Octane ratings explained!

Dear Car Talk

Dear Car Talk | Jul 01, 1995

Dear Tom and Ray:

For umpteen years, I have had a choice of 87 octane, 90 octane, and 92 octane gas. I always purchased 87 octane because it was the least expensive. And my cars have always performed well on 87 octane. I now have a car that has a sign on the dash that says "Premium Unleaded Fuel Only." Why does the manufacturer tell me to use 92 octane, and how important is it that I pay attention to this?

RAY: I can tell you're a cheapskate, Norma. They put a big sign right in front of your face on the dashboard so you can't possibly miss it, and instead of spending a few extra bucks a week, you stall, hem and haw, and write to us to ask if you really have to do it.

TOM: Ahh. A woman after me own heart!

RAY: And because you're--shall we say--frugal, you're going to be really bent if you ruin the engine on your new car and have to pay for a new one, aren't you? In which case it's very important that you follow the manufacturer's instructions for octane ratings--whatever they are.

TOM: Here's why. The lower the octane, the lower the temperature at which the gasoline explodes in the cylinders. And in certain "high compression" engines like yours, 87 octane gasoline explodes too early. Those early explosions are known as "pinging" and they eventually cause engine damage.

RAY: Premium gas (probably 91 or 92 octane, whatever is specified in your owner's manual) explodes when it's supposed to in your engine, and that's why the manufacturer of your car requires it.

TOM: In general, you should always follow the manufacturer's recommendation when it comes to octane. Using a lower octane can harm the engine over time, and using a higher than called- for octane is a complete waste of money.

RAY: And I guess that's the good news we can give you, Norma. You don't have to waste your money springing for 94 octane. You can stick with the nice, cheap, 92 octane stuff.

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