Originally Posted by rkramer
octane doesn't have much to do with the AFR. the AFR won't change (much) with different gas. what does change is if the car knocks or not, at a certain RPM it might need more fuel to prevent knock, even though it technically isn't lean, or at least any more lean then before.
rkramer is right. Please try to post accurate information especially when it pertains to something as sensitive as engine management. /channeling Uncle Scotty.
I can understand where the confusion might come from, however octane has NOTHING to do directly with AFR. Since 91 octane has a lower knock threshold (ie what the octane rating measures) than 93, the ECU will ONLY run richer on 91 than it did with the 93 if it detects the presence of knocks. Otherwise fuel is fuel and the ECU has no way of detecting what sort of octane gas is in the tank BEFORE a knock event occurs.