4.6L V-8 FourStroke Series
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Mercury 4.6L V-8 FourStroke Series Main Video

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Mercury 4.6L V-8 FourStroke Series Mercury 4.6L V-8 FourStroke Series
The new 4.6L V-8 FourStroke is a brand-new design that uses a naturally aspirated powerhead with a computer-controlled electronic throttle to dial in the power when it’s needed throughout the rpm range. With their quiet and efficient operation, the new engines are designed primarily for sportboats, pontoon boats, coastal and inshore center and dual consoles, and aluminum fishing boats, among other applications.

Key Features

  • Highest displacement in its class
  • 8 cylinders, DOHC
  • Digital Throttle and Shift (DTS)
  • 85 amp alternator
  • Idle Charge Battery Management
  • Transient Spark Calibration for more torque
  • Adaptive Speed Control
  • Advanced Range Optimization
  • Handy oil check and fill
  • Handle to easily remove the cowl
  • Recommended 87 Octane fuel

Mercury 4.6L V-8 FourStroke Series (Outboard) Specifications
Horsepower 250-hp
Type of Engine 4-Stroke
Number of Cylinders 8 Cylinders
Weight (lbs/kg) 527.00 lbs / 239.04 kg
Displacement (cu/cc) 279.00 cu / 4,571.99 cc
Shaft Length Options 20” (50.8 cm) / 25” (63.5 cm)
CARB Rating 3-Star
Recommended Fuel 87 Octane
Alternator Output 85 Amps total

Mercury 4.6L V-8 FourStroke Series Captain's Report

Mercury 4.6L V-8 FourStroke
The new Mercury 4.6L V-8 FourStroke comes in models from 250 to 300-hp, comes in three shades of white in addition to Mercury’s traditional Phantom Black.

Mercury 4.6L V-8 FourStroke
The new Mercury 4.6L V-8 FourStroke is the largest-displacement 200 to 300-hp outboard on the market. It is seen here on a Barletta L25U pontoon boat.


The new Mercury FourStroke is an all-new 4.6-liter (279 cu. in.) V8 engine. It is a versatile design so that it can be matched with all sorts of applications, including pontoon boats, sportboats, and aluminum fishing boats, among others. It has hydraulic steering, comes with mechanical or digital controls, and is available in three versions of white plus Phantom Black. A 4.8” or 5.4” (.12 m or .14 m) gearcase can be chosen.

Mercury 4.6L V-8 FourStroke
The new 4.6L FourStroke V-8 weighs 527 lbs. (239.04 kg), which is lighter than most of its 6-cylinder competition in the same horsepower range.

Mercury 4.6L V-8 FourStroke
The Mercury FourStroke V-8 300 is intended primarily for single and twin-engine installations.

Mercury FourStroke: The Big Picture

Mercury has introduced its next generation of large outboards and the company has based the new models on two displacement sizes and configurations, a 3.4-liter V6 for the 175, 200, and 225-hp FourStroke and a 4.6-liter V8 for the 250 and 300-hp engines.

The concept driving these outboards is to offer a compelling amount of power by means of displacement, fuel injection, and electronic engine control. The electronic system is used to dial-in fuel efficiency. It does this by controlling the fuel-air mixture in a closed-loop fuel system.

For more torque at the low end and better hole shot times, Mercury is using what is known as a “Transient Spark”, which the company says will give the engine 5% to 7% more torque at the low end. This, together with 10% greater displacement than its closest 4-stoke competitor can only result in greater torque.

Mercury 4.6L V-8 FourStroke
We clocked the Barletta L25U pontoon boat from 0 to 20 mph in 3.8 seconds, and 6.5 seconds to 30 mph with the new Mercury 300-hp FourStroke.

Design Innovation

The Powerhead. Consider that the 4.6-liter FourStroke is 2 liters greater than the supercharged 2.6-liter displacement of the inline-6 OptiMax that it replaces. Displacement is the way to generate more torque in a 4-stroke engine, a job that the supercharger fulfilled in the L6 2.6 models. The double-overhead cam design, with four valves per cylinder, combines with an enlarged intake to deliver plenty of air and fuel when it’s needed.

Mercury 4.6L V-8 FourStroke
The double-overhead-cam design has four valves per cylinder, and includes a balance shaft to reduce noise and vibration.

