Abstract: A method of operating an internal combustion engine generally comprises supplying a primary fuel having a first octane rating at to an intake manifold of the engine, supplying an amount of racing fuel concentrate to the intake manifold in response to a signal associated with engine load, and varying the amount of the racing fuel concentrate supplied to the intake manifold in response to variance of the signal. The racing fuel concentrate is a petroleum distillate racing fuel, such as Torco® Mach Series Accelerator Race Fuel Concentrate, that significantly increases the octane rating of the primary fuel when the concentrate is supplied to the manifold.