Mercury Diesel's new twin turbo 370hp TDI 4.2
is the first production marine engine to boast a better power-to-weight
ratio than its petrol-powered equivalent.
Mercury
Diesel’s new twin turbo 370hp TDI 4.2 is the first production marine
engine to boast a better power-to-weight ratio than its petrol-powered
equivalent. Based on the all-aluminium V8 block designed by German car
manufacturer Audi for its A8 and Q8 models, this marinised 370hp
powerplant is claimed to set new benchmarks for size, weight and
refinement.
Tipping the scales at a mere 379kg it is
over 200kg lighter than Volvo Penta’s equivalent 370hp D6 diesel engine
and even undercuts Volvo’s 8.1-litre petrol V8 by 10kg. It also has a
smaller footprint than any big-block petrol or diesel engine. Petrol
heads will argue that the petrol engine’s higher maximum power output of
420hp evens the score but with marine engines torque is the more
siginificant figure and by this yardstick the 572ft/lbs (775Nm) of
torque produced by the TDI 4.2 at 2,500rpm is unmatchable.
More significantly this engine produces a
punchy spread of power from 2,000rpm all the way up to 4,200rpm. Such
wide power bands are unheard of for marine diesel engines with most of
the good work happening between 2,000-3,000rpm. Petrol engines meanwhile
suffer the opposite problem with maximum torque not kicking in until
3,000rpm.
Whatever measurement you choose, the new
Mercury TDI 4.2 stacks up well against its closest diesel rivals, the
Yanmar 8LV-370 and the Volvo D6 370. The Yanmar weighs in at 450kg, hits
a peak torque of 590ft/lbs at 2,200rpm and is red lined at 3,800rpm.
The Volvo weighs in at 580kg, reaches a higher maximum torque of
650ft/lbs at 2,000rpm but runs out of steam earlier with a red line of
3,500rpm. However, the real crunch figure for smaller sportscruisers is
the 400kg weight saving which a pair of TDI 4.2s would give you over
twin D6s.
Mercury Diesel attributes the TDI 4.2’s wide power
delivery to the use of twin variable-vane turbochargers. As the name
suggests they work by adjusting the angle of the vanes within the
turbocharger to maximise boost pressure at low revs and reduce turbo
lag. At low rpm the vanes close up, meaning the exhaust gas has to force
its way through the small gaps at speed, causing the turbine to spin
rapidly even with low exhaust gas pressure. As the revs increase, the
vanes open up to maintain the same level of boost without fear of
overloading. Variable-vane turbochargers have proved very effective in
cars but have been problematic in some marine installations due to
relative lack of use.
The long stroke construction of the engine is another
torque booster. A long stroke means a long conrod increasing the amount
of energy transmitted to the crankshaft. The TDI 4.2 has a
stroke-to-bore ratio of 1:15 meaning the stroke is 15% longer than the
bore is wide. In comparison the 370hp V8 Yanmar 8LV-370 has a
stroke-to-bore ratio of 1:11. The downside of a long stroke is greater
piston speed resulting in increased friction. However this problem only
comes into force at engine speeds well above 5,000rpm so this will not
inhibit the TDI 4.2.
The new Mercury Diesel TDI 370 is available with
either a MerCruiser sterndrive or a ZF gearbox for shaftdrive
applications. The sterndrive options are Bravo 1, 2 or 3 in either X or
XR formats. All feature the SeaCore corrosion protection system.
SmartCraft comes as standard enabling the TDI 4.2 to be interfaced with
all digital Mercury instruments. DTS (digital throttle and shift) and
the Axius joystick system can also be specified. Scorpion is already
offering it as an engine option on its range of performance RIBs but
expect many other power boatbuilders to list the TDI 4.2 as a
factory-fit option soon. A 335hp version will also be available. All
engines are Tier 3 compliant and available for light duty commercial as
well as leisure use. Prices start at £41,000 including VAT with the
Bravo 1 XR sterndrive.