The Fuel Injection System

The new engine uses two intake valves to charge each cylinder, with the injector for each cylinder positioned between them in the intake manifold. We asked Mercury why the injectors were placed in the manifolds instead of in the cylinders themselves. The answer--

“A port fuel injection (PFI) system has many advantages, chief among those being simplicity, reliability, and cost,” says Brian Meyer, category manager for 75+ hp outboards for Mercury Marine. “Direct injection (DI) systems certainly can improve fuel economy and performance of an engine. However, Mercury has chosen to address the performance needs of our customers by optimizing the displacement and efficiency of our PFI engine, while also employing our Advanced Range Optimization (ARO) technology. We believe that the ARO system yields comparable improvements in fuel economy to a DI fuel system at a significantly lower cost.”

Mercury 4.6L V-8 FourStroke
This detail shows Mercury’s port fuel injection, which places the injector, indicated by the green arrow, between the valve stems on the intake ports. Mercury says that this design has proven to be reliable and cost-effective.

Unusual Exhaust System

This is one of the most innovative aspects of the engine because it reduces the width of the engine, reduces its weight, and helps contain engine noise – all with one design. The exhaust manifold is in the middle of the V, instead of being on the outside. It is then routed from the top of the engine, down the centerline, between each side of the V of the engine. This allowed Mercury to keep the cowl width slimmer so that the outboard could be installed on 26” (.66 m) centers – 2.5” (.06 m) less than its primary competitor.

Mercury 4.6L V-8 FourStroke
The exhaust (green arrow) emerges from the engine from inside the V abaft and below the alternator (red arrow), using the centerline instead of an outboard route.

The exhaust-system design uses a multi-chamber muffler system located beneath the “chaps,” as the manufacturer calls the panels on either side of the midsection. The muffler chambers connect the main exhaust passage to the idle relief port, and includes an electronically controlled bypass valve.

Mercury 4.6L V-8 FourStroke
Multi-chamber mufflers are concealed under midsection panels at locations indicated by yellow arrows.

Top Cowl Service Door

Perhaps one of the more noteworthy features of this new outboard line is the watertight hatch in the top of the cowl to provide access to the dipstick as well as the oil fill. There’s a decal outlining service requirements along with a QR code to allow a smartphone to link to additional service information.

Mercury 4.6L V-8 FourStroke
A watertight hatch in the cowl opens with a simple press to reveal the oil dipstick and fill.

Easy-Off Cowl

Under the top cowl service door, there’s a handle that lifts with a red button. Lifting that handle releases the latches all around the cowl, so it can be lifted off easily.

Mercury 4.6L V-8 FourStroke
Beneath the top cowl door is a red button that releases the handle to remove the cowl.

Mercury 4.6L V-8 FourStroke
The handle pops up, providing something to grip while also releasing the latches all around the base of the cowl.

Mercury 4.6L V-8 FourStroke
Using the handle, simply lift the cowl free.

Mercury 4.6L V-8 FourStroke
From the underside of the cowl, the handle, indicated with the green arrow, is connected via cables to the latches, shown with yellow arrows that secure the cowl to the engine.

Mercury 4.6L V-8 FourStroke
This image shows the cowl latches, as well as the cabling that controls them, and the seals that keep the noise in and the moisture out.

Ease of Maintenance

Once the cowl is removed, many service points are apparent. The oil filter is easy to spot near the fuse box on the port side. The spark plugs are arrayed in neat columns aft and the fuel filter is handy there as well, on the starboard side.

Mercury 4.6L V-8 FourStroke
This maintenance decal beneath the cowl shows the 100- and 300-hour service requirements in an intuitive, graphical representation.

Mercury 4.6L V-8 FourStroke spark plug
The port-side spark plugs are indicated by the four green arrows, while the yellow arrows show the long runners of the air intakes, which pack air into the engine for better combustion, according to Mercury.

Mercury 4.6L V-8 FourStroke fuse box
The fuse box and oil filter are prominently displayed on the port side of the outboard. The green arrow indicates the filter drip tray drain plug, where any oil remaining in the filter during an oil change can be recovered.

More Net Amps than Any Engine in Class

The FourStroke has an 85 amp alternator, the highest in class. But the name of the game is “net amps” – the amps left over from the running of the engine to power onboard accessories, electronics, and to keep the batteries fully charged. At WOT, these alternators produce 60 net amps, we’re told. These are high numbers and are important for owners of well-equipped boats, particularly the larger cruising boats that are now being fitted with outboard engines. But, that’s not all…

Idle Charge Battery Management

Simply put, the engine-management system detects when the batteries drop below acceptable levels due to increased power draw. When that happens, the system increases idle rpm to boost alternator output, to help charge batteries to compensate for power draw. It gently raises the idle level from 600 rpm to around 725, to get to the next level on the curve of the alternator, and in this mode, it delivers more amps. While the engine is revving slightly higher, it’s still at a speed where it’s able to shift.

