Octane rating is a fuels resistance to detonation or pinging as compared to a standard. The standard used is iso-octane, which gives it the value of 1.0, or as we know it, an octane rating of 100. If a fuels anti-detonation characteristic is better than iso-octanes, the fuels octane rating is above 100, if a fuels anti-detonation characteristic is less than iso-octanes, the octane rating is less than 100. There are two common methods to measure a fuels octane rating, the motor method and the research method. The common octane rating we're familiar with is the average of these two octane rating methods, R+M/2 or research plus motor divided by two. The motor method to determine a fuels octane rating is probably the most accurate. In this method a variable compression single cylinder engine is used to test a fuels detonation characteristics. The outside of the cylinder sleeve is threaded, and the cylinder head assembly essentially screws over the outside of the cylinder. To increase the compression ratio the cylinder head is screwed further down the cylinder, toward the piston, and to decrease the compression ratio the cylinder head is unscrewed from the cylinder, away from the piston. The engine is baselined using iso-octane and the compression ratio is adjusted just to the point of detonation, or adjusted to an octane rating of 100. The engine is then fed the test fuel and the compression ratio is readjusted to the test fuels point of detonation to determine it's octane rating. Gasoline is not just one hydrocarbon, but a whole bunch of different ones blended together. The research method is basically an octane estimate based on previous motor tests of the different hydrocarbons used in a particular gasoline blend. Tetraethyl Lead was first introduced in the early twenties as an octane increaser for gasoline by the Ethyl corporation. Lead has nothing to do with iso-octane itself, what it does do, is increase a fuels resistance to detonation, ie improving it's octane rating, of course it also causes brain damage. A couple of other ways to increase the octane rating of gasoline is to change it's molecular compound by catalytic cracking, this method essentially turns a lower octane hyrdocarbon into a higher octane hydrocarbon through the use of super-heated steam and a catlyst. The other common method is to blend the gasoline with another fuel that has a higher octane rating most commonly ethyl alchohol. When it comes to rating fuel as regular, midgrade, or premium, most consumers only consider the octane rating. As others have pointed out, it also includes other additives which can be brand specific. If you want to see what several auto manufacturers recommend, go to www.toptiergas.com.

John