Review: Mercury 90 FourStroke

By: Andrew Norton

Presented by
  • Trade-A-Boat

The 90hp Mercury FourStroke outboard motor is a dedicated marine engine, unlike most four-strokes over 70hp which are marinised car engines.

Review: Mercury 90 FourStroke
The Mercury 90hp FourStroke outboard motor is a based on a dedicated marine engine, rather than a marinised car engine.

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.

 

Mercury FourStroke outboards

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.

 

Mercury FourStroke 90

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.

 

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Maintenance

The engine oil dipstick, filler cap and canister oil filter are easily reached and an extended drain spout in the leg ensures no oil dribbles down the leg. The recommended oil is Quicksilver FCW SAE10W30 which from testing in various outboards since 2003 I believe to be the finest four-stroke outboard oil available.

Like the F150, Mercury supplies only mechanical cable throttle and gearshift control but set up correctly this works well. Recommended servicing intervals are every 100 hours or annually after the first 20 hours. Providing servicing is done by an authorised dealer the recreational warranty is a long five years.

 

Mercury 90hp FourStroke on the water

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 Trade-a-Boat verdict

Frankly, Mercury’s approach to outboard engineering is refreshing compared to the competition. The Mercury FourStroke 90hp is the first four-stroke in its power range I’d seriously consider buying as it’s the closest thing to the simplicity of an OptiMax 90. However it’s way quieter and that’s a good thing as my tinnitus only gets louder as I age and I don’t want to compound it!

Thanks Mercury Marine for supplying such a well set-up test rig. For more info, visit mercurymarine.com.au.

 

Mercury FourStroke 90 sea trials

Stacer 509 Sea Runner with single Mercury FourStroke 90 outboard motor. Average of two-way runs on Coomera River, QLD, using SmartCraft instrumentation, handheld GPS and iPad-based decibel meter

RPM

SPEED (KTS)

FUEL BURN (LT/H)

1mdBA

3mdBA

700

2.4

1.4

64

61

1000

3.3

2.1

68

65

1500

4.6

4

74

70

2000

5.8

5.9

78

74

2500

8

7.8

82

80

3000

14

9.3

83

82

3500

19.8

12.6

88

85

4000

24.4

15.1

90

88

4500

27.4

18.9

92

90

5000

31

26.3

92

91

5500

35

31.3

92

92

6000

36.2

32.6

93

93

* Sea-trial data supplied by the author.

 

Competing outboard motors

 

Honda BF90

Suzuki DF90

PRICE

$15,000

$14,000

WEIGHT

166kg

155kg

RATED HP

88.7

88.7

RPM

5800

5800

DISPLACEMENT

1496cc

1502cc

WARRANTY

Five years

Five years

OEDA STARS

Three

Three

 

Mercury FourStroke 90hp specs

TYPE Four-cylinder four-stroke petrol outboard motor

RATED HP 89.8 at 5500rpm

DISPLACEMENT 2065cc

BORE X STROKE 90 x 81mm

WEIGHT 163kg (dry)

 

See the full version of this review in Trade-A-Boat #468, August / September 2015. Why not subscribe today?

 


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