there is a common misnomer that higher octane = higher performance or is somehow a 'cleaner' fuel. If your engine was designed to run a high octane fuel then you should. However, if your engine was designed to run a minimum of 87 octane, you'll not see much difference going to a higher 95 or even 120 (you could actually see a decrease in performance).

There's a point at which under so much heat and pressure inside an engine the fuel vapor will ignite without the presence of a spark (like in a diesel engine). Higher octane fuels auto-ignite at higher pressure and temperature levels than lower octane fuels allowing engine designers to design in higher compression ratios (or use turbos and superchargers). Long piston strokes, turbos, or superchargers cram more fuel/air into an engine resulting in higher compression and additional power but require a higher octane fuel to prevent the fuel vapor from igniting before th e spark plug lights it up resulting in "knock" - a dangerous condition that can do substantial damage inside the engine if left unchecked (most modern cars have knock-sensors that detect this condition and will try to compensate by adjusting the air/fuel ratio).