ENGINE BASICS 101: TORQUE VS POWER

Torque and power – to engineers and mathematicians, it’s simple stuff. To the rest of us, it’s one of the most elusive concepts to grasp when shopping for an engine. What does it mean to have a high torque engine, and what really are those numbers that are listed with engine specs? Let’s try to boil down the math and science behind torque and power, and lay it out in terms that anyone can understand.

Torque is force causing rotation, Power is how quickly rotation occurs

That’s the simplest way of putting it, and even with that simple phrase, it’s clear that there is some overlap between torque and power, which is typically expressed as horsepower. A little more detail on the terms:

• Torque – Torque is force, but not just any force. Specifically, it’s twisting force, which is measured in foot-pounds. In other words, torque is the measure of how many pounds of force are applied over a lever (like a crankshaft lever) of a certain length. A quick example: if an engine applies 200 pounds of force to a crankshaft level that is 2 feet long, it is producing 400 foot-pounds of torque.
  
  In short, torque refers to an engine’s ability to perform work. The more torque it offers, the greater a load it can manage. So, for example, a torquey engine is one that usually provides a lot of work potential at low engine speeds. This characteristic allows operators to get a lot of acceleration.

• Power – Power, which is represented as horsepower, is a measure of how fast an engine can deliver torque. Put another way, it’s the rate at which an engine can get work done. Torque is a factor in determining horsepower, so as the engine’s torque increases, so will its power. However, horsepower is also determined by the speed at which the engine is spinning, which is listed as rpm.
  
  Horsepower best describes how fast an engine can apply force to a load. It increases as the engine’s torque increases, or as the speed of the engine in rpms increases. While torque is about acceleration, horsepower is about top speed. 

Deciphering engine specifications

The challenge when picking an engine is figuring out how to read the numbers. Because horsepower and torque are both dependent on engine speed, there isn’t just one torque or horsepower value to pay attention to. It’s easy to focus solely on peak torque or peak horsepower, but these numbers are only part of the story. If peak torque, for example, is only available at extremely high rpms, then it’s less useful than if peak torque is achieved at a lower engine speed.

The best way to get the full picture is to take a glance at the engine’s torque and power curves. Every engine manufacturer can provide them, and they offer a look at how much torque and power an engine provides over its entire speed range. This way, a vessel owner can easily judge which engine is better at providing overall performance (vessel acceleration and top speed).

This, of course, is only a cursory look at torque and power. There’s a lot more to the interaction between torque, power and speed, and the engineering principles that underlie them, but for the purposes of picking out the best engine for a vessel, one only needs to know the basics.