Suzuki DF300

Powerful but clean

Suzuki's DF300 is one of only two 300hp outboards currently available. But where it scores over its direct competitor - Yamaha's HPDI 300 - is its CARB 2008 and Australian OEDA "3 Star" exhaust emission compliance. Due to its homogenous-only combustion process, the direct fuel injection two-stroke HPDI 300 is only a "2 Star" engine and doesn’t meet CARB 2008.

TrailerBoat editor Daniel Nash covered the technical aspects of the DF300 in detail in issue 216 so there’s no need to discuss them again, but it’s worth noting that the single spherical throttle body is more reliable than twin throttle bodies which can go out of alignment over time and create uneven running qualities. Also the piston skirts are only 31.5mm to create better conrod clearance in the relatively narrow 55 degree V6 powerhead and reduce piston weight for less reciprocating mass at higher revs.

Being a four-stroke, the DF300 also has lower fuel consumption than the Yammie at WOT and 250hp DFI outboards in the 2000 to 4000 rev range, as I found out when testing twin DF300s on a Powercat 3000 Sports Cabriolet in February.

Spinning 16 x 21.5in stainless steel Suzuki props and pushing a total of 4.3 tonnes, including full fuel tanks and seven adults, the DF300s planed us at 28.9kmh and just 2500rpm without needing to "floor" the throttle levers, most likely because the variable valve timing holds the intake valves open longer from only 1500rpm. Cruising at 4000rpm the averages were 58.1kmh with a fuel flow of only 30lt/h per engine according to the digital fuel flow equipment fitted. This rate is 17 per cent less than the DFI two-stroke 250 I tested in October, 2006, amazingly low for an engine having such "grunt". No prop ventilation occurred through full lock turns at these revs.

Flooring the throttles from 4000rpm literally threw us back in our seats with acceleration almost as good as a DFI two-stroke or supercharged four-stroke. However, above 4000rpm the Suzies really started to use fuel and at WOT, averaging 95.9kmh and 6000rpm, the flow rate was 91lt/h per engine, but still 13 per cent below what an HPDI 300 uses at WOT. The engines had an exhaust note unlike any other V6 four-stroke I’ve tested and were surprisingly quiet with virtually no vibration. Due mainly to the forward rake of the lower units the legs could be trimmed right out at WOT without incurring prop blow-out.

Brendon Shaw of Powercat told me that over 15.7 hours of demos during the Haines Group media and dealer event in February, the engines used a total of 540lt or just over 34lt/h between them. Not bad for twin 300s!

As expected the demo engines started instantly hot or cold. They didn’t blow any oil smoke and, unlike some DFI two-strokes I’ve tested, there wasn’t an oil smell when backing upwind. The optional power steering and standard Suzuki Precision Control electronic throttle and gearshift controls (similar to Mercury Marine’s DTS on Verado models) were a delight to use.

Access to the powerhead is very good with the ability to separate the lower cowl for full access. The engine oil sump dipstick is easy to reach and, like all Suzuki four-strokes, plumbing and wiring are neatly routed on the powerhead. The engine oil and filter should be changed every 50 hours or six months after the 20-hour service, important as the eight-litre sump capacity is the same as the DF150 with little capacity to absorb oil sludging between changes. The waterpump impeller should be replaced every 100 hours or once a year but, because it runs in a stainless steel housing, up to 200 hours should be possible between replacements.

The warranty coverage is five years for recreational usage.