Fuel & Its Effects On DRIVEABILITY: Octane: Notes

Fig 1: Identifying Normal Combustion

Courtesy of GENERAL MOTORS CORP.

A gasoline's octane number is a measurement of its antiknock capability. Increasing the octane from 87 (regular) to 89 (mid-grade) or 91 (premium) does not increase power or miles per gallon, nor does it keep the engine's internal parts cleaner. Increasing the octane only increases a fuel's resistance to spark knock.

In 1985, when port fuel injection was new, the plugging of tiny passages in the injectors with fuel deposits became a serious concern. Because deposit control additives were not available in many regular-grade fuels, higher grades were sometimes recommended to keep injectors clean. Today, redesigned injectors are more resistant to plugging, and deposit control additives are generally used in all grades of fuels.

With today's fuels, there is absolutely no reason to use high-octane fuel except to reduce spark knock or pinging.

Fig 2: Identifying Abnormal Combustion (Spark Knock/Detonation)

Courtesy of GENERAL MOTORS CORP.