Choosing The Right Gas

By Vicky Katz Whitaker

August 23, 2011 4 min read

With gas prices as high as they are, who could blame you for choosing a cheaper grade when you fill up at the pump?

Only your car's engine.

And maybe your wallet.

It really depends on the car you drive and your driving habits, automotive experts say.

"If your manufacturer recommends that you use 91-grade gas, using a lower grade, such as 89 or 87, will not necessarily harm your engine, but it will affect your car's performance and reduce your miles per gallon by about 3 to 4 percent. Having said that, unless you are an aggressive driver, in most normal driving conditions you will likely not notice any real change," says automotive engineer Robert Hill. But, he adds, "if your manufacturer requires that you use the 91 grade of gas, you should use this grade of gas, to prevent harm and avoid the risk of invalidating your warranty if something were to happen to your engine."

Hill explains that there are three main grades of fuel in use today, "regular (87) mid (89) and premium (91 or 93). The lowest grade will burn faster than the highest, and a fuel that burns more readily has the tendency to 'auto-ignite' before it's intentionally ignited by the engine's ignition system. This can lead to what's known as 'detonation,' or 'knock,' and shock the piston and cylinder walls, seriously damaging an engine if left unchecked."

Most high-end vehicles that recommend premium fuel are outfitted with a knock sensor that measures when the engine is knocking and reduces the ignition timing, points out CarMD.com Corp. Vice President Art Jacobsen, a 20-year veteran of automotive testing and development. "The reason this system exists is that it allows manufacturers to have special settings to squeeze another few horsepower out of the engine. In order to get that nice round 300 horsepower, you may need premium fuel," Jacobsen says.

The type of driving you do can also make a difference at the gas pump, he adds. "If you're cruising on the highway at 65 mph for six hours, then premium fuel isn't going to be so important, and you're better off saving the cash. If you ever hear engine knocking (which sounds as if the pistons are rattling, kind of like a diesel), you should get your knock sensor checked out. It often can fail and may or may not trigger the 'check engine' light to come on."

High-end supercharged engines are the only type that truly need premium gas, adds Jacobsen. "The supercharger rushes air and fuel into the combustion chamber so quickly and so violently that if you slam down on the throttle, the knock sensor may not have time to react and correct the engine timing before knocking. Knocking is a really bad thing."

If you're driving a big SUV towing a heavy item -- particularly uphill -- and your vehicle recommends premium, use it. "This is a fairly taxing situation for the vehicle, and the premium may be worth it," Jacobsen says. "If you hear knocking, that means the pistons in your engine are slapping against the cylinder walls and/or head assembly." That could be an expensive fix. According to the 2011 CarMD Vehicle Health Index, a cylinder head assembly replacement would cost $1,826.

Automotive experts, including Hill, frown on using fuel that's more than 10 percent ethanol. "Consumers should be aware that anything more than 10 percent can reduce performance even more and be harmful to their engines in the long run," he says.

Dan Vespertino, director of service operations for Subaru of America, agrees. The owners manual, he adds, is your best guide to filling the tank.

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