High Octane Fuel

Hexane, Heptane, Octane, Flame
From the sludge that is crude oil, straight chain hydrocarbons are refined and lined up in a row to put the tiger in your tank. The greater number of molecules in the chain, the more resistant to compression-induced ignition the fuel is. Octane has eight molecules in the chain like an Octopus has eight legs. The real difference between 87-octane regular gasoline and 91-octane premium is the rate at which it burns when compressed and ignited while inside your engine.


The compression ratio of a given engine is what determines its octane requirements. Since most engines have a compression ratio designed for 87-octane unleaded fuel, all should be well in staying regular with regular. Even if you have a super-performance sportster with a big "91 ONLY" sticker on the gas cap, no harm will likely come if 87 is added by mistake. Modern engines come equipped with a knock sensing system, which listens for detonation and tells the engine computer to adjust spark timing until detonation is eliminated.


Conversely, the engine computer in a vehicle built to run on regular will not sense an increase in octane and adjust ignition timing to take advantage of the higher octane, so pouring in the premium in an engine not designed to take advantage of it is the equivalent of shooting money out of the tailpipe. All this is, of course, is a massive oversimplification, yet the fact of the matter is this: If your engine is designed to use 87 octane, yet rattles away like Carmen Miranda shaking her maracas every time you hit the gas pedal, then something is wrong—and it's only going to get worse.

Less Space, More Problems
High mileage engines often suffer from a buildup of carbon deposits in the combustion chamber. This buildup causes the total volume of the combustion chamber to become smaller. This decrease in chamber volume effectively raises the compression ratio of the engine, and causes low octane gasoline to detonate, and your engine to knock. If a high-mileage engine designed for regular gas knocks unless premium is used, then carbon buildup may be the culprit.


Other causes of detonation include overheating, a malfunctioning EGR valve, too far advanced ignition timing, or a defective knock sensor. Overheating caused by a defective thermostat or cooling system negates the engine's ability to channel away heat from the combustion chamber and causes the compressed fuel to detonate. If the timing is advanced too far, the spark ignites the compressed mixture too early on the power stoke, causing detonation. A defective knock sensor can send the ignition timing pell-mell, and direct sparks into the cylinders astray.