if you need octane just mix in a little TOLUENE. You can get it pretty cheap at your local paint store. 114 octane just don't mix in more than 40% or it will eat your rubber lines up quick. I hear after extended use (every tank) it will get the gaskets in your carburator in a couple years. this was a subject in the hotrodders forum. How to figure octane is the average the octane between the two fuels being mixed. 7:1 ratio of 91 octane to toluene would be 91x6 + 114/ 7= 94.2
Gas stations only have 2 tanks low and high. the octane ratings in the middle are mixxed by pulling fuel from both tanks. In Oklahoma we usually just have 87/89/91. 89 is a 50/50 mix of the 2 that you pay 15 cents a gallon more for then another 15 cents for the high grade.
Toluene..my octane booster of choice - SportsCarForums.com an interesting read for those that use octane boosters.