Fuels/Octane/Pre-ignition & Detonation

High octane or premium gas burns slower than low-octane gas (regular gas). Therefore, combustion is slowed with premium gas. Slower combustion can prevent pre-ignition, or "pinging". Pre-ignition is ignition of the fuel/air mixture before the spark plug fires. The ping of pre-ignition sounds like marbles bouncing around inside the engine. The term "pre-ignition" is sometimes incorrectly used interchangeably with "detonation". The death rattle of detonation is an erratic and intense pressure wave that occurs shortly after TDC (top dead center) of the power stroke.
When the air/fuel mixture is ignited by the spark plug, a combustion flame front spreads outward from the plug accompanied by pressure waves created by the combustion. Detonation is caused by the amplification of two pressure waves combining to form an intense wave "spike", which then bounces around the combustion chamber surfaces, causing very-high temperature spots that ignite a second flame front. The "knocking" sound of detonation results from this secondary ignition and it's pressure waves which greatly shock-stresses pistons, piston lands, rods, crank and bearings. Schlieren photography of Harley combustion chambers during actual engine operation has revealed that detonation is not the result of "colliding flame fronts" as some believe. Most everyone does agree, however, that severe detonation will quickly destroy an engine.

The octane rating system used in the USA is RON + MON/2, where RON is the Research Octane Number, and MON is the Motor Octane Number. The ratings are determined on a dynamometer for a particular fuel. The RON dyno test is done without a load on the motor, and the MON dyno test is done with a load on the motor. The two results are then summed and divided by 2 to determine the rating of the gas which is displayed at the gas pumps.

An engine's octane needs are influenced by several factors, especially the configuration of the engine (state of mechanical tune, i.e. - cranking pressure, valve timing, ignition timing, operating temperature, plug heat range, carbon deposits, etc.) and each engine has it's own distinct octane requirements for optimum operation. For example: If your engine only needs 89 octane to run properly without pre ignition, the use of 91 octane will not increase performance! Conversely, the use of 87 octane, when 89 is needed, will result in pre ignition, overheating and poor performance. Once you have established the actual octane needs of your particular engine for ping-free operation, the use of higher octane gas is only wasted money!