MerCruiser 496 Series

I must confess some sadness when Marine Power International informed me in March of Mercury Marine's replacement of the longstanding GM 454ci (7.4lt) V-eight with the new GM Vortec 496ci (8.1lt) model.

The 454 was my all-time favourite petrol V-eight because of its sheer reliability, torque and relatively good fuel efficiency.

My first experience of the 454 was in 1983 when I skippered a Halvorsen 40 for informal charters on Sydney Harbour. The twin 300hp Chrysler Golden Lions fitted utilised the 454 block, and even though the motors were then 16 years old, they proved completely reliable. Starting was also instantaneous, hot or cold, the power delivery silky-smooth across the entire rev range, and when cruising around the harbour at 1200-1500rpm they could hardly be heard.

They were also surprisingly fuel-efficient for a big block V-eight. Yet, when needed, they easily pushed the Halvo to 25kt.

One reason for their reliability was their closed circuit (freshwater heat exchanger) cooling, which ensured they ran at the optimum temperature for combustion efficiency and effective reduction of condensation. The main reason why many inboard petrol V-eights wear out prematurely is the open circuit (raw-water) cooling makes them run too cold to reduce the corrosion-inducing condensation. (In automotive applications they run 20-30?C hotter - alas, cold-running is necessary to prevent the rapid build-up of salt crystals in the cooling passages.)

Sensibly, Mercury Marine has fitted closed circuit cooling across its entire new 496 range, which comprises the 496 MAG HO Sterndrive, 496 MAG Sterndrive, 8.1S HO Inboard and 8.1S Horizon Inboard. (Note: all models utilise the same 8.1lt engine which is stroked from the 454.)

Normally when V-eight manufacturers increase piston displacement they bore-out an existing engine, as Mercury Marine did when it created the 502 or 8.2lt V-eight from the 454. Unusually, the 496/8.1S is stroked to the extent it is Mercury Marine's first undersquare or long stroke petrol V-eight. The bore of 108mm is unchanged but the stroke is now 111mm (the same as the HP500 Bulldog) compared to 102mm. In comparison the 502 had a bore of 114mm and stroke of 102mm.

To comply with automotive exhaust emission regulations, three decades ago carmakers found undersquare engines created better combustion chamber 'squish' for a more thorough burn of the air/fuel mix. In view of the 496's original purpose as power for Chevrolet, GMC and Cadillac SUVs, this makes more sense than the old 502.

According to Mercury Marine, GM has engineered the 496 for the 'unique demands' of marine environments and uses corrosion-resistant alloys, engine oil sealing systems, marine-specific camshaft designs and closed circuit cooling.

That said, the 496/8.1S is more than just a stroked 454. It has the PCM 555 microprocessor engine management system which, combined with the multipoint fuel-injection, maintains the correct air/fuel ratio and ignition timing for each cylinder. To help the PCM 555 system achieve its functions, an individual ignition coil is mounted close to each of the eight platinum-tipped spark plugs.

The PCM 555 system interfaces with the SmartCraft SC5000 system, which continuously monitors 64 separate engine functions and displays fuel flow, fueltank level for range-to-empty, revs, water depth and GPS speed, air, water and engine temperatures, and sterndrive leg position. It even provides maintenance updates and service advice!

Mercury Marine's Engine Guardian alerts the engine operator of malfunctions and in some cases it will automatically take corrective action and reduce revs to prevent engine damage.

The next stage of this management framework is the SC5000 Electronic Remote Control, available as an option for the 496/8.1S later this year. Similar to electronic systems implemented by most marine diesel manufacturers, it eliminates traditional control cables and provides electronic throttle and shift control, twin-engine synchronisation and automatic trim tab adjustment.

To reduce engine weight, Mercury Marine has dropped the old cast iron exhaust manifolds for new aluminium units which have stainless steel elbows with dry joints. Combined with the additional 50mm exhaust riser height over the 454, this should reduce condensation in the exhaust system running back into the combustion hambers. Reportedly this has caused rusting of cylinder walls and piston rings in some MerCruiser petrol V-eight sterndrives fitted to deep-vee hulls.

The 496 MAG HO model has a forged steel crankshaft for reliability, and all models have a cast aluminium oil pan with a 12mm drain plug either side.

The 496 MAG HO Sterndrive engine (pictured) measures 889mm x 838mm x 610mm compared to 991mm x 813mm x 559mm for the 454 MAG MPI.

Complete with Bravo I leg, the 496 weighs 544kg compared to 534kg for the 454 with open circuit cooling, while the Bravo II and III leg units weigh 551kg and 555kg respectively (compared to 539kg and 543kg).

The 496 develops 425hp at 4600-5000rpm compared to 385hp for the 454 in the same rev range. The 496 is also available with the Bravo I XZ leg which has a larger U-joint, stronger thrust bearings, thicker upper gear floor and heavier-duty driveshafts and propshaft (the latter fitting all square-bore Quicksilver props).

The 8.1S Horizon Inboard measures 1118mm x 838mm x 610mm, weighs 513kg and develops 370hp at 4200-4600rpm. The superseded 454 MAG MPI Horizon with closed circuit cooling measured 1219mm x 813mm x 559mm, weighed 535kg but developed 380hp at 4400-4800rpm.

The 8.1S has the same extended service intervals that have made the Horizon series famous.

As mentioned previously in Trade-A-Boat, there's a lot to be said for petrol engines in recreational boats. They're understressed, simple to maintain and service compared to turbo-intercooled diesels, and cost-effective for the majority of cruiser-owners who may clock-up only 50 running hours a year.

There's no doubt the upgraded MerCruiser 496/8.1S range will make many more potential buyers think hard about whether they really need to go diesel...