Five years with a Suzuki DF6
Judging from the way this engine has performed in the five years of controlled evaluation, Suzuki's slogan of 'The World's best four-stroke' is very close to the truth. Nothing has failed in this period, but as the engine continues to run-in, its performance is still improving.
For example, in the three-year report I described the engine's performance on my '70s 3.4m Savage Gull dinghy as adequate providing the total displacement was limited to no more than 240kg. Spinning the standard seven-inch pitch Suzuki alloy prop and pushing a total of 285kg including two adults and fishing tackle, the DF6 averaged 24.9kmh and 5670rpm at Wide Open Throttle - with this throttle opening needed to maintain a clean plane. At least due to the Savage's excellent hull shape, averaging 7.5kmh with a total of 10 per cent WOT and 40 per cent trolling operation, the DF6 used only 0.43lt/h.
But when the total weight was reduced to 235kg or one adult and one child aboard, the WOT average increased to 28.6kmh. Initially, when trials began with this hull, loading WOT was needed to get us out of the hole but by 70 hours, two-thirds throttle was sufficient to plane the Gull and maintain it with a significant increase in fuel efficiency.
Of the six hulls used for evaluating the DF6, the best performance has still been on my flat-bottomed 3.4m Sea Jay punt, due to its relatively large planing surface in relation to hull loading, although even on this hull the total displacement has to be limited to 300kg to enable the engine to reach and maintain a clean plane.
Spinning the same prop and pushing a total of 290kg including two adults, the DF6 trolled at 4.3kmh and 1200rpm using 0.2lt/h, and planed at 22.4kmh and 5000rpm. WOT averages were 28.0kmh and 5690rpm using 2.6lt/h, while across the 7.5kmh 'loop' it averaged 0.58lt/h.
Increasing the total displacement to 330kg or maximum hull loading, the WOT averages were only 15.8kmh and 4860rpm, and the 'loop' fuel consumption 0.84lt/h. Noise and vibration levels rose significantly because the engine had to work so much harder to achieve these results.
When the three-year report was compiled the engine had done 50.4 hours. Averaging 0.55lt/h with 7.5 per cent of WOT, the fuel/oil ratio was 1245:1, not bad for a small four-stroke engine. But over the next two years where 21.6 hours were clocked up with the same WOT percentage, averaging 0.53lt/h the F/O ratio was 2510:1, bringing the total for 72.0 hours and 0.55lt/h to 1460:1. Oddly enough in the period from 42.0 to 50.4 hours using 0.53lt/h and with 7.5 per cent WOT operation the F/O ratio was 2760:1, so the peak of low oil consumption had been reached at 50 hours.
In the period from 50.4 to 72.0 hours the DF6 blew oil smoke occasionally on cold starting. So although engine performance is still improving, oil consumption is starting to increase.
Apart from one period, when it was hard to start probably due to dirty fuel, the DF6 has been reliable. Since testing commenced it has needed two new waterpump impellers with the last one at 56 hours. Because the cooling water intakes under the antiventilation plate are not fitted with a removable screen, small debris can pass through to the impeller. My Suzuki dealer found that a small chunk of plastic(!) had chewed out one of the impeller blades, resulting in slight engine overheating when worked hard. Since then, the flow from the pilot water discharge has been okay, but it's worth taking care operating in weedy water as pieces of weed can reach the impeller.
Due to significant pitting, the lower zinc anode was replaced after 205 hours of saltwater leg/lower unit immersion and by 220 hours, corrosion was limited to slight rusting of one of the lower cowl retaining rivets and some paint bubbling around the dry exhaust relief hole beneath the powerhead. The upper zinc anode should survive another 100 hours of immersion.
The DF6 is the most powerful of a trio of 138cc single-cylinder OHV four-strokes, all with pressure lubrication, a rev limiter (6000rpm), 180-degree steering, in addition to a full gearshift and two shallow water drive positions. Nice touches are a large carry handle and a lower cowl oil level sight glass.
The DF6 meets with CARB 2008 and OEDA '3 Star' exhaust emission levels and has a two-year warranty for recreational applications.
|Engine type: OHV four-stroke single|
|HP at revs: 5.9 at 5250|
|WOT rev range: 4750 to 5750|
|Piston displacement (cc): 138|
|Bore x stroke (mm): 62 x 46|
|Ignition system: CD with electronic timing advance|
|Charging circuit: Optional 12V 6amp|
|Break-in period (hours): 10|
|Fuel type: ULP 91 RON|
|Fuel capacity (litres): 1.5 or 12|
|Oil type: SAE10W30 or 10W40|
|Oil capacity (litres): 0.7|
|Gear ratio: 1.92:1|
|Transom height (inches): 15/20|
|Weights (kg): 25.0/26.0|
|Rec. s/shaft retail September, 2007 $1992|