When an engine knocks, one of the likely causes is low-quality fuel. Fuel of low quality or a low octane rating results in many different problems in the engine. This fuel rating informs the consumer of the amount of compression gasoline is able to tolerate. According to HowStuffWorks, these problems are why high-performance engines require higher-octane fuel to prevent knocks.
Low-octane fuel leads to pre-ignition, causing the knocking noise. Internal combustion is a delicate process that has the potential to go awry. While the design is for the spark plug to ignite the fuel, compression issues cause it to burn before the plug has a chance to spark. These issues lead to incomplete combustion, leaving pieces of debris that stick to the cylinder, another common cause of engine knock.
Engineers design engines for use with a particular spark plug. Installing the wrong plugs in the engine affects the combustion conditions. They sometimes cause the engine to run too hot or too cold and knock. They can increase pre-ignition and the buildup of deposits in the combustion chamber, increasing the existing knock.
Dirty fuel injectors, worn valve seals or rings or a bad knock sensor are other common reasons for the engine to knock.