Lower Unit

The standard 20” lower unit gearcase on the FourStroke is the 4.8” unit. On the 25” model, a 5.4” (.51 m) gearcase is standard. We recommend this on bay boat applications where high-speed operation in choppy water is likely. The robust 5.4” (.14 m) gearcase is an evolution of the one that has been used on previous models and has proven itself in tens of thousands of hours of rugged use over the last few years.

Mercury 4.6L V-8 FourStroke
The torpedo for the FourStroke has been redesigned to reduce drag. Look closely at the tip to see the forward, lower water intakes.

Adaptive Speed Control

This standard proprietary system maintains engine speed regardless of condition, and Mercury has the system patented. The idea is that the rpm remains constant even when seas are rough, the driver puts the boat into a hard turn, or a tow sport requires steady power. Adaptive Speed Control is a function of the ECU, where operator demand and engine load are measured and the electronic throttle position is adjusted. It is standard on all of the engines.

Advanced Range Optimization – Improves Fuel Efficiency

Because the outboard uses a digital throttle control, it can also improve its own fuel economy. It begins when the system detects a steady-state mode, where the driver isn’t adjusting the throttle constantly. Once the operator variable appears to be set, the digital control tunes the throttle plate and the spark to lean out the fuel burn to maximize fuel economy, optimizing the boat’s range. This comes standard.

Mercury 4.6L V-8 FourStroke
With Advanced Range Optimization, the operator can leave the throttle alone and let the engines step up the efficiency.

Performance Test – Barletta L25U

We tested the new FourStroke 300 on a Barletta L25U pontoon boat on a freshwater lake on a 80-degree day with wind blowing about 15 mph and a chop on the water. It was not ideal conditions, but good enough to get an idea of how this new engine will perform.

Our test boat was a 27’5” (8.36 m) tri-toon with full fuel, two people aboard and gear had an estimated weight of 5,373 lbs. (2,437 kg). The engine was swinging a 16’ x 18’ Enertia ss prop.

The top speed we recorded was 47.2 mph getting 1.9 mpg at 6000 rpm, the rated maximum. Best cruise came at 3000 rpm at 17.1 mph, getting 3.3 mpg.

Our time to 20 mph was 3.8 seconds, and to 30 mph was 6.5 seconds.

Below is our complete test with numbers.

Barletta L25U w/ Mercury 4.6L V-8 FourStroke Engine

Mercury 4.6L V-8 FourStroke

Mercury 4.6L V-8 FourStroke
Our sound readings on the Barletta were reasonably low, which is noteworthy because this is a tri-toon with lots of water and air noise.

Noise Abatement

Mercury engineers have worked hard on keeping as much noise as possible from escaping from the engine, and our sound readings on the test seemed to confirm that they have been successful. Our top reading at WOT was 87 dBa, which a notch or two lower than we could have expected, but the real difference came at the mid RPM range. There we found sound readings in the mid 70s, instead of the low and mid 80s, which is typically what we record.

Decibel readings are not linier. To get a handle on how much of a drop this is on our test engine, 5 dBa is clearly noticeable, and 10 dBa is twice as loud.

A “Quiet” Block design. Mercury has worked at both the air intake, the exhaust outlet, and in between to reduce noise. The air intake plenum has baffles that have successfully worked before. The exhaust manifold is inside the V so the metal of the block itself is helping to contain sound – much more so than twin exhaust manifolds on the sides. The exhaust and the sound carried along with it, is kept in the V, then routed into one pipe that feeds into two mufflers on each side of the leg. A relief valve is tuned to reduce noise as well as back pressure, particularly at idle. Over the injectors, Mercury has plastic covers which help contain their noise.

Finally, the lightweight cowl has insulation and three clamps to make sure it is firmly fastened to its perimeter gasket to keep the sound in. Even the little hatch on top is well gasketed to make it water tight.

Mercury 4.6L V-8 FourStroke
With the exhaust manifold inside the V of the V8, directing gas and noise into one pipe seen above, both are directed down into twin mufflers, before exiting under water.

