Heed High-Octane Specifications

Many motorists believe that higher-octane gasoline, typically a premium-grade fuel, will yield higher performance and better economy.

Gasoline retailers often encourage this notion, either explicitly or through subtle advertising that extols the benefit of their higher-priced brands.

You might think that with computer-controlled engines, this hype would have passed from the scene along with carburetors and leaded gasoline. But sellers of high-octane gasoline would have you believe that higher-octane fuels can trick your computer into better performance.

The issue is fairly complex, but it goes to the heart of whether you need to use premium or regular gasoline in your car. For a motorist who drives a typical new car 25,000 miles annually, premium fuel can cost more than $1,000 over the life of the car.

Computers cannot be tricked into higher performance. The engine controllers that operate today’s modern engines are programmed to operate within specific ranges, and the octane rating on the gasoline will not substantially change the engine’s operation for the better.

But using fuel with too low an octane rating can cause a loss of power and economy. Some modern computer-controlled engines have a feature called a knock limiter that will automatically tune down an engine to eliminate or limit knock.

When an engine knocks or pings, typically during acceleration, it means that the combustion of fuel inside the engine is uncontrolled. The knock or ping can be hard on an engine and in severe cases can actually damage the piston.

The octane rating on the gasoline is a measure of the fuel’s ability to resist this uncontrolled combustion: As the octane rating of the gasoline rises, the engine is less likely to ping. If the engine does ping, however, a knock limiter inside the computer will try to stop the ping by retarding the spark plug timing.

The mistake a lot of motorists make is thinking that the opposite happens when you use higher octane than you need. They think the computer advances the spark timing to the point where the engine will ping.

But the software that controls the ignition timing has what is called a mapped range. That means that at any given speed and engine load, the computer is going to keep the timing within a fairly tight range. Some engineers call it the engine’s recipe.

Unless your engine is specifically rated for premium fuel, you should not use it.

Question: The brake pads on my 1986 Honda Accord have wear indicators that squeal when the pads wear low. Mine recently began to squeal even though there seemed to be a lot of brake lining left. Should I ignore the wear indicators and keep driving?

Answer: The pads may look like they have a lot of braking material left, but there is a layer of different braking material intended to act like an insulator, preventing heat from transferring to other brake parts, particularly the caliper.

This material is brake lining also, but its friction characteristics are different than those of the rest of the pad, and you might get uneven braking with it. For that reason, the wear indicators sound before you actually reach the insulating material.

The manufacturer did not intend for you to drive on the insulator material, though many people have done so.