Time to set the record straight about octane ratings with credit and apologies to Q45tech.

Octane is an additive that increases fuel compressability. Premium fuel is niether cleaner nor dirtier than regular but does contain more additives. "Premium" contains the same amount of energy as "regular" per unit.

The term "octane" that most use refers to the method used to calculate octane equivalence which is a measure of the fuel's resistance to preignition otherwise known as knock. Engine knock occurs when the fuel vapor in the cylinder prematurely ignites on the compression stroke due to pressure and ambient cylinder temperature resulting in severe damage to rods and bearings, etc.

The knock sensor is actually a vibration sensor that will send a signal to the ECU to retard spark timing when banging -"knocking" - from the cylinders is detected.

It all comes down to a combination of pressure and temperature which walk hand in hand. Retarding the spark reduces the power generated on the down stroke which lowers the pressure in the cylinder. While saving your engine from seld destruction, fuel energy is being wasted because there will be less energy created from the down stroke.

Gasoline companies adjust the amount of "octane" according to season. Winter requires less "octane" because ambient air temperatures are lower which means that knock is less likely than in summer. However, this means that in order to meet engine standards, the fuels react basically the same way throughout the year. Otherwise, one could run "summer" "regular" in the winter because some "octane" will be present in the fuel but this would mean that the refineries would be wasting capital on unecessary octane.

The method used to determine anti-knock rating is a mathimatical one that has little to do with the amount of octane actually present. High altitude areas have less ambient air pressure so their premium fuels only have to be rated around 87 to prevent knock. Here in Chicago where gas can contain up to 10% of the less powerful ethanol, premium is as high as 94 so that the engine can run at maximum compression to meet OEM performance specks.

I thought that I could just dash off a quick explanation about octane but it is not as easy as I thought. In short though, "octane" as we normally use the term refers to a fuel's resistance to knock. Cold air intakes help reduce knock. Fuel mixture (lean vs rich) reduces knock. An efficient cooling system also. Temperature and pressure and litterally getting the biggest bang for your fuel buck is what it is all about.