Review: Nanni Diesel 6.420 TDi Marine Engine

In the recreational boating world where more V-block automotive diesels are being marinised, it’s refreshing to encounter an inline engine in the 4.2-litre range. Inline engines offer way better servicing access when mounted in boats, particularly twin installations where V-blocks can force the inner cylinder banks close to one another.

Sure, an inline engine will never have that wonderful exhaust rumble that even V-sixes have under load, such as coming onto the plane, but being the practical nerd I am I’d always choose servicing ease over a pleasurable exhaust note.

THE NITTY GRITTY

The all-cast-iron Toyota engine is unusual for a straight six, having two harmonic balancing shafts. It is also marinised by Yanmar but Nannidiesel has managed to extract more torque and power from the same base engine without resorting to electronic engine management. Maximum torque is up 14 per cent at the same revs with peak output two per cent higher and 200rpm lower. However, with a standard hydraulic gearbox weight is up four per cent.

Having direct injection the compression ratio is a relatively low 15.7:1, but despite being mechanically controlled the engine complies with all current European and US exhaust emission control requirements.

Unlike many automotive motors, the standard 80-amp (120A optional) voltage-regulated alternator is mounted well above engine bearer height, as is the starter motor. The cam belt is protected by a full shroud. Heat exchanger cooling with an engine-oil cooler is standard and the fuel/water sedimenter is mounted at the forward end where it’s instantly accessible.

The 6.420 TDI is available with six hydraulic gearbox options; the ZF63A is standard. This ’box has an eight-degree down angle at the output flange; combined with the maximum static engine installation angle of seven degrees, this means the engine can easily cope with a normal planing hull’s shaft angle of around 12 degrees. The optional ZF63IV V-drive gearbox has a 12-degree down angle so in these installations the engine can be mounted parallel to the static waterline.

Complete with ZF63A ’box the conventional shaft drive model is 1311mm long, 669mm wide and 740mm high. The standard C3 instrument panel includes an analogue tachometer with digital hour meter, plus oil pressure and coolant temperature gauges, a voltmeter and audible alarms for low oil pressure and high coolant temperature.

Depending on the ambient temperature most likely to occur between oil and filter changes, a diesel-specific SAE 10W30 oil can be used in colder climates and an SAE 15W40 oil in tropical conditions. Alternatively and if the engine is likely to be worked hard between oil and filter changes, for temperate climates a straight mono-grade SAE 30 can be used. Mono grades don’t have viscosity index improvers to thicken the oil as engine temperature increases and so are less likely to fail (revert to the base viscosity) under high loads. The only drawback is that in a cold snap, thick oil may create cold starting issues.

ON THE WATER

Although I haven’t tried the 6.420 TDI in a boat I did test the Yanmar version of the 100 engine in a mate’s Cresta 32. The twins started instantly with incredibly low vibration levels (how I love a straight six) and no fear of flab reduction and blew black smoke from fuel oversupply only when the hull was coming out of the hole. Unusually, the engines were mounted aft-facing under the cockpit floor and swung the four-bladed 19x21in props via ZF63IV V-drives with 2:1 reductions.

Pushing a total displacement of 7.8 tonnes, the Yanmars returned an offshore troll of 9.1kt and 1500rpm using 6.0L/h each.

As with all correctly matched hull and engine installations the engines planed the Cresta at less than maximum torque – in this case 14.0kt and 2200rpm. So when the hull was fully planing at 17.7kt and 2500rpm the engines were right at their peak torque and returning excellent efficiency; only 19.5L/h each. Increasing the rpm to 3500 (maximum continuous rpm for this engine) the Cresta averaged 26.7kt using 41.0L/h each. The wide-open throttle averages were 29.4kt and 3800rpm using 60.0L/h each, and we could still talk normally on the fly bridge.

With the increased torque of the 6.420 TDI a hull like the Cresta should plane at lower rpm and deliver slightly better cruising fuel efficiency. The engines might even crack the 30kt barrier!

THE WRAP

Nanni Diesel has been very clever in using the Toyota base engine. The 4.2-litre unit, along with its 3.0-litre four-cylinder counterpart, have been some of the smoothest-running and most reliable automotive diesels ever produced and it’s comforting to know that many parts can be sourced from Toyota dealers.

SEA TRIALS

Because this engine has mechanical fuel injection it shouldn’t be operated continuously below 2000rpm or above 3300rpm. The fuel flow figures below are for displacement and semi-displacement hulls and the engine may use slightly more fuel under load during transition from displacement to planing mode.

Note the rapid torque rise from 1200 to 2000rpm and how much torque is still available at 3600rpm.

Quick specs

Engine type Turbo-intercooled direct injection straight six

Rated BHP/MHP* 315.7/320.2 at 3600rpm

Max torque 795nm at 2400rpm

Displacement 4163cc

Bore x stroke 94 x 100 mm

Weight 469kg (dry, with ZF63A ’box)

*Brake horsepower/metric horsepower