If you’ve ever taken a car to a gas station, you’ve noticed multiple octane choices for fuel. Diesel, 87, 89 and 92 octanes are most common at standard American gas stations, but not everyone knows which fuel is right for their car. Reading your vehicle’s manual helps, but have you ever wondered why one octane is better for your car than another? Thanks to our friends at About.com, here’s a brief summary to help you sort out the octane rating puzzle.

What is the octane rating?

The octane rating of gasoline is a measure of that gasoline’s resistance to knocking. A knocking sound occurs when your internal-combustion engine’s gasoline-air mix ignites prematurely instead of burning smoothly and efficiently. The octane rating is determined after a gasoline is compared to isooctane and heptane. The former sits at the high end of the octane scale (100), while the latter has an octane rating of zero (and knocks like crazy). According to About.com, untouched gasoline has an octane rating of about 70 (70 percent isooctane, 30 percent heptane).  Lead used to be added to gasoline to reduce knocking, but now that emissions standards require unleaded gasoline in the U.S., more expensive compounds are used to raise the octane rating.

Is higher octane always better?

Older engines that used a carburetor to control the air/gas mix benefited from high octane gas. The introduction of sophisticated electronic fuel injectors in the mid-1980s made going for the higher octane rating unnecessary in most cases. The standard unleaded fuel in the U.S. became 87 octane. This is recommended for most cars because gasoline with too high an octane rating can now damage an engine’s emissions system.

What about mid-grade?

There’s a common misconception in the U.S. regarding standard-, mid- and premium-grade gasoline. There isn’t a clear delineation between 87/89/92 octane ratings and gasoline grades; the categorization varies from state to state. One state may require a minimum octane rating of 92 for premium, while another might allow 90. Check your owner’s manual and watch for the octane rating on the yellow sticker at the pump.

What if your vehicle requires premium?

You no doubt have a high-performance engine on your hands. Instead of passing unburned fuel into the emissions system and catalytic converter (which happens when you use gasoline with too high an octane rating and can produce a rotten-egg smell), a high-performance engine uses the fuel efficiently. If you live in a large city with a low air quality index, reformulated gasoline may be required. This kind of gasoline is oxygen-infused and burns more cleanly. So long as you don’t drive with too little fuel in the tank, you’re less likely to dirty the intake valves and destroy your fuel filter. Nobody wants that.

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Sources:

About.com

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