The Outboard Expert: Up Close with the New Suzuki DF250 SS Engine

Exclusive first look at Suzuki's go-fast outboard.

24th May 2007.
By Charles Plueddeman

Suzuki designed the DF250SS to be a viable replacement for two-stroke power on bass boats.

Suzuki designed the DF250SS to be a viable replacement for two-stroke power on bass boats.

Excuse me while I’ll pull my socks back on-they’ve just been knocked off by the new Suzuki DF250SS, a 250-hp four-stroke designed specifically for the bass-boat market, but also intended to power pontoons and some flats and bay boats. This sounds a lot like the mission statement for the Yamaha F225TRL I reported on last week, except that Yamaha was careful to position its new 20-inch four-stroke as power for fish ‘n ski boaters and others who did not require maximum performance. Suzuki has introduced the SF250SS with no excuses and no qualifiers-this is a serious, go-fast outboard.

Suzuki says the performance benchmarks it used when developing the DF250SS were the Mercury Optimax 250 ProXS and the Yamaha Vmax 250 HPDI, both strong performers in the bass segment, and both direct-injected two-strokes. Suzuki is hoping the DF250SS is the first true four-stroke alternative in this market. In the bass segment, the two-stroke has remained dominant for two reasons. One is weight. A bass boat by design carries a lot of weight aft (fuel tanks, stowage, batteries, live well) so that it can lift the bow for maximum top speed, running on a small area of the pad near the transom when it’s really aired out. The extra 50 to 100 pounds a four-stroke adds to the transom can upset this balance, make it harder to get the boat on plane, and cause the boat to ride too low in the water at rest or when coming off plane. Hole shot is just as important as top speed to a serious basser, who hates to give a rival a head start. Two-strokes have an advantage on hole-shot performance because they make power on each down-stroke of the piston, rather than on every-other down stroke, as on a four-stroke. Two-strokes simply come up into their torque band more quickly.

Faced with these challenges, Suzuki went to work about two years ago to build a four-stroke that could match the performance of the two-stroke. Early-on, the company decided to take the “no replacement for displacement” approach and use its big 4,028cc, 55-degree V6 block, which was designed for the 300-hp DF300, a motor intended mostly for use on heavy off-shore fishing boats. The standard Suzuki DF250 uses a 3.6-liter V6 powerhead, and that motor could have been re-tuned to produce the performance Suzuki was looking for, but according to Suzuki applications manager David Greenwood, it would have been under a lot of stress. “Suzuki does not like to over-stress its motors,” said Greenwood. “By using the big block we could get all the power we needed and still be very reliable and durable.”

All of the technology found on the DF300 is retained on the DF250SS, and in fact it’s that technology that made it possible for Suzuki to re-tune this motor and give it an all-new personality. To tune the engine to make more power at the bottom and mid-range, the program that controls the variable valve timing function on the intake valves was re-coded, as was the program that controls the motor’s variable intake path function.

“This was all done in the computer,” said Greenwood. “The actual cam profiles are not changed. We were able to produce a torque curve that is very similar to that of a big two-stroke. Not quite as strong as a two-stroke, but very close. It peaks much faster that the curve on the DF300.”

The variable intake function is key to this performance. The motor has two separate paths for air to enter the engine. One uses long, curved tubes with a relatively small diameter. The other uses shorter tubes with much larger diameter. On hole-shot, air is routed through the longer tubes, which helps it gain velocity quickly. The mass of the moving air in these tubes has a mild supercharging effect as it packs into the cylinder, which boosts bottom-end torque. As engine rpm increases, the motor’s computer opens a valve and the air is redirected to the alternate intake patch, which offers a very direct path for a large volume of air to enter the engine, matching the pumping effect of the motor at higher speed. Imagine the difference of trying to blow air through a drinking straw and a garden hose, and you’ll visualize the difference between the intake tracts on the Suzuki. This motor also has a new midsection that produces less exhaust backpressure.

