As engine temperature increases, the air/fuel mixture tends to pre-ignite before the spark plug fires. If the octane rating is too low for a particular engine, it will allow pre-ignition to occur before the piston has completed the compression cycle, thus creating very high pressures very quickly.
Fuel with the correct octane rating will slow the combustion process considerably, giving the spark plug enough time to completely and smoothly ignite the mixture. Using any other fuel, even a very high quality one, is not a substitute for a properly tuned and maintained engine. If your engine exhibits any signs of pinging, detonation or pre-ignition when using regular fuel, you should check the ignition timing against the specification or remove the cylinder head to remove carbon deposits.
All Accords and Preludes covered by this book are equipped with catalytic converters and must use only UNLEADED GASOLINE. Use of leaded fuel will shorten the life of spark plugs, exhaust system, and EGR valves and may damage the catalytic converter. Engines are designed to operate efficiently on unleaded gasoline with an octane rating of at least 91. Use of unleaded gasoline with an octane rating lower than 91 may cause persistent spark ignition on impact, which may damage the engine.
A slight knock may be noticed when accelerating or when driving uphill, especially in carbureted engines. A slight knock can be considered normal (with 91 gasoline), because maximum fuel economy is achieved under conditions of minimum knock. It is possible to use gasoline with an octane number higher than 91, but this is not at all necessary for normal operation of a well-tuned engine.
The fuel requirement of your engine may change over time, mainly due to carbon deposits that change the compression ratio. If your engine is knocking, detonating - first change to another BRAND of fuel, but not with a higher octane number. Each oil refiner adds certain chemicals to the fuel; some engines prefer ONE brand.