4.6L V-8 Pro XS Series
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Mercury 4.6L V-8 Pro XS Series Main Video

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Mercury 4.6L V-8 Pro XS Series Mercury 4.6L V-8 Pro XS Series
The new 4-stroke Mercury Pro XS replaces the 2-stroke OptiMax series of engines which have been discontinued. Like the OptiMax, the Pro XS specializes in having high low-end torque and lightweight. It has been specially designed for the demands of bass boat, multi-specie, and saltwater inshore anglers.

Key Features

  • Highest displacement in its class
  • 8 cylinders, DOHC
  • Digital Throttle and Shift (DTS) available
  • 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
  • 6200 max RPM

Mercury 4.6L V-8 Pro XS Series (Outboard) Specifications
Horsepower 250-hp
Type of Engine 4-Stroke
Number of Cylinders 8 Cylinders
Weight (lbs/kg) 505.00 lbs / 229.06 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 Pro XS Series Captain's Report

Mercury 4.6L V-8 Pro XS Series horsepower
The new Mercury 4.6L V-8 Pro XS comes in models from 200- to 300-hp.


The new V-8 4.6L Pro XS engines are designed primarily for bass, multi-species, and bay boats. Here, many consumers want fast hole-shot times as well as high top speeds, something that is difficult to produce with the same prop. Mercury’s solution is light weight – at 505 lbs. (229 kg), this is the brand’s lightest V8 engine. The torpedo has been redesigned for reduced drag, and a Torque Master unit is optional on 20” shaft outboards. And a transient spark calibration is said to increase torque somewhat. This line of outboards gets a 175 based on the 3.4-liter V-6, and the 4.6-liter V8 platform is used in four models — rated for 200, 225, 250, and – for the first time – a 300-hp model. This line of engines replaces the 2-stroke OptiMax Pro XS engines, which will no longer be made.

Mercury 4.6L V-8 Pro XS Series largest
The new Mercury Pro XS 4.6L V-8 is the largest-displacement 200- to 300-hp outboard on the market.


Mercury has introduced an all-new 4.6-liter (279 cu. in.) V8 engine, specifically designed for the bass, multi-species and the saltwater bay boat markets. The company says the naturally aspirated outboards will provide smooth, quiet power and optimize fuel economy, all in a package with a dry weight package as light as 505 lbs. (229 kg.). The powerhead is a large-displacement design for more power and greater low- and mid-range torque, and for better acceleration. Even though it is a V8, it is exactly the same weight as the inline 6-cylinder OptiMax that it replaces in the Mercury line.

Mercury 4.6L V-8 Pro XS Series black
The new Pro XS comes only with the Mercury trademark Phantom Black cowl.

Mercury 4.6L V-8 Pro XS Series running
The Mercury Pro XS 250 is intended primarily for single-engine installations.

Mercury Pro XS: 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-hp Pro XS and a 4.6-liter V8 for the 200-, 225-, 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 Pro XS Series planing
We clocked the Triton 21 TrX from 0 to 20 mph in 4.8 seconds, and 7.4 seconds to 30 mph with the new Mercury 250-hp Pro XS.

The Pro XS engines are designed primarily for bass, multi-species, and bay boats. Here, many consumers want fast hole-shot times as well as high top speeds, something that is difficult to produce with the same prop. Mercury’s solution is light weight – 505 lbs. (229 kg), the brand’s lightest V8 engine.

The torpedo has been redesigned for reduced drag, and a Torque Master unit is optional with a 20” (50.8 cm) shaft length. The 25” (63.5 cm) shaft length comes standard with the 5.4” (13.71 cm) gear case.

Design Innovation

The Powerhead. Consider that the 4.6-liter V8 Pro XS is 2 liters greater than the supercharged 2.6-liter displacement of the inline-6 OptiMax Mercury and yet it weighs the same. Displacement is one way to generate more torque in a 4-stroke engine, a job that the supercharger fulfilled in the L6 2.6L 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 Pro XS Series cam
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 Pro XS Series fuel injection
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” centers – 2.5” less than its primary competitor.

