Dear Tom and Ray:

Being one of those who marvels everytime I turn the switch and the car starts, I would like to know why some vehicles need a higher octane gasoline. -- Jeffrey

Tom: The simple answer is "to stop them from pinging," Jeffrey. But since we get paid by the word, we'll give you the complicated answer, too.

Ray: You might be surprised to learn that there is no octane in gasoline. The octane rating is just a measure of how a fuel matches up to a test fuel called iso-octane.

Tom: And the most relevant thing they're comparing is the ignition point of the fuel.

Ray: Why is the ignition point so important? Well, historically, to get more power out of an engine, you increased the "compression" or pre- ignition pressure in the cylinders. But you could increase it only so far, because if you increased the compression too much, some of that fuel would ignite under the high pressure before the spark plug had a chance to fire. That's called "pinging" or "detonation," and it's not good for the engine.

Tom: So how do you avoid pinging? You increase the ignition point of the fuel, so that it doesn't ignite too early. And those gasolines with higher octane ratings have higher ignition points.

Ray: So cars that have high compression/high performance engines require high octane fuel primarily so they don't ping.

Tom: As for the marvel of why the car starts when you turn the switch, we're still working on that one ourselves, Jeffrey. Write to us again in six months. Maybe we'll have it figured out by then.

Changing your oil is the cheapest insurance you can buy for your car, but how often should you change it? Find out by ordering Tom and Ray's pamphlet Ten Ways You May Be Ruining Your Car Without Even Knowing It! To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1996 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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