Simple chemistry shows that lower octane fuel is more combustable than higher octane. Hence why you would run high octane in a high compression motor to avoid PRE IGNITION (aka spark detonation). So i can't see how a higher octane fuel alone would cause a motor to make more power unless other factors were involved. Maybe those expensive fuels that make those claims contain lead and other additives that lubricate the rings, reducing friction or something along those lines. Your altitude is also a factor, which is why airplanes use Av-Gas, which is over 120 octane I believe. It also has lead in it. I used to mix 1 gallon of av-gas with 4 gallons of 92 octane pump gas, and then mix in my oil when I raced 125's, and I can tell you that it did keep the combustion chamber cleaner. The reason for this is that it increases operating temperature and burns more completely. We ran it in 4 stroke air cooled quads once and melted the fenders....but thats how you learn I guess.

 

At 10.5:1 you can run 91-93 octane with no problems........

I would not go any lower on a 400ex with that valve set up..... running regular will have you running your valves out of adjustment a lot more often .......

 

Yes I understand what you are saying 100% and thats why I have said this the VP fuels that are for 4 strokes are low octane levels (U4 VP 92 OCTANE and VP MR9 IS 87 OCTANE) just check it out they make the HP thats why it cost so much lol