Mercruiser 8.1S HO Horizon inboard

Although the 8.1S HO has the same 90 degree V8 powerhead design as its 5.7 and 6.2 litre counterparts, it's unusual amongst petrol V8s in that it's an undersquare engine. Displacing 8.1 litres or 496 cubic inches, the 8.1S HO has a 108mm bore and 111mm stroke.

The HO designation stands for "High Output" because the 8.1 is available in two outputs. The standard 8.1S inboard develops 370hp at the crankshaft at 4400rpm and has a Wide Open Throttle rev range of 4200-4600, whereas the HO develops 419.6hp at 4600rpm with a WOT rev range of 4400 - 4800.

Both engines have pushrod-actuated crossflow overhead valves and hydraulic lifters for reliability and 9.1:1 compression ratios necessitating Premium unleaded 95 RON petrol. The standard closed-circuit or heat exchanger cooling increases engine lifespan through reduced cylinder wall condensation and cuts maintenance intervals to normally once a year for oil and filter changes. Both engines have platinum-tipped spark plugs, watercooled fuel systems to reduce vapour lock, water/fuel separators and remote oil filters mounted at rocker-cover height for easy replacement.

As with the smaller engines, a serpentine belt drives the high-mounted 14.1 volt, 65 amp voltage-regulated alternator and the cooling water pumps. The HO has an individual ignition coil per spark plug which reduces radio interference “noise”.

The sequential multipoint fuel injection system incorporates Mercury Marine's PCM 555 microprocessor control that optimises fuel delivery for each cylinder according to engine rpm and load and ambient temperature and barometric pressure. The “Engine Guardian” system has more than 40 sensors to warn of any engine operating problems and in the event of low oil pressure or high coolant temperature it will progressively reduce engine rpm. And then there's SmartCraft, which displays fuel tank level, boatspeed, fuel/operating range, fuel efficiency, total fuel used and of course realtime fuelflow.

The 8.1S is available with either Borg Warner Velvet Drive or ZF gearboxes and with the Velvet Drive the combined engine/box weight is 513kg, only 19 per cent heavier than the similarly-equipped MX 6.2 MPI Horizon inboard, which develops 320.4hp at 4800rpm. The V-drive demo units from Lifestyle Marine (02 4959 1444) were fitted with ZF 63 IV boxes, remarkably compact units considering the engine torque they handled. However, reversing the engines compared to sterndrives does limit access to the engine oil dipsticks and serpentine belts.

Compared to the 6.2, which has the same dimensions and weight as the 5.7 or 350 MAG MPI Horizon, both 8.1S models are somewhat bulkier and with a Velvet Drive box are 1194mm long (up 102mm), 838 wide (up 92) and 610mm high (up 51mm and again measured from the crankshaft centreline).

Driving the 8.1S HO
Lifestyle Marine provided twin units for evaluation in a Sea Ray 40 Sundancer, which measures 12.49 x 4.01m and has a 19 degree transom deadrise. All Sundancers above the 340 (formerly the 375) have V-drive inboards as standard with the props operating in tunnels that reduce shaft angles to around 10 degrees and raise the prop tips above the hull bottom for reduced draft and easier slipping. Spinning 21 x 22.5 inch four-bladed props and driving through 2.5:1 gear reductions, the 8.1S HO engines were very well suited to this hull, which with half tankage, two adults and various options fitted, displaced 9.5 tonnes.

Both engines started instantly, hot or cold, with lower vibration levels across the rev range than the 6.2s tested in a 340. The twin-lever controls were certainly not appealing but did provide precise low-speed control and with one engine ahead at 1000rpm and the other astern at 1500 the hull could be quickly spun in its own length without using the 24 volt bowthruster fitted.

Idling at 700rpm, the 40 averaged 3.7kt with a combined fuelflow of 12.2 lt/h and just a gentle rumble coming from the above-water exhausts. At 1000rpm we averaged 4.7kt using 16.8 lt/h and at 2000rpm, 8.8kt using 40.4 lt/h with lower noise levels, thanks to the switchover to underwater exhausts that exit outboard of the tunnels.

But what really surprised me was the low planing speed, only 14.7kt on 3000rpm using 93.6 lt/h despite leaving the trim tabs at the neutral setting. In my opinion this was due to the combination of the flat-topped tunnels which provide better lift than rounded tunnels, the down-angled propshafts and mounting the engines at least a metre ahead of the transom to allow for close-coupling of the V-drive gearboxes.

In comparison, the twin 6.2s fitted to the 340, running through Bravo Three legs and pushing 7.6 tonnes planed us at 18.3kt on 3500rpm using 100.4 lt/h in total.

With the 40 comfortably planing and maintaining 18.0kt on 3400rpm into a 20kt headwind and 50 cm chop the total fuelflow was 115.2 lt/h and this speed would be ideal for offshore passagemaking. From these rpm out to WOT with the throttles "floored" the acceleration was better than any comparable-output diesels could manage and above 4000rpm the oh-so beautiful V8 exhaust note became more noticeable but even at WOT we could talk normally at the helm.

At 4000rpm the averages were 24.2kt and a combined 176.0 lt/h, whereas the 6.2s used a combined 112.0 lt/h for a 25.0kt average at 4000rpm. Through full lock figure-of-eight turns at 4000rpm, the 8.1S dropped only 100rpm due to the fuel management system supplying more fuel for the additional load and frankly, the 40 handled much better than sterndrive-equipped hulls such as the Mariner M430 I've tested.

The WOT average was 28.9kt on 4750rpm using a combined 265.6 lt/h compared to 31.6kt on 4700rpm and 176.0 lt/h for the 6.2-powered 340, but then the 40 was suffering some bottom slime which can really knock top-end speeds. Lifestyle Marine says the 8.1S HO-equipped 40 usually reaches 31.5kt and according to Sea Ray, the HOs use a combined 249.8 lt/h at 4800rpm, so there's a lot to be gained by having a clean hull!