In relatively simple terms, it’s a measure of the fuel’s resistance to premature detonation, or the early ignition of the fuel/air mixture by compression before it would normally be ignited by the firing of the spark plugs in the engine’s combustion chamber.
This early ignition of the fuel/air mixture creates a shock wave within the cylinder which collides with the still upwardly moving piston, knocking it against the cylinder walls. The resulting noise is often detected by the operator as a “knock” or “ping” coming from the engine, and over time can cause significant engine damage.
A fuel’s octane rating can be measured in several ways. In the United States and Canada, the most common rating shown on the gas pump is the Anti-Knock Index (AKI), which is the average of the fuel’s Research Octane Number (RON) and Motor Octane Number (MON).
This is typically shown at the pump by the abbreviation R+M/2, on a bright yellow label with black lettering. In other countries, the Research Octane Number may be more common.
Your Sea-Doo engine is designed to run most efficiently with fuel of a specific octane rating. For all engines except the 300hp Rotax 1630 ACE - 300, look for unleaded fuel with an AKI (RON+MON)/2 octane rating of 87 (or an RON octane rating of 91).
Craft using the high-performance ACE - 300 engine require a higher octane level for best performance, fuel economy and engine life. Use Premium gasoline with an AKI (RON+MON)/2 octane rating of 91 (or an RON octane rating of 95).
These octane requirements can also be found in the Sea-Doo Operator's Guide that comes with each craft.