Mercury 4.6L V-8 Pro XS Series exhaust
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 include an electronically controlled bypass valve.

Muffler Cut-Out for Cool Rumble

The new V8 Pro XS engines also feature a unique sound quality. The exhaust tone has been fine-tuned to provide a brawny, performance-inspired sound that’s unmistakable on the water. Just touch a button to engage the bypass valve, and listen to the throaty rumble of the Pro XS’s eight cylinders doing their work.

Mercury 4.6L V-8 Pro XS Series mufflers
Multi-chamber mufflers are concealed under midsection panels at locations indicated by yellow arrows, easier to see on these white Verados. The system is the same on the Pro XS.

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 Pro XS Series hatch
A watertight hatch in the cowl opens with a simple press to reveal the oil dipstick and fill. Note the red button on the handle at the bottom of the access door.

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 Pro XS Series red button
With a push of the red button, 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 Pro XS Series cowl
Using the handle, simply lift the cowl free.

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

Mercury 4.6L V-8 Pro XS Series spark plugs
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 Pro XS Series 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 Pro XS 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. At high cruising speeds, they produce 65 net amps. 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” (50.8 cm) lower unit gearcase on the Pro XS is the 4.8” unit, and the Torque Master is an option. On the 25” (63.5 cm) model, a 5.4” gearcase is standard. We recommend this on bay boat applications where high-speed operation in choppy water is likely. The robust 5.4” 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 Pro XS Series torpedo
The torpedo for the Mercury Pro XS has been redesigned to reduce drag. Note the three water intakes on the side (red arrow), in addition to other intakes on the tip (green arrow) of the torpedo, which can’t be seen in this image. The lower unit on the 25” (63.5 cm) shaft unit accommodates a robust 5.4” gearcase.

Adaptive Speed Control

This 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.

Mercury 4.6L V-8 Pro XS Series rpm level
The Mercury Pro XS 250 keeps its rpm level steady on hard turns when Adaptive Speed Control is used, and our test captain was delighted that incremental throttle adjustments were no longer needed.

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.

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

6200 Max Rated RPM

One of the difficulties that all engine manufacturers have is to provide an engine that has both high torque at the low end for fast hole shot times and high top speed with the same prop. Mercury has helped solve this problem by allowing the engine to run at a higher top rpm – 6200 for the Pro XS, instead of 6000 for its other models.

Based on our test of the Triton 21 TrX, the extra 200 rpm was worth an additional 2.3 mph at the top end.

Performance Test – Triton 21 TrX

We tested the new Pro XS 250 on a Triton 21 TrX on freshwater lake on a 74-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 with full fuel, one person aboard, and gear had an estimated weight of 2,812 lbs. (1,278 kg). The engine was swinging a 24” Fury prop, which meant that it would favor top-end speed instead of hole-shot times, and that proved to be the case.

The top speed we recorded was 73.7 mph getting 3.0 mpg at 6200 rpm, the rated maximum. Best cruise came at 4000 rpm at 37.9 mph, getting 3.9 mpg. We find it noteworthy that the mpg dropped by only 0.9 mpg from WOT speed to best cruise.

Our time to 20 mph was 4.8 seconds, and to 30 mph in 7.4 seconds.

Below is our complete test with numbers.

Triton 21 TrX Performance w/ Mercury Pro XS 250

Mercury 4.6L V-8 Pro XS Series

Mercury 4.6L V-8 Pro XS Series
Here our photographer caught our test boat going in excess of 60 mph.

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 Pro XS Series active trim
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 are integrated into the 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 Pro XS Series vessel view
The Mercury VesselView touchscreen gave access to a wide range of features offered by the integrated outboard system.

Advanced Features Electronic Control

The 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 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. This feature is standard on Verado V8 models and optional on all of the other new V8 and V6 outboards.

Mercury 4.6L V-8 Pro XS Series vessel view
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.


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