Mercury - 115 FourStroke

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Captain's Report


2015 brings a new addition to Mercury Marine’s line of second-generation 4-stroke outboard engines, all based on a larger 2.1-liter displacement block. Available in 75, 90 or 115 horsepower, these refined four-cylinder engines are cleaner burning, quieter and give better fuel economy with less maintenance than their predecessors, according to the company.

The normally-aspirated fuel injected outboard is also said by the builder to be one of the lightest on the market, as much as 20 lbs. (9.07 kg) for the 115-hp version.


All models consist of an in-line four with two valves per cylinder and a single overhead cam supported by a revised multi-port fuel injection system. They also share the same 3.5” diameter bore and 3.2” stroke and have earned EPA’s 3 star emissions rating. The all aluminum block and head feature hydraulic cam lifters and roller rockers for a maintenance free valve train.

Mercury 115 FourStroke

Shown here is the ultra-lightweight composite intake manifold and injector fuel rail.

The 90 and 115 models are available with Command Thrust lower gear cases that are meant for heavier applications. A major factor with the 115 horsepower model is that twin-engine rigging is possible since it is now offered in a counter-rotating version.


To test the new platform we chose the 115-hp "Command Thrust" model and rigged it to a 17’10” (5.44 m) center console fishing boat with a 7’6” (2.29 m) beam. Fully-rigged and loaded with two people and 1/3 of a tank of fuel, our test boat weighed in at 2,312 lbs. (1,049 kg). Getting on plane took just 4.1 seconds and 30 mph was clocked at 16.6 seconds from a dead start.

Speed Runs. The 115 then responded with a 37.7 mph top speed with the engine turning at 6000 rpm to record 3.4 mpg. Throttling back to a 4000-rpm cruise speed delivered 5.1 mpg and 19.2 mph, which was the most fuel-efficient planing speed in this application.

Mercury 2.1L FourStroke 115-hp Outboard
Mercury 115 FourStroke

Maximum engine speed is 5000 to 6000 rpm, depending on the model chosen.

Comparing Noise Levels

Mercury has made several design changes to reduce the engine noise heard in the cockpit. So we tested the engine’s noise signature and compared it to the Mercury 1.7L first generation model. We arbitrarily chose 6 boats we had previously tested and compared our sound readings at idle, 3500, 4000, and at 6000 rpm.

At idle, we found the new 115 emitted 63 dbA, which is almost identical to what we found with its predecessor.

Caveat About Noise Readings. In the field it is difficult to compare "engine-only noise". Due to excessive ambient wind and water noise, and the construction harmonics of each individual boat, both of which interferes with the readings for "engine-only noise", accurate comparisons are problematic. The best comparison test would be on a test stand, which we assume Mercury has done.

Reduced Noise Levels Found. Nevertheless, our study discovered an unmistakable pattern: when we averaged the 6 readings from the previous 115-hp models tested we found that the new 2.1L Mercury 115-hp engine was from 4.5 dbA quieter at 3500 rpm to 6.1 dbA quieter at 6000 rpm. (5 dbA is audibly louder to the human ear, and 10 dbA is generally considered to be twice as loud.) From this study we can only conclude that Mercury's claim to producing a quieter engine -- even though it has greater displacement than its previous model -- is true.

Mercury 115 FourStroke

This chart shows our recorded performance numbers from the new 115 Mercury outboard engine.

Engine Management

The SmartCraft engine control unit (ECU) has been remapped to deliver improved fuel economy and reduced emissions. It was also programmed to handle up to 10% ethanol fuels blends to minimize loss in performance due to ethanols. (Industry sources state that a 10% ethanol blend is from 3% to 4% less fuel efficient than non-blended gas.)

Mercury’s new 2.1L engine can be rigged with SmartCraft Digital or Analog Instrumentation. Remote control is traditional mechanical throttle and shift and an optional Big Tiller handle with either manual or power steering is available.

Mercury 115 FourStroke

Mercury’s latest electronic control module fires individual
coil packs for each cylinder for a better, hotter spark, thus more providing more complete combustion, more horsepower and fewer emissions.

Mercury 115 FourStroke

The main fuse for the motor is conveniently located here.


Reducing maintenance was another goal for the new 2.1L outboard. Pop the cowl and located upfront is a maintenance schedule decal which details the 100 and 300 hour service intervals. For all the basic service items, color-coded maintenance locations helps the with the process. The oil dipstick and in line fuel filter are conveniently located on the starboard side for a quick daily check. Crankcase oil fill is located on the top of the motor and uses a large port for easy filling. One thing we particularly like is the fresh water flush fitting located forward on the engine housing since it makes things a lot easier when accessing it from the transom.

Mercury 115 FourStroke

All new 2.1L models have an easy to service oil system.

Mercury 115 FourStroke

The in-line fuel filter is easy to locate and check for service when required.

Corrosion Protection

Mercury’s cylinder block and heads get a chromate conversion treatment prior to paint for extended corrosion protection. Next they are coated with a special electro deposition paint that is electrically fused to the metal then baked on for a tough, durable finish. Components that are exposed to the water are power coated for a shiny finish providing extra UV protection.

Shaft Lengths and Gear Ratios

All three versions come standard in a 20” long shaft length and turn a reduction gear with a ratio of 2.07 to 1. The 90 and 115 can be ordered with the longer 25” shaft, while heavier boats needing a big prop can upgrade these two models with Mercury’s Command Thrust. This larger gear case offers a 2.38 to 1 ratio, and has a larger torpedo and bearings to support the larger wheel’s needs. It also provides greater leverage and steering control than its smaller, standard size lower unit.

Steering Options

The 2.1s are available in three steering formats: mechanical cable, hydraulic or with a big tiller. The big tiller option provides gearshift, throttle and trim controls integrated into one unit.
We especially like the adjustable troll control that allows users to set engine speed in 10 rpm increments. This feature, along with the three-position locking tiller arm eases the pilot’s workload and gives precise control.


Mercury has developed the 75 to 115 horsepower line-up with a focus on emissions control. This class of motors propels the majority of small recreational boats and should have a significant impact on overall boat emissions.

Finally, Mercury Marine has been working hard during the last decade to make all of its products saltwater-ready. Its newly formulated alloys, coatings, and painting procedures are generally recognized to make its products as corrosion-resistant as any on the market.