Some race fuels don't mix , you won't hear detonation but it WILL be there , the chamber will decipher the higher octane & could ping on the low stuff , go conservative on the timing & pull the plugs as this will tell you if you need race gas or not , the motor will slow down the more timing you put into it (race fuel) if running optimum for pump gas , the same motor running lower octane will run faster than a motor advanced for higher octane , the piston does'nt like the a/f lit off to far down the hole.