450hp outboard: Everything you need to know about the new monster Merc

Mercury Racing trumps all-comers with its most powerful model yet, a supercharged 450hp outboard

Within months of Yamaha claiming the title of the world’s most powerful mass-market outboard engine courtesy of its 425hp XTO, Mercury has stolen it back again with a new supercharged 450hp version of its 4.6-litre V8 engine.

Not only does the new Mercury 450R offer an extra 25hp over the Yamaha XTO, it’s also 129kg lighter. This impressive power-to-weight ratio should give even better acceleration and handling than the heavier Yamaha, especially on relatively lightweight high performance RIBs and sportsboats.

Performance boost

The key to its performance is a supercharger that forces denser, pressurised air and fuel into the cylinders. Mercury has extensive experience of supercharging having used a similar set up on its previous generation of 2.6-litre straight six Verado engines.

However, this is the first time it has added a supercharger to its new family of larger capacity 4.6-litre V8 engines. This helps it to deliver 40% higher torque than the already punchy Mercury 400R.


Rear tie bar brackets are ideal for race boats and high-speed leisure catamarans

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The supercharger itself is a 2.4-litre belt-driven twin scroll unit with water-cooling to reduce the temperature of the intake charge and maximise air density. A boost bypass valve automatically adjusts boost pressure to take account of the ambient air temperature and maintain peak performance regardless of weather conditions.

Crucially, it is also designed to deliver maximum power and run reliably on ordinary 89 octane (95 RON) unleaded petrol rather than the harder to find and costlier super unleaded fuel preferred by some high-performance engines.

The benefit of a belt-driven supercharger over a naturally aspirated engine is that it boosts low rev torque as well as peak power without having to increase cylinder capacity or engine speed, providing stronger acceleration when climbing onto the plane. It also provides much faster throttle response than an exhaust-driven turbo-charger.

The downside is that the supercharger saps power from the engine, so that it has to work harder and burn more fuel to deliver the required horsepower rating at the prop shaft. It also tends to generate a distinctive whining noise, which in this case has been suppressed with the aid of a special intake attenuator designed to neutralise the high frequency whine while retaining the deeper thrum of the exhaust.

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Slimline tonic

The engine is based on the same all-aluminium eight-cylinder block as the 300hp Verado V8. This has an unusually narrow angle 64 vee to keep the overall package as compact as possible. Even with the extra weight of the supercharger, the 450R weighs 313kg, 42kg more than the naturally aspirated 300hp V8 Verado but still 129kg less than the 5.6-litre Yamaha XTO.

It has also managed to pack the supercharger under the same slender cowling as the 300hp V8, keeping the centre-to-centre mounting distance down to just 26in on multi-engine installations – making it easier to fit twin, triple or even quad installations on space-restricted transoms.

To cope with the extra power and heat generated, changes have been made to the base engine including a new high capacity oil cooler and a poppet valve in the mid-section to increase the flow of cooling water.


The twin scroll supercharger sits between the V8’s two banks of cylinders

The valve train features a quad cam design with four valves per cylinder and special high-performance cam profiles and race-spec Inconel exhaust valves to allow for the increased flow of gases in and out of the cyclinders. The camshafts are chain-driven and run in an oil bath for increased durability and reduced maintenance.

The wide open throttle range is rated from 5,800-6,400rpm, allowing for an extensive range of different propeller options to suit the size and weight of boat it is fitted to.

Top gears

As well as a range of different propeller options, the 450R can be specified with two different gearcases. The 5.44 HD is designed for traditional submerged applications on boats with a maximum speed of 74 knots (85mph). The optional Sport Master gearcase is designed for surface-piercing applications on race boats and high performance leisure craft. Rear tie-bar brackets are also available for multiple applications on high-speed craft.


The standard 450R has a 5.44HD submerged gearcase for boats up to 74 knots

Despite the emphasis on performance the 450R still comes with a three-year factory warranty (extendable to five years) and all the features that make the regular V8 Verado such a popular choice.

These include electro-hydraulic steering as standard, joystick control on multi-engine installations, a pop-up hatch in the cowling for routine oil checks and top ups, a powerful 115-amp alternator for rapid battery charging, and adaptive speed control for easier handling. This automatically adjusts the throttle to maintain the same speed through corners.

The UK importer E P Barrus hopes to have the first 450Rs in time for the start of the 2020 season. However, prices have not yet been announced.


Capacity: 4.6-litres
Configuration: 64° V8
Induction: Supercharged
Bore and stroke: 92mm x 86mm
Max power: 450hp
Max rpm: 5,800-6,400rpm
Idle rpm: 600rpm
Weight: 313-324kg
Warranty: 3-year limited


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