Bichon

In a car with modern electronic engine controls, incipient knock will be detected; running less than the recommended octane will cause the engine management software to retard the timing, reducing both performance and the chance of further knocking.

High octane fuels (greater than factory recommendations) can be very beneficial, but only if the engine is tuned to take advantage of them. Otherwise they are a waste of money.

lobuxracer

iTrader: (2)

As usual Bichon is dead on the money. One thing to add, your fuel economy goes to garbage when the ECM is retarding timing. I lost 33% of my mileage in my Supra when CA went from 93 to 92 and finally to 91. It's actually cheaper for me to put in 4 gallons of 100 octane unleaded and 14.5 gallons of 91 unleaded despite the 100 octane costing more than $6 a gallon because I recover such a large amount of fuel economy.

I have a spreadsheet that calculates the temperature of the compressed mixture in an engine. If that temperature is less than the autoignition temperature of the fuel, more octane (higher autoignition temperature) will not add anything but cost. Again, as Bichon stated, if you retune for the better fuel you may see better performance, but typically this is for forced induction with variable boost. It's really rare to retune an NA engine for better fuel and get more power unless you make some mechanical changes too.