Honda’s outboard motors have evolved from Honda’s original design philosophy. Leveraging the strengths of Honda’s outboard range, which include outstanding durability, reliability, and economy, the large BF350 outboard motor was developed for leisure use and other water activities.
With an exclusive 5-liter V8 engine, the powerful BF350, which is Honda’s flagship model, produces a maximum propulsive force of 350 horsepower. In addition to excellent dynamic performance (acceleration and top speed) and class-leading fuel efficiency*1, the BF350 is quiet enough to enable occupants to chat while cruising, with minimal vibration making occupants comfortable even on long trips. Combined with the Shiphandling Assist function, it provides a high-quality boating experience.
*1 As of January 2024 and according to Honda research
High Output and Good Fuel Efficiency with an Exclusively Designed Engine
As the core of any large outboard motor, the engine used for the BF350 is a V8 engine that has been newly developed with an exclusive design. It is Honda’s first use of a V8 for outboard motors. Achieving powerful propulsion with displacement of 4,952 cm3 and maximum output of 350 horsepower, the engine delivers high torque at all speeds in combination with Honda’s VTEC system.
The engine also achieves outstanding fuel efficiency through oxygen feedback control, which is a unique Honda technology for outboard motors, 8-cylinder individual fuel injection mapping control to support the variable intake air volumes per cylinder caused by irregular interval combustion, and plateau honing*2 used for the first time on a Honda outboard motor. Using regular gasoline as well, it is extremely economical to run.
*2 Plateau honing is a cylinder surface machining process that achieves a smooth finish to the cylinder walls and increases oil retention.
High-precision Lean-Burn Control Enabled by Oxygen Feedback
Oxygen concentration in exhaust gas is detected, and the data is fed back to the ECU, by an oxygen sensor placed within the exhaust channel. The knocking limit can then be accurately estimated to achieve high-precision lean-burn control.
Outboard motors are constructed to discharge cooling water, together with the exhaust gas, from near the propeller. Conventionally, the structure generally joined the cooling water (return water) and exhaust gas flows near the tip of the exhaust pipe and within the extension case. For this reason, oxygen sensors placed within exhaust channels could be doused with water drawn into the exhaust channel by exhaust pulsation or vacuum formation when traveling at low speeds or rapidly decelerating.
With this in mind, Honda reexamined the structure of its conventional extension cases and instead adopted a structure that separates the exhaust and cooling water channels. With the BF350, exhaust gas and cooling water channels are separated until the top edge of the gearcase, while the oxygen sensor is placed at the topmost point of the cylinder head, resulting in reduced risk of the sensor being doused with water.
Exhaust Gas/Cooling Water Channel Comparison and Composition
With the conventional structure, exhaust gas and cooling water mix together within the extension case before being discharged. With the new structure, this has been changed to prevent mixing of the exhaust gas and cooling water (return water) flows until close to the bottom surface of the extension case.
Newly Designed Crankshaft Ensures Comfortable Cruising through Low Vibration and Outstanding Quietness
V8 engines generally employ a cylinder bank angle of 90 degrees when used in automobiles, with flat-plane or cross-plane crankshaft configurations available. While the flat-plane configuration achieves high output with minimal exhaust interference between the four cylinders on one side, a two-axis secondary balancer is necessary to cancel vibration. On the other hand, secondary vibration does not occur with the cross-plane configuration but there is exhaust interference between the four cylinders on one side. This creates an issue with achieving high output and a single-axis balancer is required to cancel the primary inertia couple.
With the BF350, a dual-plane crankshaft was used to achieve superiority in terms of vibration to create a more luxurious feeling. However, to mount multiple outboard motors, it was necessary to reduce the overall width of the outboard motor and set a cylinder bank angle of 60 degrees. To achieve regular interval combustion for V8 engines, it is necessary to set a 30-degree phase difference (offset) between right-bank and left-bank crank pins, but the primary inertia force and primary inertia couple cannot be canceled without a balancer under these specifications.
For this reason, a 60-degree crank pin offset was used on the BF350 crankshaft, creating a structure that completely cancels primary inertia force and primary inertia couple without a balancer. While the 60-degree crank pin offset creates irregular interval combustion in a V8 engine, peak engine noise resonance becomes distributed in this case, which improved noise and vibration performance to create a more luxurious feeling.
To ensure sufficient strength and dimensional accuracy for a crankshaft that is longer and more complex in shape, the BF350 crankshaft is made from the same high-strength material as used for the crankshaft in the second-generation NSX. A special manufacturing process is also used to create a twisted crank by twisting the journal after forging and before it cools. The result is a V8 engine with an outstanding level of quietness and low vibration.
Crank Pin Configuration
Example of a cross-plane configuration with crank pins on each bank set at 90-degree intervals with a 60-degree pin offset.
Comparison of Primary Inertia Couple with 30-Degree and 60-Degree Offsets
Example of a 60-degree pin offset with balanced weight arranged in such a way that the primary inertia couples from each bank completely cancel each other out.
