Top End: Talking Torque with Andrew Norton

Yanmar has become a master at extracting heaps of torque from relatively small displacement engines. And its 4LV 250 is a prime example with a flat torque curve from 2,000 to 3,000rpm.

At the top end of a range of 2.8-litre Toyota-based diesels from 150 to 250 MHP, the 250 can almost compete with the 3.0-litre V6 VW Amarok-origin diesel for torque and output. But it's substantially more compact and seven per cent lighter, despite developing only one per cent less torque and four per cent less maximum output. In fact the 4LV 250 has a similar torque output to the MerCruiser 6.2-litre V8 petrol engine but at way lower rpm.

The entire 4LV range is NMEA and J1939 CAN-bus compatible, which Yanmar says allows for “future proofed integration to any multifunction bridge display”. What this means is real-time information such as fuel flow, range to empty and of course engine problem solving using the diagnostic facility provided.

THE NITTY GRITTY

Of course, like most modern high performance marine diesels the 4LV 250 has electronically managed common rail fuel injection, in this case made by Denso. In my opinion Denso is a very clever engineering corporation that among achievements makes some of the best automotive aircon systems I've ever tested. The adjustable-temperature Denso aircon I had in my 1980 Corolla was the best system I've ever had in 

a car. The airflow temperature could be set precisely according to ambient temperature, although this aircon did knock the guts out of performance up hills.

Having electronic injection means that not only is the injector fuel spray volume adjusted according to engine load but also the spray timing is variable, reducing the possibility of cylinder wall glazing due to fuel over-supply at low rpm. The precise fuel management enables the 4LV 250 to meet US EPA Tier 3 and European RCD (Recreational Craft Directive) 2 exhaust emission regulations, making the 250 one of the cleanest-running diesels in its output range. Not that Aus has regulations like these yet but I'm sure most boaters don't want to pollute our waterways if at all possible. Perhaps that's just the yachtie in me.

As with its less powerful counterparts the 4LV 250 develops rated power on distillate (diesel) having a specific gravity of 840 grams per litre, which is the same density as low-sulphur diesel sold in Australia. So we get a true representation of engine torque and output, whereas previous Japanese and European diesels were rated at 860 grams per litre and their outputs had to be de-rated for Australia. 

The 250 has camshaft belt drive that should be checked annually for any deterioration and replaced every 800 to 1,000 running hours. Should the cylinder head gasket need replacing, removing the belt and cylinder head are less fiddly than engines having push rod-operated valves. And of course being an in line engine doing this task  in the bilge of a boat is way easier than with a V-block engine.

The 2 kW starter motor is located above engine mounting feet level while the massive 12-volt 130-amp alternator is mounted at cylinder head level. As the engine was designed primarily for right hand drive vehicles the engine oil dipstick and inverted oil filter are located on the starboard-side beneath the exhaust manifold, while the air intake manifold and starter motor are to port. The engine drawings don't show an engine sump oil evacuation pump so one would need to be fitted, but the fine fuel filter with hand priming pump is located to port at the forward end of the engine. The coolant drain cock is also to port but easily reached. An emergency engine stop button is located at the forward end.

Yanmar recommends an SAE15W40 diesel for temperate and tropical conditions and as the engine is highly stressed I suggest changing the oil and filter every 100 running hours or six months. Admittedly the oil sump capacity of 8.5 litres is large for a 2.8-litre engine, but when worked hard the 250 will place a lot of load on the oil, especially as the maximum torque output is 40 per cent higher than when this engine is fitted to a Hilux. The straight and V-drive boxes need a mono grade SAE30 oil, very unusual for hydraulic boxes. But understandable in that mono grades have no viscosity improvers to fail under high loads, unlike multi grade oils.

The eight degree down angle box is offered with ratios of 1.67:1 to 2.43:1 ahead and astern and for planing hulls I suggest the deepest ratio would be the most effective way of converting engine torque to thrust at the prop with 1,560rpm at the prop. Only in lightweight planing hulls or where there's insufficient space to fit larger diameter props needed to handle the engine torque (such as re-powering from a petrol inboard) would be the 2.13:1 ratio be suitable. The 12 degree shaft angle V-drive has ratios of 1.22:1 ahead and astern to 2.47:1 and again I suggest the deepest ratio in all but lightweight hulls where the 2.08:1 ratio may be more suitable. The rule of thumb when powering heavier planing hulls is that maximum prop rpm should be 1,500 to 1,600 and up to 2,000rpm for lightweight planing hulls.

The 4LV 250 has bobtail (engine only) dimensions of 921 millimetres long by 755 wide and 772 high, whereas the bobtail VW 3.0 litre measures 927 by 813 by 853 millimetres and weighs 358kg, so the 4LV 250 is way more compact. Complete with the eight degree down angle box the 4LV 250 is 1,151 millimetres long whereas the V-drive total length is 1,152 millimetres.

THE WRAP

When it comes to working on engines in bilges I must confess I prefer in-line engines. The 4LV 250 would made a good re-power option in twin installations for older cruisers such as the Mariner 31 and provide way better performance than the old 3.6-litre straight-six 165 horse power Volvo Penta diesels commonly fitted to this hull. Alternatively, it really adds some oomph to a Bertram 28 by not only developing way more torque than the old 260 HP 5.7-litre V8 petrols but also being substantially lighter. And when the 4LV 250 is available in stern drive form it would transform a Caribbean 26 into a pocket rocket.

However, I would not recommend the 4LV 250 for displacement hulls, although its 4LV 150 counterpart may be suitable for semi-planing hulls. As with all high performance diesels the 4LV 250 would need careful propping to enable it to reach slightly over 3,800rpm under normal loads. I also suggest that the maximum continuous rpm should be 3,000, which in any case is still in the maximum efficiency band.

For more recent hulls that have twin engine installations and a bow thruster, Yanmar offers its JC20 joystick control for easier docking and control when short handed aboard the boat.

Google Yanmar Marine Diesels, click on Find a Dealer, enter the type of dealer (marine) and enter your region and postcode to find your nearest dealer.

SPECS

YANMAR 4LV 250

Engine type Four-cylinder turbo inter cooled diesel 

Rated BHP/MHP* 246.6/250.2 at 3,800rpm

Max torque 560NM at 2,000 to 3,000rpm

Displacement 2755cc

Bore x stroke 92 × 103.6mm

Dry weights BOBTAIL 332 kg with 8 degree down angle 384 kg with V-drive 407 kg

* Brake horsepower/metric horsepower or PS.