Digital Control

The boat we tested was equipped with the Pro XS SmartCraft Digital Throttle and Shift (DTS), which gives the responsive control by wire. Mechanical control is also available.

A Note About Active Trim

While expert drivers may sneer at the idea of the optional Active Trim, it has been shown to work in our testing, and can even offer something to those who may think their feel for boat trim is more effective than that of a computer. Active trim synchs up the trim of the engines, letting the expert trimmer tune the rig more effectively, rather than dealing with trim motors that adjust the engines at varying rates in a multiple-outboard installation.

Mercury 4.6L V-8 FourStroke
Active Trim lets the user choose from five preset trim profiles to accommodate a variety of sea and load conditions. The green arrows show the Active Trim control window on a VesselView display. This image is from our test of a twin-engine installation.


The outboards have received a three-star emission rating from CARB, and are compliant to the U.S. and E.U. regulations as well, all on one calibration.

Mercury Marine’s Corrosion-Resistant Aluminum Alloy

Historically, outboard engines in salt water have suffered from corrosion. When electrons flow through dissimilar metals, the weaker metal is eaten away or corrodes. Outboard engine blocks, cylinder heads, gear cases, driveshaft housings and swivel brackets are made out of an aluminum alloy by all manufacturers. But all aluminum alloys are not the same.

Low-Copper Alloy is Key. Mercury's die-cast components, are all made from XK360 aluminum-silicon alloy. The same goes for the new 4.6L block of this outboard platform, which is made with a high-pressure die-casting in a 4,500-ton press, which is among the largest in the world. This process shows significant investment by Mercury and contributes to the weight loss of the new engines. Mercury is generally regarded to have the best corrosion-resistant alloy in the recreational marine industry. All four lines use the same alloy.

VesselView Simrad Electronics Integration

The propulsion system diagnostics can be integrated into the optional Mercury VesselView display, which is a Simrad unit rebadged for Mercury. Some boats will be able to integrate the engines into the Simrad helm electronics and not require a standalone display. The Simrad units have a specific, branded Mercury engine page. Screen sizes run from 7” to 24” (.18 m to .61 m).

The touchscreen unit lets the user tap with a fingertip to open a display window on the left side of the helm screen with a number of data categories which can be set to display consistently across all functions, including chartplotter, fishfinder, and radar displays. Speed data is available and any tap will bring out more detailed information. The display screen also offers analog-looking gauge treatments.

Mercury 4.6L V-8 FourStroke
The Mercury VesselView touchscreen gave access to a wide range of features offered by the integrated outboard system.

Advanced Features Electronic Control

The optional Simrad-Mercury VesselView interface allows the operator to set Mercury’s proprietary electronic control features, including Cruise Control, which allows the user to set a speed or rpm and have the engine maintain it, and Troll Control, which will let the engines slow-troll down to 550 rpm. Additionally, the Active Trim system can be set to tune engine trim to optimize efficiency.

Mercury’s Skyhook Advanced is also available, with different settings that allow different levels of control in specific situations and are easily downloaded via Wi-Fi connection – and advantage over the competitive brands that require dealer installation.

  • Heading Adjust lets joystick users adjust the heading in 1- and 10-degree increments while using Skyhook, for precise control.
  • BowHook maintains a boat’s position, while unlocking the boat from a set heading. This allows the boat to find the best position to minimize engine use while station keeping, in relation to wind and current.
  • DriftHook will maintain the boat’s heading, but let the boat drift, allowing the same 1- and 10-degree increments while drifting, if Heading Adjust is also on the system.

For in-depth descriptions of these features and videos showing how they work, see the “Control” model pages under the Mercury heading in BoatTEST’s engine scroll down. (

Remote Oil Checks

Mercury’s optional VesselView engine monitor helm display lets the user know the oil level is fine prior to the first startup of the day. The system only works if the engines are in the down position and the oil temperature indicates the engine is cold, since a tipped up engine or one that is warm wouldn’t get an accurate reading, so it won’t work each time a boat starts throughout the day.

Mercury 4.6L V-8 FourStroke
The VesselView display shows the oil level is fine prior to startup. Note this display shows a twin-engine installation.

If you have a question about these engines or any other boating subject, just let us know…click here.


Boats Tested with this Mercury 4.6L V-8 FourStroke Series
Tahoe 2150 CC (2019-) Tahoe 2150 CC (2019-) Barletta L25U (2018-) Barletta L25U (2018-)

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