Understanding Gasoline Octane Ratings - Do You Need Premium?

February 12, 2010

You may hear a variety of opinions about what grade of fuel to use in your vehicle. Believe it or not, there are different fuel octane ratings for a reason. Here's why you should know what your engine needs.
The octane rating of a fuel (gasoline in this case) is an indication of how well it resists ignition under compression. Internal combustion engines are designed to compress a mixture of fuel and air, and then ignite it with a spark. This causes the mixture to expand, forcing the pistons to turn the crankshaft. The timing of the ignition, triggered by the spark is very important. If gasoline of a certain octane is compressed too much, it will ignite, even without a spark.

The premature ignition of the fuel mixture is called knocking. When this happens, the force of the expanding mixture acts in opposition to the motion of your engine. This can cause extra stress on internal components. It will also prevent your engine from reaching its maximum power rating. Either way, knocking is not good for your motor.

Some modern performance cars--including most luxury vehicles from brands such as Acura, Lexus, Jaguar, or Mercedes-Benz--come with manufacturers' recommendations (or requirements) for higher grades of gasoline. For example, your engine might be designed to run on premium fuel. This means that it compresses the fuel mixture more than others, before ignition. Using regular gas will cause it to knock, and you'll sacrifice some of that performance.

Newer engines also may have a knock sensor. This sensor can detect early ignition and make some adjustments to prevent knocking. This will help protect internal components from unnecessary wear, but at a cost. You'll generally notice a drop in fuel economy as a result. This may even negate the cost savings of buying a lower octane fuel.

Although higher octane fuel is required by performance engines, it doesn't necessarily make normal engines any better. If your vehicle isn't designed for it, premium gas won't make it run any better. You're welcome to try it if you want, but you'll just be paying more money for the same thing.

The bottom line is that octane recommendations are there for your benefit. You can get away with going higher or lower, but it's unlikely that you will really gain anything.

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