MerCruiser MX 6.2 MPI

Developed from the Black Scorpion 377, the 6.2 MerCruiser is available in two power ratings for skiboats and one output in sterndrive form, the latter being available with either raw (salt) water cooling or the Horizon heat exchanger model. The inboard "Tow Sports" versions are known as the MX 6.2 MPI and the MX 6.2 Black Scorpion and develop 320.4 and 340.5hp respectively (measured at the crankshaft) and both at 5000 revs. Complete with 1:1 hydraulically-actuated Velvet Drive gearboxes, these engines weigh 373kg, only 4kg over the 315hp 350 MAG MPI inboard.

However, the MX 6.2 MPI sterndrive version develops 320.4hp at 5000rpm (compared to 300.3 at 4800 revs for the 350 MAG MPI) but at the prop instead. The MX 6.2 MPI is available with a choice of Bravo One, Two or Three legs - but for hulls such as Sea Ray's larger bowriders, the Three drive not only provides better out-of-the-hole performance but also faster wide open throttle (WOT) speeds because prop steer torque is eliminated by the twin counter-rotating props.

Some time ago I reviewed the 350 MAG MPI/Bravo Three combination in a Sea Ray 220 Sundeck and it returned excellent fuel efficiency for a petrol engine this powerful. Pushing a total of 2250kg including two adults, the trolling average was 6.1kmh on 600rpm and at 1000 revs we averaged 8.8kmh. At 2000 revs the average was 16.6kmh and a clean plane was achieved at 28.9kmh on 2500rpm. Cruising at 3000 revs the average was 42.7kmh and fuelflow 42.7lt/h, while at 4000 revs and 64.3kmh the usage was 65lt/h. At WOT and 5200 revs the averages were 81.4kmh and 92lt/h.

Although I have yet to test the 6.2 in a trailerboat, the twins I tested in a SeaRay 375 Sundancer sportscruiser showed the 6.2 to have a fuel efficiency increase over the 350 MAG MPI. According to the SmartCraft system fitted, at 600rpm the fuelflow for each engine was a mere 3.8lt/h, only six per cent greater than Yamaha's two-stroke HPDI 200 outboard!

At 1000rpm the average was 7.3lt/h and 1500 revs 11.0lt/h, a reasonable consumption for offshore trolling speeds. At 2000 revs the fuelflow was 16.8lt/h, but because in the 375 the hull was starting to plane at 3000 revs the flowrate jumped to 41.0lt/h and even when the hull was planing at 3500rpm the flow was still relatively high at 50.2lt/h. However by 4000 revs - cruising speed for the 6.2lt engines in the 375 - the fuelflow had risen marginally to 56.0lt/h and even at WOT and 4700 revs the engines used 88.0lt/h each. Compare this figure to a 2.6lt supercharged Mercury Verado 250 outboard, which used 109lt/h at WOT during trials.

As with the 350 MAG MPI the 6.2 engines started instantly hot or cold, didn't blow any oil smoke and were almost completely unobtrusive when trolling. Only at 4000 revs and above was there any real exhaust noise but with that beautiful petrol V8 burble in your ears, who cares?

The 6.2 engines tested were Horizon models and for an additional 23kg over the raw water cooled 6.2. I believe these engines are well worth the extra outlay. The thermostatically-controlled heat exchanger cooling 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 greater blow-by gas past the rings which increases sump oil sludging between oil/filter changes.

Maintaining and servicing the 6.2 is the same as the 350 MAG MPI, with the first service at 20 hours or three months then every 100 hours or once a year which includes waterpump impeller replacement. But if the engine is used infrequently then the oil and filter should be changed every 50 hours.