As the engine temperature rises, the air/fuel mixture tends to ignite prematurely before the spark plug ignites. If the octane rating is too low to be used on a particular engine, it will allow pre-combustion to occur before the piston has completed its compression cycle, thus building up very high pressure fairly quickly.
Fuel with the correct octane rating will significantly slow down the combustion process, giving the spark plug enough time to fully and gently ignite the mixture. The use of any other, even very high-quality, fuel is not a substitute for a normally tuned and serviced engine. In the event that your engine exhibits any signs of impact ignition, knocking, or pre-ignition when using conventional fuel, check the ignition timing to specification or remove the cylinder head to remove carbon deposits.
All Accords and Preludes covered in this book are equipped with catalytic converters and must use LEAD-FREE PETROL only (UNLEADED). The use of leaded fuel will shorten the life of the spark plugs, exhaust system and gas recirculation system valves and may also destroy the catalytic converter. Engines are designed to operate efficiently when using unleaded gasoline with an octane rating of at least 91. The use of unleaded gasoline with an octane rating of less than 91 can cause permanent ignition on impact, which can damage the engine.
You may notice slight knocking when accelerating or when going uphill, especially with carbureted engines. Small knocks can be considered normal (at 91 gasoline), because maximum fuel economy is achieved under minimum knock conditions. It is possible to use gasoline with an octane rating higher than 91, but this is not at all necessary for the normal operation of a well-tuned engine.
The fuel requirement for your engine may change over time, mostly due to carbon deposits that change the compression ratio. If your engine knocks, detonates - first go to another brand of fuel, but not with a high octane number. Every oil refiner adds certain chemicals to the fuel; some engines prefer ONE brand.