HONDA BF150

Easing down on the foot-throttle elicits a steady rush of acceleration until the sleek Triton Tr-186 bassboat is running high on the pad and the GPS is registering in the low-80kmh range. Bolted to the transom, Honda's newest four-stroke outboard - the BF150 - hums with a quietly melodic wail. What we perceive as smooth acceleration, however, actually results from complex mechanical wizardry under that familiar Honda-silver cowl.

Inside the pre-production 2.4lt inline four's lightweight cowl, a computer monitors every move we make. The paperback-sized electronic control unit (ECU) orchestrates a sophisticated range of adjustments to optimise power and fuel economy based on everything from humidity and barometric pressure to air temperature and manifold pressure. And that's just the beginning.

The moment the engine reaches 3250rpm, the ECU commands the air intake shutter valves to open within Honda's dual-stage induction (DSI) system. The electronic switch from long air-intake runners to a shorter, higher-volume design allows the inline four-cylinder block to get the air it needs to continue building torque and horsepower as rpm spools up.

As the Triton accelerates and the engine hits 4300rpm, the computer fires off another electrical burst - this time to activate Honda's Variable Timing and Lift Electronic Control (VTEC) technology, which is the key to the new 150's unusually wide powerband.

CAR-BASED BLOCK
Honda pioneered VTEC in the early '90s on the NSX sports car, and has since included iterations throughout its automotive line, as well as the BF225 outboard released in 2002. So it shouldn't be a surprise that the BF150 - which is promised for delivery in Australia late this year - is the same 16-valve, fuel-injected, four-cylinder engine found in the Accord. The only difference is the outboard's vertical oiling system.

The block is aluminum with thin-wall, centrifugically-cast iron liners that help minimise weight and overall size. Available in 20 and 25in shaft lengths, the dry weight of the 20in model we ran during our first test on Georgia's Lake Lanier in the USA is listed at 217kg. It uses a maintenance-free, chain-driven camshaft and forged-steel crank with flat-top and cast-aluminum pistons that easily handle its 6200rpm rating. The marinised block makes 150hp and 150lb/ft of torque at 5500rpm, but the power curve serves up a big, juicy plateau from 2500 to 5000rpm, thanks to DSI and VTEC.

VTEC MAGIC
Suzuki's new 3.6lt V6 four-stroke family (see "Big Bertha" test in Trailer Boat issue 172) also employs variable valve timing and a multistage tuned air-intake system on its flagship 250 model. And while the two competitors use slightly different technology, the results are the same - punchy performance.

The BF150's 16-valve DOHC design uses an intake cam with three rocker arms to control each cylinder's twin intake valves. Below 4300rpm, the outer lobes and rockers provide the optimal air/fuel mix for strong low-end torque. During that period, the middle rocker arm just floats up and down on its lobe, doing nothing.

However, when the computer senses 4300rpm it signals the VTEC solenoid to pressurise an oil line feeding directly to the small, pin-like pistons located inside the rocker arms. The pistons lock the three together to follow the lead of the middle lobe and rocker arm, and intake valve operation instantly changes from a low-rpm torque cam to a high-lift, long-duration, high-performance cam. Higher valve lift and longer opening times allow more air/fuel into the cylinders at higher rpm for more power.

Incidentally, the only difference between the BF150 and then equally-new BF135 (both are based on the 2.4lt block) is that the latter doesn't have the ground wire hooked up to the VTEC spool valve, or the ECU mapping to control the variable valve timing.

From holeshot to Wide Open Throttle, the new Honda is smooth and, in terms of outboards, relatively quiet in its show of performance. If you pay attention, however, there's a slight increase in the sound level and tone when DSI and VTEC activate - eerily similar to the "old days" when the secondaries of a four-barrel carb opened up.

KEEP IT COOL
According to Honda officials, the new inline fours will earn CARB Ultra-Low, 3-Star emissions ratings, making them among the cleanest outboards available. The new family utilises what Honda calls "lean burn" technology, in which a linear air/fuel feedback (LAF) sensor allows the computer to adjust the air/fuel ratio, contributing to the BF150's excellent economy.

Yet when an outboard runs lean, it creates heat - which is detrimental to engine life and the ability to develop horsepower. As a countermeasure, Honda has incorporated a twin-thermostat, three-way cooling system, with separate systems for the cylinder head, block and exhaust manifold. An external oil cooler also is fitted.

Another cool part of the new Honda four-stroke is an automotive-style alternator that produces 50-plus amps above 3000rpm - of which 44 are available to charge the battery and run electronics. Other aspects that have appeal for boaters include a lightweight cowl, side covers that can be removed individually, and a front-access flushing device. There's also a spark plug access port that allows you to reach the inline-four's bottom (No 4) plug without removing the midsection covers.

Honda developed a new gearcase - a large-diameter type that houses a 2.14:1 gearset - specifically for the BF135/150. It isn't nearly as hydrodynamic as the bullet-nosed units offered on the BF200/225, but it boasts water pickups that are about 1cm lower than found on smaller Hondas.

PRE-PRODUCTION WOES
When we arrived at Lake Lanier, the Triton was in the water and ready to go. Pre-test runs by both Honda and Triton reportedly netted speeds in the high-80kmh mark with three aboard and the engine spinning 6000rpm. So we figured our test speeds should be similar. Not so. Somewhere between the initial setup and our arrival - a span of four days - the rig apparently lost 600rpm and nearly 12kmh. The best any of us could muster was 83.7kmh. Something was definitely amiss.

It turns out that a factory tech from Japan had changed the fuel pumps on all the prototypes just prior to our arrival. So, off we went to Honda Central to bolt on another pre-production 150. After seven hours of break-in and a lot of head scratching, we ran our test again. Both GPS and radar showed 88kmh at 5800rpm. Where did the speed go?

The techs at Honda's Georgia facility believe that the engineer made mapping changes to the ECU and altered fuel pump pressures - changes that may or may not reflect how production outboards will perform.

All we know is that the engines never missed a beat during testing, and from our seat-of-the-pants perspective, both felt like they were running at 7/8th throttle.

STRONG FUEL NUMBERS
We still came away with favorable impressions of the Honda's general performance - particularly fuel economy. Our optimum cruise speed of 65.3kmh (4500rpm) netted a whopping 3.1km/lt, and came just as VTEC engaged. At speeds in the 3000-4000rpm range, when the DSI air-intake system kicks in, we recorded numbers close to 3.0km/lt. At wide open, the BF150 still managed a thrifty 1.7km/lt, giving our 18ft Triton a range of more than 207km flat out. Cruise at a leisurely 64.4kmh, and you should be able to run nearly 386km before refueling. Such efficiency is a key selling point.

With the BF135 replacing the old 130 design, and a 150 to fill the void between its 130 and 200hp models, Honda's new family offers anglers better choices in smooth-running, quiet four-stroke power - joining similarly-sized offerings from Suzuki, Yamaha and Johnson. And with the prototype listed at only 26kg more than the lightest two-stroke 150 DFI (Evinrude's Ficht Ram), the weight penalty is becoming more of a non-issue with each new release.

Coupled with 0-50kmh times in the high nine-second range, Honda's latest entry in mid-level outboards proves that advanced four-stroke technology is putting a serious squeeze on these engine's traditional V6 rivals.