Why not mix 50/50 with 108 race gas. Give you 100 octane. 12:1 should run on that. May have to back off the timing a few degrees. Are your heads aluminum? If so, you can get away with 11.5:1. to figure out the mix for a required octane use:
(octane A*number of gallons)+(octaneB*number of gallons)/total number of gallons. So if you need 100 octane and using 10 gallons of 93 and 5 amount of 108:
Octane of mix =(10*93)+(108*5)/ 10+5
=930+540/15
= 1470/15
Octane=98
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