Greenwood said that Suzuki also spent a lot of time developing and testing a new line of propellers for the DF250SS. Line other Suzuki models, the DF250SS has an offset driveshaft that mates to the crankshaft on the bottom of the motor through a gear set. This offset allows Suzuki to locate the powerhead a few inches forward (closer to the boat), so its mass is closer to the transom bracket. This reduces the leverage effect of the weight of the motor on the transom, and also reduces vibration. The other benefit is that Suzuki can accomplish some of its gear reduction between the crank and the drive shafts, and get further reduction in the gearcase. The DF250SS has a 2.08:1 gear ratio, which lets it run much larger props with more pitch than other outboards of similar horsepower. The Yamaha Vmax and Merc 250 Opitmax both have a 1.75:1 gear ratio.

Tests revealed that a 4-blade stainless prop with 14.5-inch diameter and a specific blade shape worked best with the DF250SS. The pitch range goes from 27 to 31 in one-inch increments.

“These props are designed to slip a little on hole shot to help the engine rev more quickly into its powerband,” explained Greenwood. “We also tested props with vent holes in the hub, but found they just don’t work on a four-stroke. We also found that these four-blade props cost us very little top speed.”

The gearcase is the same designed for the DF300, and has a pronounced bullet shape that Suzuki says reduces drag by 18 percent compared its standard case. Greenwood said the shape of the water pick-ups was changed slightly to insure good flow on a trimmed-out, jacked-up bass boat.

Suzuki also worked hard to get the weight down on this motor. It comes in at a reported 578 pounds dry (no oil or prop), still heavier than the 3.0-liter Mercury 250, which weighs just 505 pounds, or the 3.3-liter Yamaha Vmax at 539 pounds. Excess weight is never a good thing, and its significance in this case will probably depend on the boat.

My first test of the Suzuki DF250SS came in a Blazer 210 Pro V, a pretty blue-flake, 21-foot 2-inch bass boat that has a lot of set-back built into the transom and had even more with a jackplate. Running a 14.5 x 31 prop, the boat planed off in about seven seconds and topped out at 75.4 mph with about 36 gallons of fuel on board. It’s never possible to make a performance comparison in a situation like this – you need the other motors rigged to the same boat on the same day to do it right. But I don’t think the performance of the Suzuki DF250SS would disappoint anyone on this boat.

I also ran the motor on a 20-foot 4-inch Bass Cat Puma that planed off smartly and ran 74 mph with me at the wheel. Also impressive is the mid-range power of the DF250SS. Putting the boat into a big 180-degree turn, and I could settle the hull and then get on the throttle early and just power the boat around. There’s no hesitation and no lag. The power is available instantly.

Fuel economy will also be a big selling point for the Suzuki. At wide open throttle on each of the bass boats at this test, the DF250SS burned 24.4 gallons per hour (GPH) at wide-open throttle and got 4.7 mpg at about 3500 rpm and 32 to 34 mph, which might be about 15 to 20 percent better than a DFI two-stroke, though its really hard to compare without conducting a head-to-head test.

One feature that does not carry over from the DF300 is digital throttle and shift, a disappointment as I’ve become a big fan of that feature on any outboard. Suzuki points out that many of the big bass rigs use a foot throttle, which requires a cable (I hope someone comes up with a digital foot throttle soon). It also does not yet have a side-mount digital control.

If Suzuki has missed the mark in any regard with this motor it’s in the styling department. The DF250SS is the conservative charcoal color of an undertaker’s suit and has the same subdued graphics as all other Suzuki V6 models. What it needs it some styling that screams “high performance,” like the big spoiler air-scoop on the Vmax cowl, or the racing-style graphics on the Mercury high-performance motors. Maybe paint the gearcase silver, and the rest of the motor the same shade of blue at Suzuki racing motorcycles. And I think many bassers would like this motor to make a little more noise, a bit of snarl to go with all that performance.

The Suzuki list price for the 2008 DF250SS will be $21,250. A 2007 Mercury Optimax Pro 250XS is priced at $18,920, while the Yamaha Vmax 250 is $18,790. Those are 2007 prices, and could go up slightly for the 2008 model year. That’s a significant premium for the Suzuki if dealers stay near the MSRP price.

Styling of the Suzuki DF250SS is subdued.

Styling of the Suzuki DF250SS is subdued.

Editor’s Note: Charlie Plueddeman is the editor at large for Boating, the nation’s largest boating magazine.


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Charles Plueddeman

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Charles Plueddeman is Boats.com's outboard, trailer, and PWC expert. He is a former editor at Boating Magazine and contributor to many national publications since 1986.

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