or RON+MON / 2 = AKI). This equals the minimum octane rating for unleaded motor fuels and is the number you see at the pump at your local gas station.
In the example pictured, we are using aviation fuels, since this is a common and affordable way to increase the octane of pump gas by blending.
Here you can see here, for example, 100LL AV GAS is the equivalent of 105 AKI, so a 50/50 blend with 91 from the pump gets you 98 AKI octane (105 + 91 / 2 = 98).
In Europe (EU), the octane rating on the pump is simply the RON figure. EU 95 octane = US 91 octane and EU 98 octane = US 93 octane.
You can apply this simple math to any fuel once you know the octane rating method to get to the desired number required for your set-up.
While I prefer the race fuel over AVGAS for land-based racing outboards, I have successfully run blends of AVGAS with pump gas in older 2-stroke outboards to increase to octane rating.