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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. 


400R and Automatic Knock Protection 

All Mercury Racing outboards and sterndrives require a certain minimum fuel octane to protect against knock while maximizing performance. The Verado 400R outboard takes it one step farther with an advanced computer controlled knock protection system. The 400R produces its advertised horsepower at 7,000 RPM on 91-octane (98 RON) pump fuel; however, the engine control unit will automatically adjust spark timing on individual cylinders should it start to detect engine knock. The amount of spark removed and subsequent power reduction is highly dependent on ambient conditions (water and air temperature, humidity) and other factors. 

The great thing about the 400R’s knock protection system is it is designed to always give you as much power as possible under all conditions while still protecting the engine from knock damage.  Running your 400Rs with the recommended 91 octane fuel will help ensure you always have the full 400 horsepower at your fingertips, but sometimes on the water, 89 octane or Rec 90 is the best you can find.  Don’t sweat it . . . your 400Rs will run safely and reliably on this fuel as well, you just may not see the same top speed you will with the premium fuel.  This 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. 

What About Other Race Product? 

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. 

Fortunately, all but three of Mercury Racing’s outboard and sterndrive products are designed to run on 87 or 89 octane (see chart below for octane requirements by engine model).  Since most gas docks carry 89 or Rec 90, the vast majority of Racing product may be operated virtually anywhere.  The consumer Race product which requires 91 octane includes the 300XS outboard and the QC4 1350 sterndrive.  Finally, the dual cal 1350/1550 QC4 consumer model requires 91 octane in 1350 mode and 112 octane race fuel in 1550 mode.  Typically race fuel (112+ octane) is not readily available and must be ordered in advance for speed runs or competition race activities.  Recommended race fuels per the chart below are Sunoco Supreme 112 AKI or Sunoco 117 MON, VP C16 or equivalents. 



Can’t I Just Add Octane Boost? 



As previously mentioned, 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, one that often leads to serious engine damage. 

As a result, it comes as no surprise that many of our customers turn to aftermarket products (fuel additives) promising to raise the octane in their boats to acceptable levels which comply with our specified fuel requirements.  Internet forums are full of discussion threads arguing whether these products actually work.  Here at Mercury Racing, we are frequently asked for our opinions on these additives. 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. 

Many of these products advertise that they boost the octane by a certain percentage or factor when mixed with the fuel at a certain ratio.  It is unclear how much of the product is truly required to boost the fuel octane in the tank by a full point.  Higher volume fuel tanks found on most powerboats or center consoles would likely require a substantial amount of octane booster, and even at that, there is risk the final octane in the tank may still not be enough to meet the specified requirement. This is a major risk to take with your high dollar investment, and one that would not be covered by Mercury Racing’s limited warranty. 

Our advice?  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.