TNT Custom Marine is well known as the Platinum Mercury Dealer in the South Florida region. They have been the recipient of the Mercury CSI Award for the past 10 consecutive years. If you have any questions or service needs contact TNT Custom Marine. In the meantime, check out this recent story released by Mercury Racing.
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.