8-Cylinder Individual Fuel Injection Mapping Control for Irregular Interval Combustion
It is known that engine output can be affected by exhaust interference between the four cylinders on each side of a V8 engine, even when using 8-cylinder regular interval combustion employing a cross-plane crankshaft with a 30-degree pin offset. In the case of the BF350 however, which employs 8-cylinder irregular interval combustion with a 60-degree pin offset, the intake/exhaust efficiency is different for all eight cylinders, making it impossible to appropriately manage combustion at all cylinders using a single fuel injection map. For this reason, the BF350 employs 8-cylinder individual fuel injection maps. In combination with Honda’s VTEC system, this achieves torque and maximum output at low- and mid-range speeds, and good fuel efficiency at cruising speeds through lean-burn control.
Low-Resistance Gearcase for Converting Engine Power into Propulsive Force without Waste
The gearcase of an outboard motor is always underwater, so reducing underwater resistance for the gearcase can greatly impact motoring performance, top speed, and fuel efficiency.
Accordingly, Honda’s goal was to make the case as small as possible while creating a shape that has low underwater resistance. It optimized tooth contact, shim thickness, and other aspects of the gears (pinion, bevel, etc.) that change rotational direction when transmitting engine rotation to the propeller, and it increased tooth contact area by increasing the number of clutch dowels on the clutch shifter switching between forward and reverse gears. It also optimized the outside shape of the gearcase to achieve uniform pressure distribution across the case during high-speed cruising while ensuring sufficient toughness to handle cavitation*3. The result is a gearcase that is an equivalent size to the BF250 while being able to efficiently transmit the higher horsepower of the BF350.
*3 Cavitation is a phenomenon where a fluid vaporizes and forms bubbles at low pressure.
Gearcase Size Comparison
BF350 gearcase is an equivalent size to the BF250.
Pressure Distribution Comparison
BF350 gearcase shape has been optimized and pressure has been made uniform.
Unique Honda Durability and Reliability
One of the characteristics of Honda outboard motors is that they have the durability and reliability required for professional use as well. This is the result of Honda’s commitment to detail.
Dual Water Intake System
Outboard motors take in water through the gearcase for engine cooling. In addition to the conventional left and right intake ports, the BF350 also has ports located lower and further forward on the gearcase. Angled forward instead of sideways, the intakes have improved water-drawing function at high speeds. Located at four different points as well, the risk of overheating caused by blockages is also reduced.
Water Intake Layout
Enhanced Anode Metal and Lead Wires
The BF350 includes expanded use of anode metal, which is a sacrificial electrode to prevent corrosion of aluminum parts in seawater. Consideration has been given to shapes and layout to ensure anode metal around the frame is fully submerged when at anchor, while corrosion resistance has also been improved by increasing the number of lead wires electrically connected to each aluminum part. Anode metal is located within the new intake ports as well.
Anode Metal Layout
Twin-throttle Bodies
Two throttle bodies are provided in a side-by-side arrangement. In addition to ensuring stability when idling and at full throttle, this arrangement can maintain power on a single throttle if one of the throttles malfunctions during use.
Comprehensive Boat Handling Support Worthy of a Flagship Model
The BF350 comes with a full range of assist functions to enable as many users as possible to enjoy a smooth and comfortable boating experience. It also includes a large capacity (12V-93A) alternator to power a range of electrical devices in the boat. Maintainability has also been improved, including the ability to conduct some inspections when on the water.
Automatic Tilt Function
Outboard motors are tilted up when stored on a mooring to prevent fouling by seaweed, shellfish, or other organisms, and to prevent corrosion by seawater. The BF350’s automatic tilt function can be operated with the engine stopped by double-clicking a button on the remote control lever to automatically tilt the outboard motor up or down. Toughness in harsh environments is ensured with features such as a high output motor that maintains tilt speed even in cold weather and a special water discharge structure that prevents freezing.
Automatic Tilt Range
Trimming Support Function
The angle of the outboard motor when viewed from the side of the boat is called trim, and trim/balance (boat hull attitude) changes motoring performance. The BF350's trimming support function automatically controls trim, which is difficult for less experienced boat users in particular. It optimizes the trim angle of the outboard motor in line with the boat hull attitude, which changes according to the operation, such as acceleration or cruising.
The function comes with three customizable presets. Trimming support offers remote control button operation that automatically adjusts the motor to a pre-set trim angle matching engine and boat speed. It is also possible to fine-tune the preset settings while motoring and to save those changes as well. The function makes handling boats easier and more comfortable.
Illustration of Trim Angle Adjustment
Cruise Control Function
The BF350's cruise control function maintains a set engine speed when motoring, with adjustments in 50 rpm increments possible with a button on the remote control. If combined with a GPS device, boat speed can also be controlled in increments of 1 km/h. It can be used across a wide range of speeds, from trolling speed to maximum engine speed, making it useful for various boating activities, including fishing, wake-boarding, and cruising.
