It is important to use fuel of the proper octane rating in your car. Octane rating is based on the quantity of anti-knock compounds added to the fuel and it determines the speed at which the gas will burn. The lower the octane rating, the faster it burns. The higher the octane, the slower the fuel will burn and a greater percentage of compounds in the fuel prevent spark knock, detonation (pinging) and pre-ignition (dieseling).
As the temperature of the engine increases, the air/fuel mixture exhibits a tendency to ignite before the spark plug is fired. If fuel of an octane rating too low for the engine is used, this will allow combustion to occur before the piston has completed its compression stroke, thereby creating a very high pressure very rapidly.
Fuel of the proper octane rating, for the compression ratio and ignition timing of your car, will slow the combustion process sufficiently to allow the spark plug enough time to ignite the mixture completely and smoothly. Many non-catalyst models are designed to run on regular-grade fuel. The use of some super-premium fuel cannot hurt, but is no substitute for a properly tuned and maintained engine. It is a good practice, however, to use fuel with detergent additives that can help to prevent combustion chamber deposits. This is particularly important for vehicles with fuel injection. Chances are that if your engine exhibits steady signs of spark knock, detonation or pre-ignition when using regular, detergent fuel, the ignition timing should be checked against specifications or the cylinder head should be removed for decarbonizing.
Vehicles equipped with catalytic converters must use UNLEADED GASOLINE ONLY. Use of leaded fuel shortens the life of spark plugs, exhaust systems and EGR valves and can damage the catalytic converter. Most converter-equipped models are designed to operate using unleaded gasoline with a minimum rating of 87 octane. Use of unleaded gasoline with octane ratings lower than 87 can cause persistent spark knock which could lead to engine damage.
Light spark knock may be noticed when accelerating or driving up hills. The slight knocking may be considered normal (with 87 octane) because the maximum fuel economy is obtained under condition of occasional light spark knock. Gasoline with an octane rating higher than 87 may be used, but it is not necessary (in most cases) for proper operation.
If spark knock is constant, when using 87 octane, at cruising speeds on level ground, an ignition timing adjustment or other service may be required.