Engines Exposed: What Is Octane (and Why Does It Matter)?

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Gas pumps | Thinkstock

Few things in the automotive world have become as murky as what octane actually is and how it affects an engine. Perhaps this is because the choice between regular, mid-grade, and premium is presented to us every time we fill up, and most of us aren’t really sure what the difference actual is: Chances are you’ve heard a variety of unsolicited advice on the subject from your know-it-all uncle or co-worker, but how do you sort out fact from fiction?

To start with, let’s answer the question about what octane actually is. Octane is a hydrocarbon chain with eight carbon atoms. Simple, right? Gasoline, in theory at least, is a combination of octane and heptane (seven carbon atoms). The numbers on the fuel pump, usually 87, 89, 91, 92, or 93, refer to the percentage of octane to heptane the fuel contains. So, regular gas is generally considered to be 87% octane, and therefore 13% heptane. In practice, though, there are wide assortment of other ingredients and additives in our fuel. So the numbers refer to the performance of the different fuel grades, and not their actual content.

So how does the ratio of octane to heptane affect how the gasoline performs? While both hydrocarbons burn rapidly and produce power, they handle compression very differently. This is important for internal combustion engines because in order to create an explosive charge in the cylinder, the engine must compress the fuel-air mixture quite a bit. While octane handles this pressure quite well, heptane tends to detonate prematurely. This phenomenon is called knock and can severely damage the engine. The greater the ratio of octane to heptane, the more compression the gasoline can handle before it spontaneously combusts.

2015-porsche-918-spyder-engine

A Porsche V8 engine | Porsche

This brings us to the subject of compression ratio (or CR), as it’s the single most important factor in determining what grade of fuel an engine should be fed. The compression ratio refers to the difference between the volume of the combustion chamber when the piston is at the bottom of its stroke, to the volume when it reaches top-dead-center. The greater the difference, the higher the compression ratio, and the more power an engine can produce for a given size.

Most car engines run CRs in the neighborhood of 10:1. In recent years, though, that number has been creeping up as manufacturers chase higher levels of performance and fuel economy. Some modern cars now have compression ratios in excess of 13:1. It’s worth mentioning that diesels use extremely high compression ratios, generally from 14 to 25:1, to initiate combustion, which is why they’re able to achieve greater levels of efficiency compared to gasoline engines.

Miguel Villagran/Getty Images

Miguel Villagran/Getty Images

So, what does all this mean for your car? Basically, the tighter your car’s engine squeezes its fuel, the higher fuel grade you’re going to need to avoid the dreaded pinging or knocking of early detonation. If you drive a car that’s designed for high performance or extreme efficiency, chances are it’s going to need at least mid-grade if not premium fuel to keep it happy. The good news, though, is that if your car isn’t as high-strung, there’s absolutely no advantage to putting a higher-grade fuel than necessary in the tank unless you’re experiencing pinging or knocking.

One persistent misunderstanding about octane ratings is that premium is somehow “cleaner.” Another is that you can get better fuel economy with the pricier stuff. Both of these beliefs are completely false: If your car will run fine on cheap gas, save your money and keep buying 87. Of course, you should never stray from the fuel grade recommended by your owner’s manual, as this could potentially void your warranty.

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