What Gas Is Better for Your Car?

When selecting which grade of gasoline to use in a car, various facts and rumors about the different grades of fuel come into play. While some of these common beliefs are, in fact, true and important to engine maintenance, others are no longer relevant (and some are complete fabrications). Knowing the truth about fuels will help you choose what is best for your car.

Octane

  • Gasoline is sold in a number of different grades, which are named for their octane rating. Octane ratings refer to how resistant to combustion the fuel is. For example, gas with a 91 or 93 octane rating is more resistant to ignition, while 87 or 89 octane fuel will burn more quickly or at a lower temperature. Octane ratings come from tests in which a sample of the gas is compared with a mixture of iso-octane (an isomer) and n-heptane (an alkane). The final octane rating refers to the percentage of iso-octane needed to reproduce the characteristics of the sample gas.

Engine Knocking

  • The most common reason cited for the use of higher octane, or "premium" gasoline, is to reduce or eliminate engine knocking. Engine knocking is actually premature ignition within the combustion chamber and can cause long-term engine damage over time. In most older cars, it occurs when carburetors begin to lose their calibration and allow too much fuel into the combustion chamber, resulting in an early ignition. Switching to a higher-octane gasoline will, in fact, solve this problem when it is too costly or inconvenient to adjust or replace the carburetor.

Manufacturer Specifications

  • Most cars come with manufacturer specifications that include a minimum octane rating to be used. Since most modern cars use computer-controlled fuel injectors instead of carburetors, this rating in 87 (often the lowest octane sold at a fueling station). If higher octane fuels are used anyway, the fuel injectors will be able to self-adjust. Many cars that come with recommendations for mid-grade fuels (89 or 91 octane) can actually be run using 87 octane gas. While these cars may experience a slight loss of power, if no engine knocking is experienced when 87 is used, then the fuel injectors have proven to adjust to the lower grade.

When to Use Premium

  • In few cases, premium gas is actually necessary. This includes cars with high-performance engines, such as some sport or luxury models. In these cases, manufacturer specifications will require (rather than simply "recommend") using a 91 or 93 octane-rated gasoline. This is due to a higher compression ratio in the combustion chambers, meaning that gas with a lower octane is likely to cause early combustion and, consequently, engine knocking.

Detergents

  • One popular myth involves the presence of more and better detergents in higher-octane gasoline, promising a clean engine and more efficient operation. In fact, federal mandates require all grades of gasoline to contain the same cleaning agents to limit the amount of harmful emissions coming from cars. While detergents may vary from one brand of gas to another, most commercially available gasoline performs the same amount of cleaning.