Most internal combustion automobile engines, whether they're four, six or eight cylinders, operate on a four-stroke cycle known as the Otto cycle. The four strokes are: intake, compression, combustion and exhaust. To put it in the simplest of terms, each of the vehicle's pistons moves up and down within a cylinder. As the piston moves to the bottom of the cylinder, a mixture of fuel and air flows in. The piston then moves upward, toward the top of the cylinder, compressing the air and fuel mixture as it does so. Just as the piston reaches the top of the cylinder, that cylinder's spark plug ignites. The spark creates a small, controlled explosion that forces the piston to the bottom of the cylinder. In the final stroke of the cycle, the piston moves upward to push the exhaust gas out of the cylinder. Once the exhaust gas has been pushed out, the entire cycle begins again. For a much more in-depth look at how an internal combustion engine operates, you may want to read How Car Engines Work.
As long as this process works as described above, the engine runs smoothly. But occasionally the pressure of the piston itself will cause the air and gas mixture to ignite prematurely during the compression cycle, creating a smaller, less powerful explosion. This is called preignition and it's the cause of engine knock, the erratic rattling or pinging sound you may occasionally hear underneath your car's hood. A little bit of engine knock isn't necessarily bad for your engine, but it's not desirable, either. It means that your engine isn't running as efficiently as it could be, and left unchecked, it could eventually cause damage. Engine knock reduces your car's performance, too, so you definitely want to avoid it. How, you may ask? Well, low-octane gas is more likely to ignite under the pressure of the piston alone, so it's also more likely to produce engine knock.
Does this mean you should always use high-octane gas? Not necessarily. It really depends on the compression ratio of your engine. This is the ratio of the volume within the cylinder when the piston is at its lowest point to the volume within the cylinder when the piston is at its highest point. The higher the compression ratio, the more compressed the air and fuel mixture becomes and the more likely it is to ignite before it's supposed to due to pressure alone. Cars with a low compression ratio don't need premium gas because there's little danger of the air and fuel mixture igniting improperly. But high-performance engines, which have a high compression ratio, are more prone to preignition and can truly benefit from premium fuel. This would include the engines in most luxury cars.
Even so, premium gas isn't always necessary for these engines. We'll find out why on the next page.