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Anti-knock sensors provide the maximum allowable spark advance depending upon what grade fuel you use. Did try 89 octane ethanol free gas in my Cruze, both my fuel economy and performance suffered.
So you notice a difference on 91 octane? all that aftermarket tune does is better utilize higher octane fuel, pushing the timing advance more and adjusting the fuel trims. Remember GM stock ECU perimeters are to only make XXX torque and to dial things back if that is exceeded, I'm sure this is where the aftermarket tune is able to get all that extra power.So what exactly is a tune doing? One thing I did read, is you have to use at least 91 octane fuel, already doing this.