1 year ago
Last edited at 4:12PM on 2/5/2013
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.