Octane is the active ingredient in gasoline. When your car burns gas, the part of the gas that gives the car energy is the octane burning. There is an octane rating on gasoline at the pumps. That rating is the percentage of octane in the gasoline. There are ways of increasing the octane rating without actually increasing the amount of octane, but that involves complicated chemistry. Higher octane gasoline will burn more efficiently, therefore give you better mileage, and that's why it costs more for higher octane gasoline.

Octane is C8H18, meaning it's a chain of eight carbon atoms and 18 hydrogens. Unlike other combinations of carbon and hydrogen (such as butane, pentane, hexane and heptane - 4, 5, 6 and 7 carbon atoms respectively), octane handles compression very well. Your engine has four strokes - intake, compression, combustion and exhaust. On the intake stroke the piston goes down and the air-fuel mixture is let in through the intake valves. On the compression stroke, the one we're concerned with, the piston moves up and compresses the air-fuel mixture. During combustion, the spark plug ignites the mixture, producing the combustion (or power) stroke which moves the piston down forcefully, and finally we have the exhaust stroke when the piston moves back up to push the spent gases out. The reason octane is important is because engines can produce more power if they have higher compression ratios. A 10:1 compression ratio means that the engine compresses the volume of air it sucks in - say X, into a volume on tenth that size. The problem is that when air compresses, it increases in pressure and this heats it up. A low octane gasoline is prone to spontaneously ignite prematurely in a high compression engine, which causes damage. If the gasoline ignites too soon, the power of the burning gas and air mixture will push down on the piston as it's still compressing, losing you power and over-heating the piston (as well as valves, head, and cylinder walls), eventually leading to catastrophic engine failure. This is known as pre-detonation, pre-ignition or "knock". With a high octane rating, engines can run with higher compression. You may be asking yourself why turbocharged or supercharged engines with low compression (sometimes as low as 8:1) would need high octane gasoline. The reason is because turbocharging and supercharging compresses the air even before it reaches the engine proper, meaning it's already very hot when it enters the cylinder. In summary: higher octane gasoline allows engineers to design engines that have more power. However, that doesn't mean that an engine designed for regular gas will gain power just because you added high octane gasoline.

Octane is C8H18, but it is not a chain molecule, it is a <see correction below> molecule. It is this configuration that makes it better for high-compression gas engines. The molecule geometry is more stable than chain molecules like heptane and requires more energy to break apart before the constituent carbon and hydrogen become available for combustion. Heptane is great for diesel engines because, as a chain molecule, the end atoms on the molecule quickly and easily react with oxygen in the combustion chamber when injected. The octane rating for gasoline essentially measures the heat energy required to start the chemical recombination of the fuel with the air. An octane rating of 100 has the same threshold as octane. Ratings lower than 100 begin burning with lower initial energy than octane - the lower the number the lower the ignition threshold. Ratings higher than 100 require more energy to start combustion and can be achieved with fuel molecules more stable than octane. The octane rating of any fuel can be changed by adding chemical agents, or additives, that stabilize (or, I suppose, de-stabilize) the fuel. The compounds found most effective at this stabilization include lead atoms, which is why leaded fuels were so common and popular till concerns about health and environmental effects killed those additives. At the pump, some companies may mix a higher and lower grade to sell an intermediate grade - this is legitimate, not cheating. As other posters pointed out, higher octane allows engine designers and modifiers to use higher compression ratios and engine temperatures to get more performance out of an engine. Also as mentioned, using higher octane fuel is unlikely to improve performance or operation of an engine design (or modification) that does not require it. _______________________ Thank you for the correction. I was certain that I was taught in a internal combustion engine class that it was a ring. According to: http://chemistry.about.com/cs/howthingswork/a/aa070401a.htm the correct form of octane for octane ratings is isooctane which is still not a chain molecule, but is not a ring either. It is more of a compact branched molecule. See http://chemistry.about.com/library/graphics/blisooc.htm for a diagram

Octane is a radio station on Sirius satelite radio and it is important because without it YOU WOULD ALL BE DEAD!