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Honda's BF225 VTEC Outboard

A look at the most sophisticated marine outboard engine in the world.

Words by Michael Knowling, Pix courtesy Honda Marine

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Honda Marine's latest creation - the BF225 - has debuted as the most technically advanced outboard engine in the world.

The BF225 is a 3.5-litre 4-stroke V6 with a pair of 12-valve SOHC heads boasting - for the first time in a marine application - variable valve timing and lift. A unique dual-stage intake plenum and complex fuel and ignition system also serve to improve its efficiency. The result is a 225hp output at 5500 rpm - together with unrivalled levels of flexibility, fuel economy, noise, vibration and emissions.

Perhaps most interestingly, the BF225 draws from the engine design of the road-going Honda Odyssey V6 and US-spec Acura MDX. Several components - such as the conrods, crankshaft and piston rings - are common to these engines. Honda stresses, however, the BF225 is not simply a quick adaptation - it's completely "marine ready".

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Just like in a road-going application, Honda's variable valve timing and lift electronic control (VTEC) provides increased peak power together with an even spread of torque throughout the rev range. It is also said to improve fuel efficiency and emissions.

The B225's VTEC system employs three in-line camshaft lobes for each pair of intake valves. During low-rpm operation, the two outer lobes - which deliver low-lift and short-duration - control the intake valves via their own set of rocker arms. A third set of rocker arms - which align with the centre camshaft lobes - are left idling during this stage; their movement controlled by a so-called 'lost motion' spring. Then, during high rpm operation, the engine management system locks the centre rocker arm to the outer arms using a hydraulic synchronising pin. This sees the centre camshaft lobe - which delivers high-lift and long-duration - taking control of the intake valves and giving increased engine airflow at high rpm.

The VTEC switchover point is set at 4500 rpm - 1000-1500 rpm below the maximum rpm cutout.

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This graph compares the BF225 VTEC power curve to a typical 4-stroke outboard with a compromise single camshaft profile. Note how the red area reflects the improvement in torque during the first stage of VTEC, while the yellow area reflects the top-end gain when the system has switched to its second cam profile.

On the water, Honda has seen acceleration and top-speeds virtually line-ball with a 2-stroke competitor (2-stroke outboards being the traditional high-performance leaders). The BF225 is, however, claimed a full 5dB quieter at maximum speed and notably quieter under all other conditions.

Honda has also taken a dual-stage approach to the BF225's intake tuning. The new engine features twin butterfly valves in the plenum chamber to control volume and airflow velocity into the combustion chambers. Below 4000 rpm these valves are closed, reducing the plenum volume and increasing air velocity. Above 4000 rpm (just before the VTEC switchover), the valves open to allow the engine to draw air from the entire plenum volume. Again - just like VTEC - this system reduces the compromise associated with a fixed-geometry system.

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Both VTEC and dual-stage intake systems are controlled by a single engine management computer - which also takes care of ignition and fuelling. Sparking the Honda BF225's combustion process are direct-fire coils (eliminating radio interference from high tension leads), while a knock sensor prevents detonation. In terms of fuel delivery, the BF225 features sequentially fired multi-point injection with closed-loop mixture feedback from an oxygen sensor in the exhaust. The quantity of fuel injected into the engine is based primarily on engine rpm and the signal from a manifold pressure sensor. Such accurate fuel control serves to benefit maximum power, response, emissions and smoothness - in addition to reducing the chance of spark plug foul-up while trolling for extended periods.

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Inadequate battery charging can also be a problem when trolling - especially when the vessel is decked out with equipment such as radar, GPS, echo sounder, radios and a bilge blower. To overcome this, the Honda BF225 uses an 'on-demand' electric charging system. Using a V-belt driven alternator, the system can deliver 48 amps at only 1000 rpm (a typical trolling engine speed) and a maximum of 60 amps from 2300 rpm. Honda claims no other outboard engine can provide as much current at such low engine rpm.

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The BF225 is also extremely eco-friendly. This is the first outboard engine in its class to meet both CARB (California Air Resources Board) 2008 Ultra Low Emissions standards and USA EPA 2006 regulations. The latter dictates manufacturers reduce outboard emissions every year, until - finally in 2006 - their emissions are 95 percent lower than 1997 standards. The BF225 already meets the level set for 2006.

There's no doubt Honda Marine's long-established philosophy toward using 4-stroke outboards has helped to meet these standards. However - from a typical consumer's point of view - it's likely the biggest advantage of the 4-stroke design is the absence of oil deposited in the exhaust system. This eliminates unpleasant (not to mention harmful) blue smoke and toxins.

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Honda suggests their 4-stroke BF225 consumes 10 percent less fuel than an equivalent horsepower direct-injection 2-stroke outboard and up to 40 percent less than a carburettor-fed 2-stroke competitor. Note that the BF225 requires at least 95-octane unleaded fuel - though the extra cost of this high-grade fuel is more than offset by its minimal oil consumption.

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Protecting all this technology is what Honda calls its "4 Front Corrosion Protection System." The list of protective features includes a double-sealed multi-layered paint process, strategically placed sacrificial anodes plus an assortment of marine grade stainless steel, chrome plated and specially treated Dacro components. Salt water is kept out by sealed wiring connections and a specially designed one-piece engine cowl, in addition to an easy-to-use freshwater flushing device for after-use.

The engine management computer also serves to protect the engine; whenever there's low oil pressure, over-temperature, over-revving or an excessive amount of water detected in the fuel, a helmsman alert is activated and revs are automatically cut. Note that the BF225's in-built water-fuel separator already combats the presence of water in the fuel supply line.

Finally, Honda offers a 3-year limited warranty in all recreational applications.

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Despite the amount of technology it employs, the Honda BF225 is both attractive and flexibly packaged. The orientation of the 60-degree V6 engine block has allowed the overall maximum width of 625mm to taper into a quite narrow leading edge. This enables the BF225 to be mounted in a narrow engine well, and increases potential for a twin-BF225 installation in a slightly more generous well. Its sleek design also improves on-water aerodynamics.

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The engine is available in Long, Extra Long and Extra Extra Long shafts (measuring 508, 635 and 764mm respectively). The option for counter rotation - ideal in a twin-engine configuration - is available on the Extra Long and Extra Extra Long models only. Overall mass varies little depending on shaft length - the BF225 weighs 267-277kg.

Of course, it's inevitable this leading-edge technology comes at a cost - an Extra Long shaft Honda BF225 will currently set you back around AUS$27,400. Rest assured, though, you're getting a lot of engineering effort in exchange for that cheque...


Honda Marine Australia
+61 3 9270 111

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