While you're joy riding around your town your car's engine is hard at work repeating its four-stroke cycle: the intake stroke, the compression stroke, the power stroke and the exhaust stroke.
The intake stroke occurs when the piston drops from the top of the cylinder which fills with gasoline and air. The piston then moves up, compressing the mix, creating the compression stroke. The spark plug then ignites the mix and shoots the piston down, which is the power stroke.
The engine then expels what's left of spent fuel, called the exhaust, during the exhaust stroke and continues the process over again...and over and over and over again.
Engine Knocking
Under normal conditions the cycle continues on without a hitch or a problem. Sometimes, though, the mix of gasoline and air loses its patience and ignites on its own because of compression and heat rather than the spark plug getting the job done.
This is called "pre-ignition" but most call it "engine knocking." The result is an actually pinging or knocking created by your engine. In extreme cases there have been instances of pre-ignition burning holes in cylinders.
Gasoline Explained
Gasoline is a gathering of hydrocarbon molecules and each different kind of molecule acts differently under pressure. Depending on your assortment of hydrocarbon molecules in your gas you could be knocking, or you could be driving smooth, and until the early 20th century there was no way of knowing if that gas you were pumping would knock or not.
In 1927 Graham Edgar of General Motors suggested using a ratio of iso-octane and heptane (two hydrocarbons) to measure the gasoline's tendency to knock. He found the more heptane to iso-octane meant more knocking.
Over time a wide array of tests have popped up to measure the octane of gasoline. What comes from these tests is the numbers on the yellow stickers you push to pump you gas: 87, 89, and 93.
What the Numbers Mean
Here's where you can impress your friends with your vast knowledge of trivial facts. Those numbers are a direct ratio to the amount of heptane to iso-octane in the gas. For instance, the usual regular gas number is 87, meaning that for every 87 iso-octane molecules there are 13 heptane molecules, or 87 percent to 13 percent.
States vary with their regulations to what "regular" means. Some may have an 85 to 15 ratio to be acceptable while others (and most) find at least a 87 rating regular gas.
What Makes #93 So Great
As stated above: not much. You might expect premium gasoline to make your car go faster, or get better gas mileage. All it does is resist knock. Most cars today have a knock sensor as well, which does even more to resist engine knock, meaning the premium gas may just be a premium rip off.
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