Don’t Knock It!

Mercury Racing issues informational article on detonation caused by low-octane fuel.
Mercury Racing 400R
The 400R’s knock protection system provides the ultimate flexibility by allowing you to maximize performance on 91 octane without having to compromise where you run your boat based on available fuel grades at the gas dock. Mercury Racing

Mercury Racing ace communications manager Rick Mackie has produced an excellent article explaining the effect of detonation on Mercury’s high-performance engines caused by low-octane fuel. Mackie also addressed the use of octane booster products, which Mercury Racing does not recommend. The bottom line: Protect your engine by feeding it quality fuel with the specified octane rating. A segment of the article below is posted below. To read it all, go to Mercury Racing.

Currently, the 400R is the only Mercury Racing engine with a built-in knock protection system. This means with other Mercury Racing outboards and sterndrives, it is absolutely necessary to comply with specified fuel requirements for each engine. Using a fuel with an octane number lower than an engine’s specified rating will likely result in engine failure, which is not covered by Mercury Racing’s limited warranty. While most fuel grades are readily available for boats that are trailered, it is important to understand what is sold at the gas docks for boats that are kept in the water or run for long distances.

Mercury Racing Fuel
Mercury Racing Fuel Requirements Mercury Racing

Most marinas today are carrying Rec 90, a cross between 91- and 89-octane pump fuel. In other areas, the highest available fuel rating is often 89 but sometimes 87 octane. This is a huge gamble if your engine requires 91-octane fuel and is not equipped with knock control, a problem that often leads to serious engine damage.


As a result, it comes as no surprise that many of Mercury Racing customers turn to aftermarket products (fuel additives) promising to raise the octane in their boats to acceptable levels that comply with our specified fuel requirements.

To date, we have not been able to validate the effectiveness of any aftermarket octane-boosting products on any Mercury Racing product. Thus, we do not recommend or support using them as a substitute for using the specified minimum-octane fuels our products require.

Don’t risk it. Top off your tanks with the correct fuel grade specified for your Mercury Racing engines prior to your day on the water. Plan your route to ensure you have access to your required fuel when needed. Your engines will thank you with top performance and unwavering reliability.