Yanmar's six-cylinder diesel sterndrives

Six and out

Like the 4BY series of diesel sterndrives, the double OHC 6BY engines were released in Australia at the 2006 Sanctuary Cove Boat Show and are a cooperative effort between BMW and Yanmar. Two output ratings are available, either 217.2 or 256.0 crankshaft horsepower, with 1hp equalling 646W and both at 4000rpm.

The base engine is a straight six-cylinder heat exchanger-cooled direct injection unit that displaces 2993cc and has a cylinder bore of 84mm and a piston stroke of 90mm. The electronically-managed high pressure common rail fuel injection system and wastegated turbocharging and intercooling with the charge air being cooled by seawater for maximum air density enable the engine to meet US EPA Tier 2 exhaust emission regulations, scheduled for introduction this year.

Standard features include a 12V, 150-amp voltage-regulated alternator and, in line with automotive practices, the top of the engine has a removable cover cut away at the sides for access to the engine oil dipstick and oil and fuel filters. The alternator is mounted reasonably high above the engine bearers and the drive belt is encased in another quick-remove cover.

For owners of sportscruisers in the 11 to 12m range, in twin installations the 6BY competes directly with the pushrod OHV 6.2lt V8 multipoint EFI petrol sterndrive competition for peak torque output. For example, the 6BY220Z has a peak torque of 495Nm at 2500 to 2700rpm, and the 6BY260Z has 550Nm at 2500rpm, whereas the 320hp (at the prop and about five per cent higher at the crankshaft) V8 produces around 500Nm in the 3000 to 4000 rev range.

The torque rise of both 6BY engines is rapid, with the 220Z producing 225Nm at 1000rpm, 290 at 1500rpm, and 450Nm at 2000rpm. The 260Z produces the same torque at 1000 and 1500rpm and 470Nm at 2000 revs. Even at 4000rpm the 220 produces 390Nm, while the 260 produces 470 at the same revs.

As torque curves are not available for the 6.2lt V8, it's fair to assume the torque curve is peakier and falls off fairly rapidly above 4000rpm. The V8 develops maximum output at 5000rpm with a wide open throttle rev range of 4800 to 5200rpm, substantially higher although with bore/stroke dimensions of 101.6 and 95.3mm the engine is an oversquare design compared to the 6BY series, so the piston speeds will be only marginally faster.

Only available with MerCruiser Bravo X3 legs, which have twin counter-rotating props, the 220Z and 260Z both weigh 408kg, compared to 486kg for the similarly-equipped 6.2lt V8 with heat exchanger cooling. The engine-only dimensions of both 6BY models are 1007mm long, 769mm wide, and 740mm high, whereas the V8 is 814mm long, 740mm wide, and 559mm high. However, as the V8's height is measured from the crankshaft centreline and not the bottom of the oil sump, about 200mm should be added to the overall height. For straight sixes, which should be narrower than comparable-output V8s, the width of the 6BY engines is surprising. Perhaps the 15-degree cylinder inclination has something to do with this, but I would still have expected the 6BY series to be narrower.

As all owners of petrol-powered sportscruisers know, the downside of having the instant acceleration, low noise levels, absence of exhaust fumes, and relatively low maintenance and repair costs is relatively high fuel consumption.

For example, twin 6.2lt V8s I tested a while back in a SeaRay 375 Sundancer, spinning 22-inch pitch stainless steel props through 2.2:1 gear ratios and pushing an estimated total of 7.6 tonnes, averaged 4.2kts and 7.3lt/h per engine at 1000rpm. Based on a standard prop power curve, the 220Z and 260Z would use 2lt/h at these revs.

Increasing the revs to 1500 returned 6.5kts and 11lt/h per engine, while the 220Z would use 4lt/h and the 260Z 5lt/h. At 2000rpm, the V8s averaged 16.8lt/h, each for 8.1kts compared to 8.5 and 10lt/h for the 220Z and 260Z respectively. But at 3000rpm and a semi-planing 12.8kts, the V8s used 41lt/h each, where to achieve this speed the 220Z and 260Z would be revving at around 2500 due to the higher torque at these revs. Even allowing for increased fuel flow due to the high engine loading, the 220Z would use 16lt/h and the 260Z would use 20lt/h.

At 3500rpm, with the 375 planing cleanly at 18.3kts, the V8s used 50.2lt/h each, whereas assuming the 220Z and 260Z would achieve this at 3000rpm, the fuel flow should be 22 and 27lt/h respectively. Increasing the revs to 4000rpm returned 25kts and 56lt/h per engine, compared to 32 and 39lt/h for the 6BY engines.

The maximum continuous revs for both 6BY engines are 3600 - only 400 below maximum, whereas the V8s should not be operated continuously above 4000rpm. At 3600rpm, the 220Z uses 34lt/h and the 260Z uses 41lt/h.

Slightly over-propped, the V8s returned WOT averages of 31.6kts and 88lt/h each at 4700rpm, although with finer-pitch props allowing the V8s to reach their recommended WOT rev ranges, the fuel flow would be slightly less due to the decreased engine loading. At WOT, the 220Z uses 45lt/h and the 260Z uses 53lt/h at 4000rpm. That’s a big saving for owners of sportsboats who like offshore passagemaking, especially as the V8s need 95 RON premium unleaded petrol, which can be hard to get from marina pumps.

The 220Z and 260Z come with instant readouts of fuel flow, range-to-empty etc, and a multifunction LCD display with analogue readings of engine revs, oil pressure and coolant water temperature.

For more details on the 6BY 220Z and 260Z, contact Michael Blair at Power Equipment on (03) 9764 0711, or email power.equipment@yanmar.com.au