Dear Tom and Ray:

I've been enjoying your amusingly informative column now for many years thinking I might never need your help. This despite the fact that I gave my stepdaughter my '82 Chevy Caprice station wagon with over 200,000 miles on it! But it was a good swap. I now have her mom's '89 Volvo 240 (as well as her mom!). Both my car and I have much more mileage on us than she or her car! Now for the question. The Chevy had a much more powerful eight-cylinder engine and ran great on regular gas. The Volvo is only a four-cylinder engine, yet both the manual and the Volvo employees say it must run only on 89 octane gas. Are they nuts? Or do they know something I don't know? -- Floyd

Tom: They know about compression ratios, Floyd.

Ray: You've probably noticed that the Volvo is more sluggish than the Chevy was, right? But it's not HALF as sluggish, even though it has only half as many cylinders.

Tom: And that's (partly) because the Volvo engine is squeezing more power out of each cylinder in part due to a higher compression ratio.

Ray: The compression ratio is the difference between the volume of the cylinder before and after the compression stroke. What does that mean in plain English? It's a measure of how much the engine compresses the fuel-and-air mixture just before the spark plug ignites it. And the more compressed the mixture, the more power it produces when it "explodes."

Tom: And our Mitchells database tells us that the Chevy's compression ratio is 8.6 to 1, whereas the Volvo's is a much higher 9.8 to 1

Ray: So why the higher-octane gas? The more you compress the gas and air, the more likely they are to pre-ignite; that is, ignite just from the pressure and heat, before the spark plug even makes a spark. That's called "knocking," "pinging" or "detonation," and all those extra, early explosions are bad for the engine.

Tom: Higher-octane gasoline is simply gasoline with a higher ignition point. That means it takes a higher temperature to ignite the 89 octane than it takes to ignite the 87. So using 89 eliminates the pre-ignition in this engine.

Ray: Some high-compression engines require even higher-octane gas, 91 octane, to prevent detonation. And that's why not all cars have them. High-compression engines are usually found on more-expensive cars, where the buyer cares more about a few extra horsepower than about a extra few bucks at the gas pump every week.

Tom: So swallow hard and fill it up with 89, Floyd. Don't worry, you still made out OK. Even without factoring in the kid's mother, you still got the better end of this deal.

To buy or not to buy -- options, that is. Are options worth what you pay for them, or are you better off just going with the basics? Order Tom and Ray's pamphlet "Should I Buy, Lease, or Steal My Next Car?" to find out. To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1997 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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