Premium in your tank may be a waste

About one-third of all motorists who pull up to a Mobil service pump put in premium fuel. Are some of them wasting their money? Some people in the auto and gasoline industries think so. Joseph Colucci, head of the fuels and lubricants department at the General Motors Technical Center here, says, ``We believe the oil industry is selling a lot more premium fuel than is necessary.''

Charles Morgan, a fuels expert at Mobil Oil Company, agrees, saying, ``We recognize that there are some people who buy super unleaded and who do not need it.'' On the other hand, other motorists refuse to upgrade their fuel despite engine knocking and poor performance, Mr. Morgan reports.

Oil companies and carmakers participate in an annual survey of octane requirements in new cars. Morgan reports that ``about one-third of our customers are dissatisfied with the performance of their cars with our 87-octane regular unleaded fuel.'' At the same time, he adds, ``about one-third of all its fuel sales are super unleaded.''

The ``overbuyers'' appear to offset the ``underbuyers.''

``If someone has a special requirement in terms of octane, knocking, or detergency, he can pay the higher price for super unleaded,'' Morgan explains. ``Otherwise, in our view, it's a waste,'' he adds.

``If your car is knocking, you don't have to go from regular to premium fuel to solve the problem,'' says GM's Colucci. ``All you have to do is be your own blender if you patronize a self-service station. Put in regular as well as premium unless the station allows you to blend it at the pump. Or you can put in a tankful of regular-grade fuel and then a tankful of premium the next time around.'' The GM fuels expert says a little bit of knock won't hurt anything, anyway. He warns, however: ``If you get into persistent high-speed knock, you'd better do something about it.

``For the past 15 years, we haven't seen any indication that there is a knock-induced engine-damage problem with unleaded gasoline in the hands of the general public, although there may be in isolated cases.''

The size of the engine does not determine the fuel requirement for a car. Instead, it's the design of the combustion chamber, spark timing, how the exhaust-gas recirculation is timed, temperature at which you run the cooling water in the radiator, and the temperature and humidity of the ambient air.

Up to now, GM has called for regular unleaded fuel in all its vehicles, although with two engines, both turbocharged, it has given the motorist a choice. The two engines, the 3.8-liter turbocharged Buick and 2.8-liter turbocharged Pontiac, account for less than one-half of 1 percent of GM's total engine output. For 1987, GM is expanding its recommendation -- either regular unleaded or premium unleaded -- to the 5.7-liter, port-fuel-injected engines in the Camaro, Firebird, and Corvette. The Cadillac Allante, with its 4.1-liter Cadillac engines, calls for premium unleaded only.

``If you can spend $50,000 for the Allante,'' Colucci quips, ``the difference between premium and regular gasoline shouldn't make a difference.''