Tohatsu TLDI 50

Released late last year, the MW TLDI 50 utilises the same gearcase as the carbie two-stroke Tohatsu 60 (reviewed some time ago in Trailer Boat) but has a 2.15:1 gear ratio. And although I would have preferred the same 2.33:1 ratio as the 60, the MW's ratio is far better suited to pushing semi-displacement hulls than the 1.85:1 ratio of the standard TLDI 50.

Since I first tested the "A" model TLDI 50 almost five years ago the model has undergone several improvements. The engine's electronic control unit processing power has increased four times to 32 bits and inductive ignition fitted, along with an electric oil pump for more precise control of the fuel/oil ratios. These changes make the "B" model more throttle-responsive and smoother-running across the entire rev range while reducing exhaust emissions due to a more thorough air/fuel burn in the combustion chambers.

But the same basic Orbital Combustion Process used in the "A" version has been retained, which functions by injecting air into the fuel delivery rail at 5.7 bar (80psi) then mixes it with fuel pressurised to 6.4 bar (90psi) and directs the mix to three combustion chamber injectors. This results in complete air/fuel atomisation (down to a fuel droplet size of only five microns) without having to resort to the much higher fuel injection pressures used in Bombardier's E-Tec and Yamaha's HPDI DFI systems.

A single throttle body air intake incorporates Throttle Position Sensors for load and barometric pressure (to ascertain the correct air/fuel ratio). The stratified combustion system below about 3000 revs gives an air/fuel ratio of about 40:1, richening to 25:1 when the engine operates on homogenous combustion. In comparison, an EFI four-stroke has an air/fuel ratio of about 14.5:1.

Standard are warning systems for engine over-rev (6000 revs), low oil, engine overheat and low battery voltage. Should any of these problems occur, after a pre-determined period the ECU will progressively reduce the engine revs down to 900.

Mounted on a Stacer 640 Pontoon Boat supplied by In-Tune Marine of Long Jetty NSW, spinning a 12x9 inch alloy Tohatsu prop and pushing a total of 1550kg including two adults, the demo MW from Lakeside Marine performed much better than expected, even against a carbie two-stroke Mariner 75 I tested a week earlier on this hull under similar conditions. With its rounded-bottom pontoons and no chines to provide hull lift, the 640 is a difficult boat to push and will never achieve planing speeds.

But whereas spinning a 14in stainless steel Quicksilver Vengeance prop the Mariner had severe prop ventilation through tight figure-eight turns at Wide Open Throttle and forced the 640 to lean outwards through the turns, the 27 per cent lighter weight of the MW on the transom eliminated any ventilation and the hull cornered flat.

Surprisingly, the MW had better throttle response than the Mariner at low speeds and averaged faster cruising speeds. Providing the engine revs were limited to 2000 the MW had very good power astern and across its entire revs range was a beautifully quiet and smooth engine, much better than the "A" model.

Powerhead access is very good and the air compressor drive belt, fuel pump, vapour separator, injector block and throttle body assembly are easily reached. The first service is at 10 hours, then every 50 hours or six months. The waterpump impeller should be replaced and the air compressor drive belt checked for damage every 100 hours or yearly.

The warranty coverage is valid for two years for recreational usage.