Originally Posted by bartinmi
Fill your tank with half 93 and half 89 = 182 divided by two equals 91! It's my understanding that most stations actually create their 89 (Plus) by mixing the correct ratio of 93 & 89. I'm no expert, but can't using too high of an octane also cause problems? The octane number doesn't necessarily mean that it's "better", just that it's formulated to burn at a higher temperature.
Close, the function of the octane rating is to indicate the temperature and pressure environment at which the fuel will ignite. Higher compression, or "coked up" engines require a higher octane content to prevent pre-ignition. Pre-ignition is the so called "ping" or "knock" that people can often hear when the fuel-air mixture ignites before the spark plug fires. Over time, a severe pre-ignition problem will cause the piston pin bearings and connecting rod bearings to wear.
Most modern engines, the xfire included, have knock sensors that will detect this pre-ignition and retard the timing to allow operation with lower octane fuels. However, this should only be trusted in an emergency situation where the lower octane fuel is all that is available. Even with the engine trying to watch out for itself, if the octane is sufficiently low it cannot compensate.
The "premium" designation was added as real octane is produced in lower quantities in most refining processes and hence the oil companies could charge a premium price for the rarer component. For many years, the anti-knock properties of octane were obtained by adding tetra-ethyl lead to gas until the catalytic converter, which is chemically fouled by the lead, came into widespread use. Now, ethanol is often added, as it also causes the octane number to be increased. Both octane and ethanol actually decrease the energy content per liter of fuel.
Actually, the amazing part is I can still remember these discussions from the chemistry class I took way to long ago.