When Choosing an Outboard, Don’t Overlook Torque

When it comes to outboard performance, obviously horsepower matters. Your engine’s horsepower needs to be appropriate for powering the boat you own. Still, there’s much more to performance than just horsepower. Torque is another vital aspect of performance that all boaters should understand at least at a high level.

To break it down, we got some help from Brian Fregonese, manager for product R&D engineering at Mercury Marine. Fregonese leads the thermodynamic development team at Mercury, which is responsible for testing engines to ensure they meet overall performance goals, fuel economy targets and emissions requirements.

He and his crew spend many hours running engines on a torque dynamometer – better known as a dyno – which measures the torque generated at the prop shaft.

In other words: He knows torque.

What is torque and why is it important for an outboard?

Torque is a measure of twisting force. Specifically, it’s the amount of force that causes an object to rotate. When you consider that an outboard moves a boat by rotating a propeller, you see why torque is so important.

“Because the engine is turning a prop, more torque basically equals more thrust,” said Fregonese. “There’s a direct correlation between torque and thrust. Therefore, when you have more torque, you accelerate faster.”

Acceleration is the performance metric where torque has the most impact, and it’s a priority for Mercury’s product development team.

“Acceleration is always in the top three of our pillars when we’re doing a new program,” Fregonese explained. “From all the customer surveys we do, acceleration almost always rises to the top of what boaters want. And that’s coming from customers.”

Acceleration is vital for hole-shot performance and getting a boat on plane. By increasing acceleration, the engine can level off the boat sooner, giving the driver more control and a better, safer view of the water in front. Torque becomes especially important when accelerating a heavy load – say, a pontoon with a full crew or a heavily loaded offshore fishing boat. It’s the muscle that gets a heavy load moving.

Fregonese and the rest of the product development team at Mercury keep these and many other boating scenarios in mind as they’re testing and developing engines. The Mercury V8 250hp Pro XS® outboard is a great example. When developing the 250hp Pro XS, the team set high targets for torque and acceleration to make sure the final product satisfied the demands of performance-obsessed bass boat owners.

“In the bass market, those boaters get very laser-focused on performance,” Fregonese said. “Top speed is huge, obviously, but hole shot is a big deal for them, too, especially if they’ve got a tournament load.

“With that engine, we were able to generate 12 to 15% more torque than a leading competitor engine at mid-range, which we measured at 3750 to 4750 rpm on that engine. Then you can see the correlation. There’s more torque, and the 250hp Pro XS leads its class in acceleration.”

The torque curve and what is says about performance

When testing an outboard on a dyno, engineers run the engine at various rpm, from slower to faster. At each rpm level, they use the dyno to measure torque and record those readouts on a chart known as a torque curve. Generally, there’s a steep spike at low rpm and then the torque curve levels off. Engineers use this curve to predict real-world performance.

“Mid-range torque is most important for hole shot,” added Fregonese. “It’s where the engine runs from 0 to 20 mph to get on plane, and then up to about 30 mph.”

Engineers also cross-reference torque against the amount of engine power (horsepower) generated at each rpm level. Adjusting the engine’s design or tuning to increase one factor can decrease the other, so the development process is always a balance between meeting peak horsepower requirements and optimizing torque.

Most recreational boaters don’t really need to know the specifics of an engine’s torque curve. Just know this: Mercury ensures the torque is optimized for hitting performance goals, particularly at the mid-range to achieve optimal acceleration.

How Mercury optimizes torque when designing outboards

How does Mercury optimize torque? In addition to a team of talented engineers and decades of experience, the foundational element of most Mercury outboards is a high-displacement powerhead.

Displacement is the volume of all the engine cylinders. More volume means more space for air and fuel to mix in the combustion process, which translates to greater ability to generate power and torque. So, it’s no coincidence that most Mercury outboards have the highest displacement in their class.

There are other ways to generate torque instead of with displacement, usually with the addition of different types of technology that control fuel flow or fine-tune cylinder and valve timing. That technology can increase cost and complexity. Mercury’s high-displacement designs reduce complexity while enabling the engines to dole out serious power and torque.

“Our engines are relatively simple,” Fregonese said. “We don’t use complex additional technology to achieve our torque goals. We don’t add additional fuel pumps and things like that. We do it with brute force, if you will, through relatively higher displacement. And usually we’re still either best in class or among the leaders for fuel economy. ”

Another way Mercury optimizes torque specifically on the popular V6, V8, V10 and V12 engines is with lengthened intake runners. These are the “tubes” that channel intake air to the cylinders as part of the fuel-induction process. The lengthened design helps make the most of all that displacement.

“The longer you make the intake runners, the more low-speed torque you’ll make,” Fregonese said. “Essentially, you can pack more air into the cylinder at lower speeds.”

Torque’s impact beyond hole-shot acceleration

One final note about torque is that it can also noticeably impact performance outside of just hole shot. It also impacts acceleration from cruising speed to wide-open throttle – that mid-speed “punch.”

Anyone who’s had the chance to run the new Mercury V10 Verado® outboard has probably experienced this. With all its torque, the V10 can get a boat up to cruising speed at a relatively low rpm. Then, because the engine isn’t “strung out,” it has a lot more muscle left to continue accelerating.

“If you’re at cruising speed, you still have a lot more torque reserve than you would on a smaller displacement engine,” Fregonese said. “Torque reserve is the difference between the torque required to cruise and your wide-open throttle torque.

“On the V10 engine, people really like the mid-range acceleration. You’re cruising 25 mph and you put the hammer down, and it really accelerates well. And again, that’s because of the shape of our torque curve and what we did there.”

Mercury wins with torque

At a high level, torque matters a lot when it comes to marine engine performance, but it’s also one piece of a bigger puzzle. Mercury also balances the gearcase design, rpm propping range, max rpm, fuel flow, hydrodynamics, prop design and many other factors to achieve its performance goals, from efficiency to top speed to quietness at cruise.

But having a high-displacement powerhead capable of a lot of torque sets a really solid foundation for overall performance. So think of torque like muscle, or strength. When you have that solid foundation, performance comes more easily.