Mercury FourStroke 90HP Command Thrust EXLPT

Mercury FourStroke outboard motors are incredibly powerful and efficient, and they feature a wide range of applications. Crafted for less weight and high displacement, they generate abundant torque and immediate power. And because they don’t have to work as hard as heavier engines or those with lower displacement, they make boats plane faster and accelerate better, and they provide optimal fuel efficiency at cruising speed.
Forget everything you know about FourStroke outboards. Mercury changes the game. Completely. They’re tougher, smaller, and lighter than any other FourStroke on the water. With a combination of performance and efficiency that will take your time on the water to a whole new level.

In the sixties and seventies US car manufacturers persisted in making big displacement, lazy, naturally aspirated engines that with a bit of TLC would last a lifetime. Watch specialist TV programs like Horsepower TV and Muscle Car and you’ll see just how many cars from that period still have reliable engines.

In more recent years European and Japanese car manufacturers have gone down the track of high revving small-displacement engines having features such as quad camshafts and variable valve timing to gain power and efficiency. This has filtered through to the outboard industry where most four-strokes over 70hp are marinised car engines.

But Mercury Marine has taken a completely different tack, starting with the Mercury F150 FourStroke released three years ago. This dedicated marine engine has a big bore and short stroke with chain-driven camshaft, gear-driven balance shafts and only two valves per cylinder. Favoured by builders of NASCAR race engines, the roller cam followers fitted eliminate the need for valve clearance adjustment for the life of the engine. Yet under similar review conditions to the direct competition the Mercury F150 performs just as well and uses no more fuel relative to engine loading.

Released in June, the 90hp Mercury FourStroke follows the same design principles, though as it just squeezes in under the magic 2.2lt it doesn’t need balance shafts. It has the same bore/stroke ratio as the F150, ensuring way lower piston speeds than the long-stroke competition. And being designed for marine usage its maximum torque is developed down low to give unbeatable holeshot and midrange cruising efficiency.

Developing 89.8 brake horsepower (at 1bhp equalling 746 Watts) at 5500rpm with a wide open throttle range of 5000 to 6000rpm, the four-cylinder 2065cc powerhead has a 90mm bore and 81mm stroke. Maximum torque is developed at only 3000rpm compared to 4000 to 4500rpm for the direct competition.

The multipoint EFI fitted gives instant turnkey starting but like the Mercury F150 there’s no oxygen sensor in the exhaust to advance ignition timing when premium (95 RON) is used. The engine is tuned to run on standard (91 RON) unleaded but premium is a better quality fuel and lasts way longer in underfloor fuel tanks. The FourStroke 90 has an OEDA 3 Star exhaust emissions rating, with similar carbon monoxide and oxides of nitrogen levels to the 3 Star OptiMax 90 outboard motor.

Two gear ratios are available, the standard 2.07:1 and Command Thrust 2.38:1, with hydrodynamic efficiency increased in both by simply making the gear case torpedo more pronounced. The dry weights for long-shaft models are 163 and 165kg.

Unlike the F150, Mercury has opted for an under-flywheel alternator that pumps out a maximum of 35amp but at least it has a water-cooled voltage regulator. The powerhead layout is clean with only a short intake manifold, blowing away the theory that a long intake is needed to develop bottom-end power and torque.

Rated from 60 to 115hp the Stacer 509 Sea Runner was a perfect match for the review FourStroke 90 which had the standard gear ratio and was swinging a 16in pitch Quicksilver Vengeance stainless steel prop. It started instantly hot or cold with no oil smoke appearing and quickly settled down to an almost vibration-free 700rpm, so no flab reduction here. With the antiventilation plate ¾-immersed power astern was good, very handy for backing off all the shoals in the Gold Coast Broadwater where we tested the engine.

Despite total weight including two hefty adults being 950kg, easing the throttle forward popped the 509 on the plane, with the maximum planing fuel efficiency (distance travelled for fuel used) in the 3000 to 4000rpm range, great for cruising offshore over a lumpy sea. Run briefly out through the Gold Coast Seaway the 90hp engine had instant throttle response for keeping the 509 on the back of waves travelling in through the breakwaters. But fuel flow at WOT was also very good for a 90, so Mercury has just got the engineering right!

Through tight turns at 4000rpm there was no prop ventilation or drop off of rpm and even at WOT we could talk normally at the helm.

The 90hp Mercury FourStroke outboard motor is my pick of the new Mercury FourStroke engine range. Maximum torque is developed at only 3000rpm with plenty of torque at 2000rpm, which means there’s a ton of grunt down low. This outboard motor is available in standard and CT gearcases.

The FourStroke 90 hp Mercury develops 89.8bhp at 5500rpm and is almost as quick as the Mercury F115, but way more fuel-efficient relative to output, even at or near WOT.