Is Premium Fuel Worth the Premium Price?

(AP Photo/Charlie Neibergall)

In our supersized society, we tend to assume that more is better. If a big gulp is better than a small gulp, then premium fuel must be better than regular unleaded. Many drivers believe that octane is a performance enhancer: that's a misconception. Octane's purpose is to prevent engine damage that can occur due to pre-ignition or detonation, or what most people call “engine knock.”

Internal combustion engines create power through a series of controlled explosions called combustion. Here's how it works: a fuel injector sprays gasoline into the engine cylinder, and mixes with air entering through the intake valve. The intake and exhaust valves close, sealing off the cylinder. The piston moves down in the cylinder, compressing the vaporized gasoline to make it more volatile.

A spark plug fires, igniting the mixture. The explosion pushes the piston back up in the cylinder. As each piston moves up in the engine cylinder, it creates energy to spin the crankshaft: the power that eventually finds its way to the wheels. The spent fuel exits the engine cylinders through the exhaust valves and makes its way to the tailpipe.

In order for the engine to work properly, the fuel must only ignite when the spark plug fires. But because gasoline is so flammable, explosions can occur in the engine cylinders at other times as well.

Internal combustion engines are inherently inefficient: some gasoline passes through the cylinders without burning. If this gas vapor auto-ignites after the spark plug fires, the result is detonation. Detonation can burn pistons, shatter spark plugs, or even crack the engine's cylinder heads.

Pre-ignition is caused by a hot spot in the combustion chamber that ignites the fuel mixture before the spark plug fires. It sound like pinging or knocking, and over time, can cause similar damage to detonation.

Octane reduces gasoline's tendency to auto-ignite, protecting the engine against detonation or pre-ignition. Manufacturers will recommend the minimum amount of octane necessary to protect the engine against damage. High compression engines tend to need more octane, because they are more susceptible to detonation. Supercharging, which uses a blower to increase the volume of air moving through the engine, can also raise the car's octane requirements.

Octane requirements increase when vehicle weight goes up significantly: for example, when a truck is towing a large trailer. Extreme heat can make an engine more likely to detonate than moist, cooler air, raising the engine’s need for octane.

The bottom line is this: don't use gasoline with a lower octane rating than the manufacturer recommends because the engine may not run as efficiently, and the likelihood of damage from detonation or pre-ignition increases. On the other hand, using premium fuel when the manufacturer recommends regular is a waste of money.

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