MerCruiser 6.2 MPI Horizon

Starting life as the Black Scorpion 377, the TRB-MCM 6.2 Horizon is a 'stroked' version of the long-running 5.7 litre or 350 Chev engine. At 101.6mm the cylinder bore remains unchanged but at 95.3mm the stroke is 6.9mm longer.

This allows the 6.2 to fit into the same space as the 5.7 and although the weight is no greater the torque delivery and fuel efficiency relative to engine output are considerably better due to the longer stroke which improves combustion chamber 'squish'. In both the inboard and sterndrive versions the maximum power output has risen by 20hp to 320, although in the sterndrive version it's measured at the prop and not the crankshaft. The sterndrive develops maximum power at 5000 revs and the inboard at 4800 with Wide Open Throttle revs ranges of 4800 to 5200 and 4600 to 5000 respectively.

Complete with a hydraulically-actuated Velvet Drive gearbox the inboard version weighs 431kg and measures 1092mm long, 740mm wide and 559mm high, although the height is measured from the crankshaft centreline and not the bottom of the lube oil sump. The engine-only dimensions of the sterndrive model are 814 x 740 x 559mm and it's available with a choice of Brave One, Two or Three legs, with the three-leg version weighing 486kg.

Sensible engineering
Although the Horizon version weighs only 23kg more than its raw/sea-water-cooled counterpart, it features thermostatically-controlled heat-exchanger cooling which allows the engine to operate at more consistent temperatures and emit fewer exhaust emissions because the engine is running within the temperature range for which it was originally designed. Running an automotive-base engine too cold not only increases engine wear but also internal corrosion due to condensation forming on the cylinder walls and greater blow-by gas past the piston rings which increases sump oil sludging between oil/filter changes.

With the 6.2 Horizon the first service is at 20 hours or three months then every 100 hours or once a year, but I would do an oil/filter change every 50 hours or six months if the boat is being used infrequently.

Unlike its 5.7 litre counterparts the 6.2 is available only with sequential multipoint fuel injection, which maintains more consistent running than carburetted engines due to the ignition timing advance and air/fuel ratio being constantly altered according to barometric pressure and ambient temperature. However as with the carbie engines Mercury Marine recommend using 95 RON Premium Unleaded fuel.

MPI provides turnkey starting just like a car engine and the system is compatible with the brilliant SmartCraft system which has more than 40 sensors to monitor engine operation and warn of any irregularities or even reduce engine revs should the need arise. SmartCraft provides digital fuelflow, range-to-empty and fuel used since the last time the tank was topped-up.

On the water
In February my local SeaRay dealer, Hirecraft Marine of Toronto, NSW, provided twin 6.2 Horizons in a 375 Sundancer sportscruiser.

The 375 is available with twin 6.2 vee-drive MerCruisers (not Horizon models) as standard but according to Hirecraft this version performs nowhere near as well as the sterndrive version. Perhaps it's the relatively steep shaft angle or that the props are operating in tunnels which reduce hull buoyancy aft, forcing drivers to use the trim tabs more frequently to get the boat out of the hole. Either way, I was glad that Hirecraft had the sterndrive version for me to test instead.

Unlike the inboard version, which has a lightship draft of 813mm, the sterndrive version draws 940mm with the legs down but only 686mm with the legs tilted up.

The demo engines started instantly hot or cold and transmitted virtually no vibration through the hull structure. But manoeuvring with the twin Bravo Three legs was very different to inboards or even sterndrives having single props, where prop steer torque can be used to pivot the hull on its axis using the one ahead/one astern technique. Far better to use the steering wheel and optional bowthruster (Hirecraft orders this as standard equipment in the 375) which enables the bow to be swung, just like driving a car, or the hull moved sideways.

Really the only manoeuvring aspect of the 375 I didn't like was the twin lever throttle and gearshift levers - when will US boat manufacturers realise that combined single lever throttle/gearshift systems are far more user-friendly?

Spinning 22 inch pitch props through 2.2:1 gear ratios and pushing an estimated total of 7.6 tonnes including the 5.0 kW Kohler petrol genset and Hirecraft's Geoff Hurt and myself, the 6.2s idled us out from Hirecraft's marina at a quiet 2.7kt (measured on the Raymarine GPS fitted) on 600 revs using 3.8lt/h each. At 1000rpm and 4.2kt the fuelflow was 7.3lt/h and at 1500rpm we averaged 6.5kt and 11.0lt/h each engine. Even at 2000 revs and 8.1kt the fuel usage was only 16.8lt/h, but at 3000 revs the hull was semi-planing at 12.8kt and the engines were working fairly hard, using 41.0lt/h each. Similarly at the clean plane average of 18.3kt on 3500 revs the fuelflow was 50.2lt/h each.

When the throttles were floored and the legs left on neutral trim the 375 planed almost as quickly as a trailerable Sea Ray bowrider, despite the trim tabs being left up. And from 4000 revs to Wide Open Throttle the acceleration was unlike any diesel I've tested in this power range. Through full-lock figure-eight turns at 4000 revs prop ventilation was non-existent providing the legs were maintained at neutral trim. Even at plus four degrees of out-trim the outboard engine's revs rose to 4500 but dropped back immediately the trim angle was reduced.

Cruising quietly on 4000 revs the average speed was 25.0kt and fuelflow 56.0lt/h each engine, very reasonable considering the distance per hour we were covering. At WOT the brand new (and consequently tight) engines averaged 4700 revs using 88.0lt/h each with a speed over ground of 31.6kt. At these revs and this speed we could still talk normally at the helm with just that oh-so-beautiful petrol V8 rumble in the background.

Considering the performance, fuel efficiency and relatively low maintenance costs, for recreational boaters who sprint out to their favourite destination then anchor overnight the 6.2 Horizon makes great economic sense compared to an equivalent-output diesel!