The New V-6 Four-Strokes Are Here!

September 21, 2007
¿ Formidable Four-Strokes: * Cleaner running * Quiet operation * Plenty of power * Worth the weight * Worth the wait

Yamaha has won the large four-stroke outboard race with the introduction of the new F225, a 3.3-liter, 60-degree V-6, the largest four-stroke outboard engine available to date. It received rave reviews as a milestone in outboard development when it was introduced to the press last fall-even before anyone had a chance to run one-as Yamaha revealed the painstaking process its engineers went through to design this new and larger four-stroke from the ground up. Now the new F225 is in dealer showrooms, and the comments have been no less glowing from those who have finally gotten an opportunity to run one (see sidebar).

Yamaha won’t have the only large four-stroke for long, however. Honda just unveiled its own 225-hp model (which is not yet available), and virtually every other outboard company, including Mercury and Suzuki, is working on a large four-stroke of its own. That’s because, for better or for worse, the two-stroke engine will most likely disappear from the market in the not-too-distant future, and manufacturers simply have no choice.

Both four-strokes and the current direct-injection two-strokes are “clean” enough to meet the 2006 EPA emissions mandates, but there are additional, tougher EPA standards coming down the road in 2008. Most knowledgeable industry observers believe that without a significant new technological breakthrough to make them run cleaner, two-strokes simply won’t meet these more stringent emissions guidelines, and that four-strokes will become the predominant outboard technology.


Yamaha F225

Rather than modifying an existing automotive design, Yamaha chose to design its new F225 four-stroke outboard from scratch. The result is an “In-Bank” water-cooled exhaust system, which directs the exhaust straight down and out through the prop.

As we mentioned, Yamaha started with a clean sheet of paper when designing its new F225 (also available in a 200-hp version, the F200). This is not a converted automobile engine, but is instead a four-stroke specifically designed for marine use. Yamaha carefully studied four-stroke configurations, as these engines are inherently heavier than two-strokes because of all the extra moving parts, such as valves and overhead cams. The company’s goal was to eliminate as much weight as possible, and Yamaha decided that existing automotive designs were just too heavy and bulky.

So, its engineers created a block that basically reverses the traditional intake and exhaust design used in most automotive four-strokes. They built what became known as the “In-Bank” exhaust system, which places the exhaust tracks on the rear of the engine between the cylinder heads. The exhaust system is water-cooled and the exhaust is directed straight down and out through the propeller. Air is inducted at the front of the engine into the air silencer box, and is then channeled into six long tube intake tracks. These tracks are precisely tuned for maximum air velocity so that the fuel atomizes better, and each intake track contains its own throttle valve. The fuel injectors are located on the inside of the intake tracks, just downstream of the throttle valves.

The F225 features double overhead camshafts on both cylinder banks. The engine has four valves per cylinder, with individual intake and exhaust camshafts providing precise valve timing control. The entire engine is controlled by a central microprocessor dubbed the Engine Control Module (ECM) which receives input from six sensors placed around the engine. The ECM delivers just the right fuel/air mixture to each individual combustion chamber for any load or speed, warns the operator about problems that may arise, and allows Yamaha service personnel to download diagnostic information and engine history onto a laptop computer.

### Yamaha F225
Displacement 3352cc
Bore X Stroke 94 X 80.5 mm
WOT rpm range 5000-6000
Horsepower 225 hp @ 5500 rpm at propshaft
Compression ratio 9.90:1
Alternator output 45 amps
Lubrication Wet sump
Gear ratio 15:30 (2.00:1)
Oil capacity 6.1 quarts
Weight without prop 583 lbs.

Other innovative design features include a high output, marine-style alternator located beneath the flywheel. It puts out 32 amps at 1000 rpm, and 45 amps from 2000 rpm up. A blow-by gas re-burning system removes oil from the blow-by gas, returning only gasoline to the combustion chamber, which helps the engine run very clean. And, an Idle Speed Controller, which is controlled by the ECM, makes starting easier and stabilizes the engine at idle and while slow trolling. The F225 and F200 are available in 25- and 30-inch shaft lengths, with counter-rotation models available, and come with standard three-year limited pleasure-use warranties.

As Barry Gibson points out in the sidebar, the F225 provides excellent acceleration throughout the power band, and is extremely quiet. It also posted some impressive numbers. For example, Yamaha’s own tests with a single F225 on a Regulator 23 showed a cruise speed of 25.5 mph at 4000 rpm, while burning a miserly 8.3 gph of fuel. At WOT, it reached 43.5 mph at 6100 rpm and was still only burning 21.2 gph. That’s impressive performance, and efficiency that no two-stroke could come close to.

