Honda Marine's BF75 and BF90 outboard motors

Honda BF75 and BF90

Outboard Motors

Honda Marine’s newest outboard motor is significantly more important than its horsepower rating. Without exaggeration it’s the four-stroke outboard of the future, available today. Honda’s newborn twins, the BF75 and BF90, are founded on an inline four-cylinder block, the same basic castings and reciprocating mass under the hood of Fit, Honda’s best selling car. While the Honda fit car engine is rated 109 horsepower, it does not benefit from the sophisticated firmware that the outboard does. And it is the onboard computing power and sensors that allowed Honda Marine engineers to tool an outboard motor that melds brutal torque with enviable fuel economy. Here’s how they did it:

The future in marine engines lies with Electronic Control Modules and new subroutines that analyze an engine’s vital signs. An example is Lean Burn Control that adjusts the air to fuel ratio for either best mileage or maximum horsepower. It does this by monitoring the percentage of oxygen in the exhaust gas, which in turn reveals the air to fuel ratio in the combustion chamber. Since the ECM knows what happened in the last power stroke, thanks to Lean Burn Control, it can accurately adjust the air/fuel ratio. So Lean Burn Control leans the air to fuel ratio. The benchmark standard is 12.5 to 1. Honda leans it to a range from 14.5 to one to as lean as 18 to one. No big surprise, the payoff with a lean burn is superior fuel efficiency in the 2500 to 4000 rpm

Boosted Low Speed Torque is another shining star. BLAST monitors the throttle, not only throttle position, but also how quickly the throttle lever advances. Under easy acceleration the BLAST algorithm advances ignition spark just a little. You get a little more torque. But jam the throttle forward and BLAST advances spark to its maximum number. Big ignition advance equals a boatload of torque, and very strong acceleration

Variable Valve Timing and Lift Electronic Control incorporates both a street camshaft and a racing cam in the same engine. At lower rpm the street profile does its good work, lending strong acceleration. Above 5300 rpm the radical grind comes on line. The extra lift and duration, opens the valves wider and hold them open longer, allowing the engine to draw in big gulps of air and build maximum horsepower.

BF75 BF90 fuel injection is of the programmed sequential multi port variety. Programmed means instant starts, crisper throttle response and enviably good fuel economy. Multi port means one injector per cylinder, mounted closer to the combustion chamber. One injector per cylinder means crisp throttle response. Also important, fuel flows to the injectors via a cooled fuel cell. Honda Marine cool fuel just about eliminates any chance of vapor lock.

Specifications: Honda BF90

Horsepower	90 @ 5800 rpm
Operating range	5300 to 6300 rpm
Displacement	91.4 CID
Configuration	InLine-4
Weight	363 pounds
Alternator	44 amps
Gear ratio	2.33 to 1

Honda BF90 studio image

Honda BF 90

Two separate, individually controlled, cooling systems maintain uniform operating temperature. That means one cooling circuit and thermostat each, one for the block and another for the cylinder head. Running the cylinder head at a cooler temperature than the block allowed Honda engineers to dial in maximum horsepower without chancing catastrophic engine failure Valve train longevity also increased.

Honda's outboard motor factory

A sneak peak inside the Honda outboard motor factory.

Under the cowling of a BF90

BF75 and BF 90 special firmware manages vital engine functions.