MerCruiser 4.3 MPI

Available in both raw- and freshwater (heat exchanger) cooled versions, the long-running MerCruiser 4.3 sterndrive is an effective compromise between its 3.0lt counterpart and the 5.7 and 6.2lt V8s in the MerCruiser range.

Essentially a six-cylinder version of its 5.7lt counterpart, the 90-degree V6 4.3 develops 190.3hp at 4600rpm with a two barrel carburettor and 219.8hp at the same revs with sequential multipoint EFI.

Both engines have a Wide Open Throttle rev range of 4400-4800. Combined with an oversquare design with a bore of 101.6mm and stroke of 88.4mm they are understressed engines that, maintained regularly, should provide years of low-cost motoring.

As with their V8 counterparts, the V6 models take 95 RON premium unleaded fuel instead of standard 91 RON unleaded.

While the carbie 4.3 is rapidly replacing the 3.0 litre sterndrive in trailerable bowriders around 5.5m, the MPI version is more popular in cruisers because of its sealed fuel system that provides a higher degree of safety against fuel vapour venting into the bilges. It also provides true turnkey starting just like an EFI car engine and is SmartCraft compatible, providing features such as fuel flow and range-to-empty monitoring not available with the carbie version.

As with its V8 counterparts, the 4.3 MPI is available with Alpha, Bravo Two and Bravo Three legs and the dry weights are 393, 414 and 419kg respectively.

Heat exchanger cooling adds 24kg - the additional weight is worth it because the engine operates within the temperature range for which it was designed and not too cold. Because the engine runs hotter it also creates fewer exhaust emissions.

In my opinion, the best feature of the heat exchanger cooling is the engine protection it provides. Because the engine runs hotter there's less cylinder wall condensation that occurs in raw-water-cooled engines and should the cylinder head gasket fail, only coolant will reach the cylinders, and not saltwater.

DRIVING THE 4.3 MPI
The SeaRay 315 Sundancer comes standard with a single MerCruiser 496 Magnum Bravo Three sterndrive which has prop rated output of 375.3hp and weighs 557kg. So it was very interesting to test the 315 with twin 4.3 MPIs, which added another 277kg to the hull displacement. A rule of thumb with twins versus singles is that due to prop blow out or wash interference between the props, a single 375hp engine is equivalent to twin 205hp engines - according to this rule, with lesser weight aft the single engine should perform almost as well as the twin 220s tested.

I would have thought Bravo One or Two legs better suited to this application, but surprisingly the Alpha legs performed very well with 1.62:1 gear ratios and spinning 14.5 x 17in stainless-steel Quicksilver Vengeance props.

With single props, the one engine ahead/one astern technique enabled the 315 to be spun in its own length without the need for a bowthruster, whereas the twin 6.2s with Bravo Three props tested in a 375 Sundancer merely 'crabbed' the hull sideways.

As expected, the 4.3 MPIs started instantly hot or cold but seemed much better balanced than expected of V6 engines. No oil smoke appeared at any time and providing the legs were kept in the run position, power astern was very good.

Because the trim tabs fitted to the 315 aren't supplied with gauges, we elected not to use them, and to rely on trimming the legs instead. Pushing a total of 4.6t including two adults and three quarters fuel, at an almost silent 610rpm we averaged 2.5kt using 3.3lt/h each engine. Increasing the revs to 1000 returned 4.4kt and 4.9lt/h, a useful speed for sightseeing on our crowded capital city waterways. At 1500rpm the averages were 5.8kt and 7.3lt/h each side; at 2000rpm, 7.6kt and 12.9lt/h. Trimming the legs right in we achieved a clean plane at 13.2kt on 2800rpm but because the engines were working hard to keep us up the fuel flow jumped to 26.2lt/h each engine.

However at a slow cruise of 19.5kt on 3000rpm with the legs slightly out the flow had increased to only 28.7lt/h. Even at a fast cruise of 30.1kt on 4000rpm the usage per engine was 41.0lt/h. Through full lock figure of eight turns at these revs no prop ventilation occurred. The standard power steering was light but reasonably direct.

The WOT average of 35.3kt on 5000rpm was not as big an increase in speed as we had anticipated and at 61.6lt/h per engine the fuel flow was much higher, but the ability to cruise quickly to a favourite anchorage is more important than outright speed in a boat like the 315. At WOT we could still hold a normal conversation at the helm but the exhaust beat was nowhere near as sweet as a V8!

SENSIBLE ENGINEERING
As with its V8 counterparts, the 4.3 MPI has a serpentine belt for the cooling water and power steering pumps and a voltage regulated alternator that produces up to 65A at 14.1V. A remote oil filter and a seawater cooling drain system for when the boat is being slipped are standard. Individual lever-valve seacocks and water strainers for each engine are a more sensible approach to minimising blockages than using the normal Alpha leg intakes.

Because the 4.3 MPI has the same overall width as its V8 counterparts, shoehorning the engines into the 315 - with a beam of 2.87m compared to 3.66m for the 375 previously tested - is not an easy task. Access to the inboard injectors and spark plugs was tight, but the main service points are at the forward end of the engine.

According to Hirecraft Marine of Toronto, NSW (tel (02) 4959 1444), which supplied the demo engines, servicing is normally required only once a year or every 100 hours after the first 20 hours or three months, although I would change the oil and filter every six months if using the engine infrequently.

For boaters who clock up less than 150 hours a year (and that's generous for sportscruiser usage) the 4.3 MPI is a cost-effective alternative to diesel engines.