First and foremost, premium gas really is a better fuel in terms of the power it provides in the right engine. All gasoline is a heady brew of many different hydrocarbon molecules, ranging from heptane (seven carbon atoms and 16 hydrogens) [see endnote] to decane (10 carbons and 22 hydrogens) and beyond. The hydrocarbon clearly identified on the pump is octane (eight carbon atoms and 18 hydrogens). This number, however, is not a measure of the percentage of octane actually in the gas itself. Rather, it is a measure of how that gasoline compares with a pure mixture of octane and heptane. At special laboratories across the globe, chemists concoct such reference fuels and then use them in comparison with refined gasoline following the dictates of standardized measures. "The American Society of Testing and Materials has this thick document on how you determine octane rating with this specialized one-cylinder engine," explains Joseph Shepherd, a mechanical engineer at the California Institute of Technology. "The higher the number the harder it is to have knock."
"Knock"—an unregulated explosion in a chamber designed for highly regulated combustion—is the bane of an internal combustion engine. During the four-stroke cycle of a typical car motor, the piston drops in the cylinder, allowing it to fill with a mixture of gasoline and air. The piston then moves up again, compressing the fuel mix and, when it reaches the top, the spark plug ignites the explosive vapor, driving the piston down again. As the piston returns to the top of the cylinder it expels what remains of the spent fuel out through the exhaust valves and the whole process starts again. Knock occurs when the compression of the fuel and air mixture alone, and not the spark plug, sets off an explosion. This results in a very loud noise and a lot of vibrations in the engine itself; "it's very bad for engines mechanically," Shepherd notes, driving the piston down before it has reached the top of its cycle. Each hydrocarbon molecule behaves differently under pressure, but octane resists the temptation to explode better than its volatile cousin heptane. "You rate the gasoline about how it knocks compared to this reference mixture," explains William Green, a chemist at the Massachusetts Institute of Technology. "One's that don't knock very much are the premium." That is, they behave in an engine as if they have a high proportion of octane, even if they don't.
Most modern cars, however, are designed to employ a specific compression ratio, a measure of how much room is available to the fuel when the piston is at the bottom and the top of the cylinder. This compression ratio—somewhere in the neighborhood of eight to one—tolerates lower octane fuels (such as regular gasoline, good old 87 octane) without knocking. "The compression ratio is fixed by the designer of the engine," Green says. "The regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money." High-performance engines, such as those in some sports cars or older, heavier automobiles, often boast much higher compression ratios. These cars—for example, Shepherd's Subaru WRX—require premium gasoline and will definitely knock without it. "I have to put the 92 octane in," he says. "It has a turbocharger."
Such high compression ratios—and the premium fuels that go with them—could be turned to efficiency, rather than speed, Green notes, especially if put into the engines of lighter cars like his Honda Civic. Other automotive fuels, such as ethanol, can also offer high octane ratings, allowing oil companies to use more volatile gasoline in such blends. But for standard cars on the road today, purchasing premium gasoline is simply paying a premium for a fuel that delivers no added benefits. "If you think you need it," Green says, "you're being very eccentric."
Note: This article was changed after publication to correct an error. It originally stated that heptane has 14 hydrogen atoms.