Honda’s BF225

Honda’s BF225 (above and inset) is aspirated with a Variable Intake System that powers the Mako 232 to 42.4 mph at 5600 rpm.|


I recently spent a day running Honda’s prototype 225-horsepower V-6 four-stroke, the BF225. This 3.5-liter, 60-degree engine is not yet on the market, but Honda officials say it should be by this coming January.

The BF225 is a single overhead cam design with four valves per cylinder that borrows technology from Honda’s NSX automobile engine. This includes what Honda calls the “Variable Valve Timing and Lift Electronic Control System”, or VTEC for short. The VTEC system utilizes a double cam profile to maximize power at all rpm. A low-profile cam lobe opens the valves at speeds under about 4500 rpm, providing low valve lift and opening duration at low speeds to allow the engine to run smoothly at idle and low rpm settings.

At speeds above 4500 rpm, a more radical cam lobe is activated by hydraulic pressure (a solenoid controlled by the engine’s microprocessor initiates the changeover). This provides higher valve lift and longer duration so that the cylinders can be adequately filled with both air and fuel at high speed, and top-end performance is thus greatly enhancing. The VTEC system provides a broader, smoother torque curve throughout the power band.

### Honda BF225
Displacement 3471cc
Bore X Stroke 89 X 93 mm
WOT rpm range 5000-6000
Horsepower 225 hp @ 5500 rpm at propshaft
Compression ratio 9.4:1
Alternator output 60 amps
Lubrication Wet sump
Gear ratio 15:28 (1.86:1)
Oil capacity 7.9 quarts
Weight Unspecified: projected to be under 600 pounds

The BF225’s air intake system, known as the Variable Intake System, doesn’t use long intake tracks, but instead uses an intake manifold plenum mounted on the back of the engine. The plenum is an air chamber that resonates at a characteristic frequency. At certain rpm settings, the resonance enhances cylinder filling and improves torque. At other rpm settings, the resonance interferes with cylinder filling and reduces torque. Honda overcame this problem by designing a two-stage plenum.

At low rpm, butterfly valves between the two chambers of the plenum (front and rear) are closed, and each cylinder bank is isolated. Waves of high-pressure air are created by the resonance effect, increasing low rpm torque by improving cylinder filling at low engine speeds. At higher speeds, the butterfly valves open and the resonance effect disappears. Precisely tuned intake runners with funnel-shaped openings take over, decreasing manifold turbulence and maximizing the amount of air that each cylinder receives at high speed.

We drove the BF225 on a Mako 232 center console, and its performance was outstanding. The engine was exceptionally quiet and free of vibration (four-strokes are inherently smoother than two-strokes), and the acceleration seemed only slightly different than what you would expect from a two-stroke. At 4000 rpm, the BF225 pushed the Mako to a cruise speed of 29.4 mph at 4000 rpm with a burn rate of 9.9 gph. When opened up, the Honda pushed the Mako to 42.4 mph at 5600 rpm and was burning 18.4 gph while doing so. The BF225 will come with a standard three-year limited pleasure-use warranty.

Look for more large four-strokes to be announced in the near future. It’s a trend that’s here to stay, and will be good for both consumers and the environment. And I, for one, will not miss the hassle of adding two-stroke oil one bit!

# Running the Yamaha F225: First Impressions¿ I got the chance to run the F225 at Yamaha’s test facility on the Tennessee River outside Chattanooga back in early May, and as the owner of a two-stroke Yamaha 225, I was interested in how this new model might stack up. Test boats included a Regulator 23 with a single, and a Grady-White 263 CC Chase with twins.The first thing that struck me as I stepped aboard the Regulator was that the idling engine produced no vibration at all. The second was that there was absolutely no trace of smoke or corresponding fuel-rich “exhaust smell” we associate with two-stroke outboards. Third, the engine was eerily quiet. Performance was exceptional. Acceleration from a standing start was torquey and smooth, and appeared to be on par with that of a similar two-stroke. Tight turns at full throttle were clean. The engine noise level at cruise speed and above was fairly significant-although nowhere near the snarl of my carbureted two-stroke-but I think it could be attributed to the fact that the Regulator had a notched transom and not even a splash well gate to act as a sound barrier. The Grady performed equally as well with the twins, propelled along at nearly 52 mph at WOT. At cruise speed, about 35 mph, the decibel level was so low that Chuck Butler, Yamaha’s Senior Applications Engineer, and I could sit on the upholstered stern seat and converse as if we were riding in a car. Here, however, the Euro-transom design really seemed to enhance the engines’ inherent quietness.One thing I did notice, while simulating docking maneuvers in both boats, was the distinct sound of the shift dogs engaging. The reason? Simple. The F225 is so quiet at idle speed that the shifting noise just seems loud! -Barry Gibson

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