Positive Spin – Suzuki DF350A

Suzuki raises the bar on high-horsepower outboards with a testosterone-laced engine that has a pair … of propellers.

The concept of high-horsepower outboards was originally hatched primarily to push large, offshore boats, but that has changed. During Suzuki’s rollout of the DF350A in June, that notion was further disabused by the wide variety of boats it was mounted on. The lineup included pontoons, a bay boat, a catamaran and medium to large center consoles, from 24 to 39 feet long with from one to four outboards. The message? This 4.4L, twin-propeller motor that puts out 350 horsepower is a good fit for whatever scratches your boating niche.

Contra-Rotating Props

The option to have twin counter-rotating props (or contra-rotating, as Suzuki calls it) on an outboard is available from boutique engine producer Seven Marine (CR gearcase) and as an accessory from Yamaha (TRP gearcase), purchased mostly by Texas wade anglers for its ability to get a boat on plane in really skinny water. But Suzuki is the only mass manufacturer to offer it today as a standard feature on all its 350s.

Having lots of horsepower is great, but if it can’t be translated to traction, it’s wasted, like a dragster with passenger tires. Twin screws create more prop surface without increasing the size of the prop itself, and more prop surface gives a motor extra bite to get a boat out of the hole and keep it hooked up in turns or in lumpy seas, eliminates the prop torque caused by a single prop, and helps the boat track straighter when on plane or when docking.

One of the advantages of Suzuki’s system over Yamaha’s is found in reverse, where both of Suzuki’s props rotate. Pontoons can be a handful to dock, especially when the wind is blowing, so we appreciated the added maneuverability on the South Bay 525RS pontoon we tested with a single DF350A. Despite the added hardware necessary, Suzuki managed to keep the lower unit’s bullet hydrodynamic by moving the shift linkage above the waterline. The water intakes are at the front of the bullet for greater effect, and another is just in front of the skeg.

Big Easy Power

Suzuki probably could have squeezed close to 350 hp out of the 4.0L block it uses on its DF300/DF250 without doing much more than tweaking the engine’s internal computer mapping. It could have added a supercharger too, but both could affect engine reliability, which goes contrary to Suzuki’s number-one goal. Instead, the displacement was increased to 4.4L by lengthening its stroke while keeping the bore the same diameter. In addition, its compression was raised to 12:1, which is the highest in the industry. Higher compression increases horsepower and torque but can cause engine knock unless steps are taken.

One of the main reasons for knock is uneven fuel detonation in the combustion chamber, so Suzuki provided two small injectors per cylinder rather than one and increased the number of fuel flow openings to 10 per injector, instead of four. That way, all the fuel can be injected at once for a more uniform and complete burn. This sudden rush of fuel creates a cooling effect, which reduces the engine’s propensity to knock. The air intake system is more efficient, which allows denser cool air to enter the combustion chamber to create more power. This dual-louver, direct-air intake traps water from spray or rain as it enters the cowling and prevents it from reaching the engine.

To accommodate the added power, various components were beefed up, including the connecting rods and adjacent hardware. The pistons were redesigned and made stronger by “shot peening” the face, which creates dimples that more evenly distribute the pressure created during combustion.

The DF350A has many of the features that other Suzuki outboards employ, such as Lean Burn, which senses when a boat isn’t under extreme load and makes the air/fuel mixture leaner for better economy. The DF350A remains compact thanks, in part, to its offset driveshaft arrangement and seems even narrower because it’s slightly taller than the DF300. Its two-stage gear reduction gives it a really low 2.29:1 gear ratio that allows it to swing a bigger prop, which in the case of the Avalon was a 15¼-inch-by-24-inch matching prop set.


Specs

Type Normally aspirated V-6 four-stroke Displacement 4.4L Full-Throttle Range 5700-6300 rpm Weight 727 lbs. (25 in. shaft) Gear Ratio 2.29:1 Price $32,000

Performance

Tested with Avalon Windjammer 2785 QL Time to plane 2.5 sec. 0-30 mph 4.6 sec. Top speed 61.5 mph Decibels @ idle 58 dBA Cruise 37.0 mph/4000 rpm/83 dBA Peak 61.5 mph/6300 rpm/93 dBA

Suzuki Marine, suzukimarine.com


Weights and Measures

The DF350A 25-inch model weighs 727 pounds, which is 77 pounds less than the Yamaha F350 V-8 but 49 pounds more than Mercury’s supercharged 2.6L Verado 350. Both the Suzuki and Yamaha meet the California Air Resources Board (CARB) Three-Star Ultra-Low Emissions standard, while the Mercury is a Two-Star Low Emissions outboard.

Let’s Take a Ride

The first boat I tested it on was an Avalon Windjammer 2785 QL, a beefy 4,950-pound pontoon with a 28-foot, 6-inch LOA and equipped with twin DF350As. Even with six people on board, its performance was outstanding. At idle, the outboards were really quiet, measuring only 58 dBA. Throwing down the drive-by-wire throttles resulted in a rush of power that accelerated really smoothly. On plane in 2.5 seconds, the Avalon reached 30 mph in 4.6 seconds, which was only two-tenths slower than its time with two passengers. Speed peaked at 61.5 mph at the engines’ 6300 rpm max, where their satisfying growl registered 93 dBA.

The next ride was on a South Bay 525RS, a 2,970-pound 25-footer powered by a single DF350A. In a way, it was even more impressive than the twin-powered Avalon. The ease with which this motor hooks up will make it a favorite of boaters who participate in watersports. With practically zero bowrise, it was hard to tell when the boat reached plane, but 2.7 seconds was my best guess. It reached 30 mph in 5.7 seconds, and the South Bay hit a top speed of 54.7 mph.

The wildest ride of the day was aboard a 390Z SeaVee powered by quad Suzuki DF350A outboards, which is 1,400 hp (for the math-challenged). The 390Z is a twin stepped-hull boat that weighs 9,670 without engines and has 22 and a half degrees of deadrise, which tends to settle a boat down in the water more than a flatter-bottomed boat. When I jammed the twin throttles, the 24 prop blades pushed the 39-footer smartly to plane in 4.3 seconds. Time to 30 mph was just over eight seconds in moderately choppy conditions (later time trials in calmer waters had it in the low seven-second range). Top speed was a face-distorting 72 mph.

The DF350A comes in either white with gray panels or black with gray panels, which seems appropriate since the case for twin props on an outboard has never been so black or white.

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