The contra-rotating propeller provides more “grip” underwater, and because contra-rotating props distribute the engine torque evenly over two propellers, the torque per propeller decreases and gear diameter can be reduced. This makes a reduction in gear diameter possible, which could then lead to the design of a smaller, more hydrodynamic gear case.
Injecting fuel does two things, it atomizes the fuel and, incidentally, it cools the cylinder to minimize knock. To achieve the power we wanted, we needed to inject 100% of the fuel into the cylinder all at once, at a precise time, and at a precise angle to both cool the cylinder and to allow combustion to take place in the center of the combustion chamber.
The new Dual Injector System using two smaller injectors provided the required precision as well as improved atomization, increasing output by 3% without causing knocking.
12.0:1 COMPRESSION RATIO
Our solution to developing 80 horsepower per liter in a V6 350 horsepower engine was to increase the compression ratio to 12.0:1, the highest compression ratio ever for a production outboard engine. In order to make this work without knocking (a typical problem at this ratio), we developed systems to mix cooler air with well-atomized fuel to provide optimal conditions for complete, and controlled combustion.
DUAL WATER INTAKES
On the DF350A, the best results were achieved when the main intake was positioned at the front of the gear case, with the secondary intake located just above the skeg. This was our ultimate solution to arranging the main and sub intakes as far apart from each other as possible, and designing them to ensure a reliable amount of cooling water, especially at high speeds.
DESIGNING STRONGER REVERSE THRUST
The gear material was changed and heat treatment was added to endure the added thrust and inertial mass from the reverse thrust, which was stronger on a contra-rotating propeller. In addition, the reverse gear was moved above the cavitation plate and the position of the lower engine mounts was modified from parallel to V-shape to fit the reverse gear above the cavitation plate.
BETTER STEERING IN ALL DIRECTIONS
Normally, when using multiple outboards a combination of standard and counter-rotating engines is mounted. The Suzuki Selective Rotation, available on our AP series outboards, eliminates the need for different models, as any model can be easily programmed to run in either direction. The DF350A’s contra-rotating propeller technology takes this process one step further by eliminating steering torque and maximizing true and straight propulsion forces.
PROPELLER BLADE DESIGN
A new three-blade front/three-blade back propeller set-up was developed that provided improved performance on the water. This new set up provided not only the highest recorded speeds for the DF350A, but also delivered incredible acceleration, even under heavy load and at high rotation speeds.
DIRECT INTAKE SYSTEM AND DUAL LOUVER SYSTEM
At high speed, water particles could be directed, trapped, and drained away. Our experienced engineers worked on applying this theory to practice by increasing the intake flow to convert water vapor to particles and designing blades to capture and deflect those particles away from the intake flow. The Dual Louver System incorporates a double shield of blades, each one designed in a dog-leg shape. The outer row of blades removes the spray from the boat and the inner louvers capture and drain the remaining mist. As a result, intake air temperature is free of moist and no higher than 10° above ambient.