Higher octane gasoline did reduce engine knock in older engines that used carburetors to regulate the air/gas mix. The older engines could not regulate the air/fuel mix going into the engine as efficiently as a computerized fuel injector. A carburetor in need of adjustment could cause too much fuel to be mixed with the air, which meant the gasoline would not burn completely. The excess gas soaked into carbon deposits and caused a premature ignition of the gasoline from the heat of the engine cylinder. The premature ignition made a sound that came to be known as 'engine knock.' When this happened, people would change to the higher octane/slower burning gasoline to resist the premature burn, thus minimizing the knock. Upping the octane was beneficial then, but engines and gasoline formulations changed.
Since the mid-1980s engines use fuel injectors with computers to accurately control the air/fuel mix over all temperature and environment ranges. The accuracy of the fuel injectors and computers is based on using the recommended gasoline for that engine. Most cars are designed to burn regular unleaded gas with an octane rating of 87. If the vehicle needs a higher octane rating this requirement is noted in the owner's manual and usually under the fuel gauge and by the gas tank.
Gasoline Factors That Matter
The quality of gasoline and the additive package usually affect the rate of engine wear more than the octane rating. Basically what this means is that it matters more where you buy your gas than which grade you purchase.
Regular Unleaded Gasoline
The recommended gasoline for most cars is regular 87 octane. One common misconception is that higher octane gasoline contains more cleaning additives than lower octane gas. All octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against engine deposit build-up. In fact, using a gasoline with too high of an octane rating may cause damage to the emissions system.
Gasoline and Octane Ratings - This article defines octane ratings and provides an overview of how gasoline in made.
The octane ratings 'regular', 'mid-grade', and 'premium' are not consistent. In the United States, for example, one state may require a minimum octane rating of 92 for premium gasoline, while another may allow an octane rating of 90 to be premium. Check the octane rating on the yellow sticker on the gas pump rather than relying on descriptive labels.
Certain high performance engines benefit from use of high octane fuel. For other engines, using a fuel with a higher octane rating than the vehicle requires sends unburned fuel into the emissions system and catalytic converter. This puts unecessary stress on the emissions system. For some vehicles, a rotten egg smell coming from the tailpipe signals use of too-high octane gas.
Many countries continue to use leaded gasoline, even though lead exposure has significant health and environmental consequences and the cost of switching to unleaded gasoline is relatively low. Although greatly improved, research indicates significant health and environmental effects from use of leaded gasoline remain even in countries that have switched to unleaded fuel.
Synthetic and Reformulated Fuels
Some major cities with air pollution problems require the use of reformulated gasoline. Reformulated gasoline is an oxygenated fuel that burns cleanly but can lower fuel economy and engine performance slightly. Reformulated gasoline may cause pinging or premature burn in engines with excessive carbon deposits. Older/dirtier engines may benefit from stepping up to the next grade of gasoline.