So, what exactly is engine knock? Well, put on your engineering hats for a moment, as we’re going to get a little technical here. Simply put, engine knock (aka “detonation”) is an undesirable phenomenon that occurs when a “left over” pocket of air-fuel mixture in the combustion chamber ignites after the spark plug has already fired. When this happens, cylinder pressure jumps as high as 25 times that of normal combustion, and in doing so creates a sharp metallic noise audible to the human ear. This noise is referred to as “knock”, and left unchecked, it can lead to engine damage ranging from relatively mild to complete engine failure. The extent of engine damage that can occur from knock is highly dependent on the specific output of the engine (horsepower/cubic inches or liters) . . . the higher the specific output, the more extensive the knock damage may be. In addition, external factors influence an engine’s propensity to knock. For example, higher air and/or water temperatures make it harder to cool the engine, and therefore create an easier environment for knock to occur, while higher humidity helps reduce the chances for knock. By now you’re probably wondering “how can I protect my engine from the weather?”
The good news is using high quality fuel with an octane rating that complies with your engine’s requirement is your single best defense against engine knock. This is why certain Mercury Racing consumer outboards and sterndrives require a minimum of 91 octane (98 RON) pump fuel. The higher-octane fuel allows your Mercury Racing engine to safely produce maximum power while protecting against engine knock.
Octane, in fact, is a measure of gasoline’s antiknock performance. There are two test methods used to measure gasoline octane rating. One method results in the Research Octane Number (RON); the other produces the Motor Octane Number (MON). Octane ratings at the pump are typically determined by the following equation:
(RON + MON)/2; commonly written as (R + M)/2. This is called the antiknock index (AKI). In general, a higher-octane fuel, such as 91, provides greater protection against engine knock than a lower octane fuel, such as 87 or 89.