by Jim Kerr

Deciding what grade of gasoline to use in your vehicle has been a question asked by many readers. Should I use regular, or is it worth spending more for the premium grade fuel? While premium grade fuel may contain a few more additives, most vehicles will operate fine on regular grades. There are some vehicles out there where premium fuel is specified by the manufacturer. What happens if regular fuel or gasohol is used in these vehicles?

Premium fuels have a higher resistance to pinging or detonation in the engine’s combustion chamber than regular fuels do. Higher compression ratios, higher heat loads and higher vehicle loads all create an environment in the engine where the fuel tends to self ignite before the spark plug can properly fire the mixture. When this occurs too much, severe engine vibrations occur internally that can break piston rings, pistons and even damage bearings. We sometimes hear this as a rattling sound from the engine compartment when the vehicle is accelerated hard.

When looking at vehicle fuel requirements in the owner’s manuals, it will state whether regular fuel can be used, premium fuel is recommended, or premium fuel is required. It also often has this printed right at the fuel filler. If the specifications say premium fuel is recommended, you can use regular fuel for many driving conditions, especially if you are not placing high heat or acceleration loads on the vehicle. If any pinging does occur, the engine knock sensor will detect this and cause the ignition timing to retard. This will reduce the pinging, but can decrease performance and fuel economy. If premium fuel is required, use what the manufacturer recommends.

Typically, when the manufacturer shows fuel requirements, they also list the minimum octane rating. It may typically be 87 octane if regular fuel is required or 91 octane rating for vehicles requiring premium fuel. Gasohol, a common name given to gasoline with small percentages of alcohol added to them has a higher octane rating than the gasoline by itself. This is because the alcohol evaporates quickly in the combustion chamber and absorbs heat that would cause pinging or detonation. Therefore, any gasohol type fuel with an octane rating equal or higher than the premium fuel will work fine in vehicles requiring premium fuel grades.

Using fuel with alcohol in it does tend to lean the fuel mixture, but the oxygen sensor in the exhaust system detects that the engine is operating slightly lean and enriches the air/fuel mixture slightly, so the engine will burn slightly more fuel. However, there is probably more difference in fuel economy made by other driving factors such as outside temperatures, tire pressures, or road conditions than the fuel would make.

Using gasohol in engines such as cars from the 70’s or earlier, older snowmobiles, motorcycles or lawn equipment that do not have oxygen sensors will cause the engine to operate lean. If the air fuel mixture was already on the rich side, then it will actually run better, but if the air fuel ratio was already lean, then leaning the fuel mixture further can cause pistons to melt. The carburettor may need to be adjusted to compensate for the different fuel.

All auto manufacturers allow gasohol with up to 10% alcohol in the fuel in vehicles equipped with oxygen sensors built in the last 15 to 20 years, so using gasohol is not a problem. As a point of interest, you may notice that some manufacturers such as GM, Ford and Chrysler are also marketing vehicles compatible with E85. This fuel is 85% ethanol and 15% gasoline. Currently, there are only a couple filling stations in Ottawa that are supplying this fuel, but the situation could change rapidly as ethanol production is ramped up to offset gasoline shortages in the future.

E85 is used instead of straight ethanol for a couple reasons. First, the addition of gasoline to the ethanol makes it unfit for human consumption (a potential problem with pure ethanol). Secondly, alcohol does not evaporate as easily as gasoline at very low temperatures, so adding gasoline makes the engine easier to start when the thermometer drops.

When pulling up to the pumps, check out the octane rating for each grade of fuel shown on the pumps. For most of us, regular grades will work just fine. Don’t spend money on premium grades unless your car really needs it.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist’s Association of Canada (AJAC).

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