MerCruiser HP500 Bulldog and 350 MPI Horizon

Though few people would describe me as a revhead, I've always loved the concept of lazy, understressed petrol V-eights in (selected) marine applications.  From personal experience of operating a twin V-eight-powered cruiser several years ago, the sound of big block V-eights burbling along is a joy to behold.  And frankly, for most pleasure boat owners who clock-up about 150 operating hours per year, petrol's arguably the way to go.

MerCruiser's Chevrolet-based petrol V-eights have long been the main staple for US boat operators.  Even cruising and some fishing craft come equipped with petrol engines as standard equipment stateside.

In the sportsboat arena, diesels are often well down the options list.  With powerplants like MerCruiser's recently-released 470hp HP500 Bulldog, that's easy to understand!  The HP500 is the largest capacity petrol V-eight yet offered by Mercury and displaces a whopping 8.9lt.

It's an almost 'square' engine with a bore of 112.8mm and stroke of 111.1 and develops 469 propshaft horsepower in the 4800-5200rpm Wide Open Throttle (WOT) rev range.
Nonetheless, despite this prodigious output - thanks to a relatively low 8.75:1 compression ratio - the HP500 runs on standard unleaded petrol.

At a time when engine manufacturers are opting for electronic engine management, the HP500 is a refreshingly simple change.

Unlike the Mercury HP500EFI, which develops the same amount of maximum power from its 8.2lt - though obviously nowhere near the torque - the HP500 has a single 800 cubic feet per minute Holley carburettor and relatively simple Thunderbolt V electronic ignition.  The only drawback is the all-up weight, 700kg compared to 516.

To give some idea of how understressed the HP500 is, its supercharged HP900SC counterpart (with which the Bulldog shares its basic architecture) develops 850 propshaft horsepower at 5900 revs and is fed by twin 1050 cubic feet per minute Holleys.

The HP500 is the smallest MerCruiser V-eight to be blueprinted and has a polished crankshaft and its bearings, connecting rods, pistons and rings are all hand-fitted to exact tolerances.

Like the lower-spec MerCruiser V-six and V-eight sterndrives, the HP500 has pushrods with roller bearings on the camshaft lobes to reduce friction and increase component lifespan.  The exhaust risers are stainless steel.

Cleverly, MerCruiser has fitted a thermostatically-controlled oil cooler to sterndrive models.
This system quickly raises oil temperature to its most effective level and keeps it there for maximum engine protection under load.

Complementing the cooler is a Positive Crankcase Ventilation valve which draws air out of the crankcase and in conjunction with the cooler reduces condensation levels in the oil for longer oil life and less internal corrosion.

The PCV valve also reduces oil vapour build-up and improves engine idle quality.
Standard are colour-coded, hand-removable coolant drain plugs and service points for the engine and drive oil and coolant and power steering fluid.

A single serpentine belt drives the water and power steering pumps and 55 amp alternator.
The HP500 is available with a Bravo One or Bravo One Performance Drive and reduction ratios of 1.36:1 or 1.50:1.

The Bravo One is the smallest MerCruiser drive to have the gearshift located at the top of the drive for easier servicing access and is a great hydrodynamic compromise between the Alpha and Bravo Two drives.

If you plan to operate in aerated (rough) water or shallow water with the drive well past maximum trim range, consider fitting the Performance Drive.  The water intakes are located at the bottom edge of the gearcase torpedo and ensure a more uninterrupted flow of water to the powerplant, particularly when running hard.  The end result of the HP500 Bulldog concept is a package that provides plenty of grunt when needed, but with regular maintenance should last countless seasons of on-the-water enjoyment.

The warranty is one year for general components and three against corrosion perforation.

350 Mag Mpi Horizon

Slotting into the same low-maintenance mould as the innovative 454 MAG MPI Horizon inboard, MerCruiser's new multipoint-injected 350 Horizon is based on the venerable Chevrolet small block V-eight.

Displacing 5.7lt, the powerplant still develops an impressive 300hp in the 4400-4800rpm WOT rev range and develops comparable torque to the 3.5lt turbo-intercooled Yanmar 230hp diesel.

At WOT the bent eight consumes around 80lt/hr, dropping to around 25 in the 2500-3000rpm range.

Stroke is 88.4mm, while the bore of 101.6mm dimensions shared with V-six 4.3lt sterndrive engine, improves spare parts commonality across the MerCruiser range.

Although the small block Horizon's compression ratio is a relatively high 9.4:1, an anti-knock sensor in the electronic engine management system allows the powerplant to happily run on standard ULP.

Complete with a hydraulic Borg Warner 1:1 Velvet Drive 71C, the 350 Horizon weighs a reasonable 426kg.

However, for installation in cruisers, most pundits would recommend the 1.5:1 or 2:1 ratio Velvet Drive 5000A which has an 8° down-angle at the propshaft flange.
With a 71C the 350 Horizon is 1143mm long, 762 wide and 559 high from the crankshaft centreline to plenum chamber cover.

Sensibly, freshwater cooling is standard, along with a top-mounted flushing device for the heat exchanger and rawwater-cooled exhaust system and stainless steel constant tension hose clamps.

The exhaust elbows and manifolds are ceramic-coated and a serpentine belt system runs the cooling water pumps and massive 65 amp alternator, which is mounted well up on the motor away from bilgewater.

A water-cooled fuel system (to prevent vapour lock), water-separating fuel filter and remote oil filter are also standard, as is an audio alarm warning system.  The spark plug electrodes have platinum tips for long life.

Indeed, the biggest selling features of the 350 Horizon is its extended maintenance intervals.
According to Mercury Marine, the 350 requires tuning, checking drive belt tension and replacing the PCV valve once every three years and changing the coolant every five years.
Ignition timing is set at pre-delivery then left untouched, along with the exhaust system.
The water pickup, seawater strainer and pump, plus the battery and ignition system, need checking only if an engine malfunction occurs.

Surprisingly, Mercury recommends changing the engine oil and filter only once a year instead of every 50 operating hours for the 5.7lt 260hp carbie inboard.

Frankly, I'd do the change every six months regardless of how few hours are clocked up.
For owners who clock-up less than 150 operating hours a year, the 350 Horizon makes great economic sense - particularly if you have an older craft and want to upgrade from the existing carbie motors to low-maintenance EFI units without going to the expense of diesel.