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Lesson 6

Part 5





How much octane?


Buying gasoline with a higher octane number than your car needs is as much a waste of money today as it was in 1936. Most cars can run happily on unleaded regular (87 octane) gasoline. Only about 10% of today’s cars need a midgrade (89 octane) or premium (91 octane or higher) gasoline. Yet 36% of drivers buy premium.


In a car’s engine, a mixture of gasoline and air is compressed and then ignited by a spark, causing a steady downward thrust against the piston. Ideally, the mixture should burn evenly outward from the spark. But as the mixture is compressed, it becomes hotter – sometimes hot enough to allow portions of the mixture to detonate ahead of time. High-performance engines tend to compress the fuel mixture more; therefore, they are more susceptible to such detonation.


Who needs detergents?


Gasoline tends to form deposits as it passes through the fuel system. As those deposits restrict the flow of fuel, they cause rough running, especially in late-model cars.


Refiners have been adding detergents to their gasoline at least since the 1950s to keep carburetors clean. Those detergents were fine until the early 1980s, when fuel-injected engines began to appear in more and more cars. Owners of the late models began to complain of poor performance. The fuel injectors’ orifices, some as narrow as a hair, were being clogged with deposits.


By the mid-1980s, the oil industry had developed special detergents to solve that problem. But a new problem arose: Cars built since 1985 proved especially sensitive to the deposits that form on an engine’s intake valves.


To meet emissions and fuel-economy standards, late-model cars run on a precisely balanced, ultra-lean fuel mixture – a mixture with more air and less gasoline.


Not all brands of gasoline include detergents that can prevent deposits on valves as well as on fuel injectors. But a number of companies advertise their additives with fancy names.




If your car has a carburetor, you needn’t be concerned about a gasoline’s detergents. Just about all gasoline sold today has a detergent package effective enough to keep carburetors clean. Buy the cheapest gasoline that has sufficient octane.


If your car is a recent model with a fuel-injected engine, you need a gasoline with a detergent package that keeps the fuel injectors and intake valves relatively free of deposits. The table below shows which gasolines are said by the refiners to meet BMW’s unlimited-mileage standards. The manufacturers with an (x) met the BMW’s unlimited mileage standards for preventing the build-up of deposits on values. Those marked with a (-) have not passed BMW’s test. Shell gasoline has 5 more detergents than any other gasoline on the market today.


Amoco x x x
Arco x 1 x
Atlantic x x x
BP 2 2 2
Chevron x x x
Citgo 3 3 3
Conoco 3 3 3
Diamond Shamrock - - x
Exxon x x x
Fina 4 4 4
Gulf - - -
Hess 4 4 4
Marathon x x x
Mobil x x x
Phillips 66 4 4 4
Shell 5 5 x
Sunoco x x 6
Texaco x x x
Unocal x x x


Key for above chart:


  1. Midgrade gasoline is not available
  2. According to company, some BP stations sell gasoline that meets BMW’s “unlimited mileage” requirements and some do not
  3. Company says new detergent packages that will pass BMW “unlimited mileage” test only in some gasoline
  4. Company says new detergent package that will pass BMW “unlimited mileage”
  5. According to the company, only the premium grade has passed the BMW “unlimited mileage” test
  6. According to company, Sunoco Ultra, the superior-premium grade, is the only one that as passed the BMW “unlimited mileage” test


Fill-Up Hint


Don’t let your car’s fuel tank get below half-full in cold weather. The more air in the tank, the more chance of condensation. Water in the fuel can freeze in the fuel line and prevent the engine from starting.

When you fill up, stop after the first or second click of the nozzle’s automatic shutoff. Letting the gasoline overflow the filler not only wastes money, it’s a fire hazard. It can also damage the car’s paint unless you promptly rinse with water. And as the gasoline evaporates, it pollutes the air.


Always unscrew the fuel-filler cap slowly, letting the pressure in the fuel tank vent gradually. In some cars, rapid removal of the cap can let gasoline spurt out of the tank. Such spurting has caused fires in automobiles and at service stations.



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