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While the terms horsepower and torque are used extensively in the automotive industry, both terms have nothing to do with cars or trucks. In fact, both are physics terms, used extensively, the field of rocket propulsion and even motors. Yes, that’s right, motors, i.e., electric mixing motors, use the terms to express almost the same values and ideas as the automotive industry. That said, not many people understand the relationship or even the definition of both terms. Some even use these terms interchangeably. 

Fortunately, even if you weren’t paying attention in grade school physics class, it is never late to learn what these terms mean. Understanding these terms can open up your understanding of motors and cars alike. 

The Basics of Horsepower and Torque 

If you have a gasoline engine, diesel engine, or even an electrical engine, its power is referred to as horsepower. In physics, the definition of horsepower is the rate at which work is done. In the automotive world, where it’s more commonly used, it refers to speed. In other words, if you want to get up to speed faster and go faster with each passing minute or hour, you will need an engine that can output more horsepower. 

Torque is a term used to express rotation or some type of twisting force. If you apply this to a car’s engine, it (the engine) rotates around a certain axis, which creates torque. In other words, you can think of torque as being the strength of a vehicle. It is the same force that will rocket a sports car, taking it from 0-100mph, pushing you back in the seat. It is the same power that helps push those million-pound trucks. 

Now that you are familiar with the basics, how are these concepts measured and the relationship between them?

What’s Behind the Numbers?

Horsepower is mathematically expressed as the force required to move a 550-pound weight a single foot in one second or move 33,000 pounds one foot within a minute. The horsepower in cars is measured using a dynamometer, which measures the torque output by the engine and its RPM (revolutions per minute). The formula used is torque x RPM / 5252, which results in the vehicle’s horsepower value. We use torque as a measurement because it is easier to measure and then calculate. 

As mentioned earlier, torque is a twisting force, which is measured in units of force multiplied by the distance from the axis of the rotation. For instance, if you have a 1-foot long wrench to exert a 10lbs force on the end of the bolt, then the torque applied will be 10 pound-feet or 10lb/ft. 

What is the Relationship Between Torque and Horsepower?

We’ll start off by stating the obvious (for most people), both torque and horsepower affect the speed of a vehicle, which is why many people mix both terms. That said, in the real world of hauling and driving, there is a difference, as there is with a vehicle’s design, which can have an impact on both horsepower and torque. 

Take, for instance, if an engine produces a more horsepower, it also has more potential for torque. The way you see this translate is via the vehicle’s axle, transmission. That’s why, even though both a race car and a tractor may have the same horsepower, but their speed and power are very different. The race car converts all of its torque to speed via a system of gears. A tractor is converting its horsepower into more pushing and pulling power, making it possible to push heavyweights. 

Another way to understand the difference between horsepower and torque is that the latter is like twisting off a pickle jar. When you put all your strength into opening the jar, you’re applying torque, that’s regardless of if the cap comes off. You only get horsepower with movement, so in this case, torque is used to loosen the lid, and the horsepower takes over to spin it (the lid) open quickly. 

Is Having More Torque Better for Acceleration, While More Power is For Speed?

In a way, we alluded to the answer to this common question above. However, it’s worth diving a little more into it. The more horsepower your vehicle, for instance, has, the faster you’ll be able to hit its top speed. Take, for instance, a 2000 horsepower vehicle with a 300-mph top speed. The problem is that torque is the other side of the same coin because what matters is when the vehicle’s peak torque is met and then how long that’s sustained. 

Take, for instance; you have a vehicle with 200 horsepower capable of delivering over 200 lb-ft of torque, at 6000 RPM, which is up against another 200 horsepower vehicle with just 150 lb/ft of torque at 1500 RPM and hitting a straight 6500 RPM redline. The second car will most certainly outpace the first one from the start, even though it has much less torque. The reason is that the engine can hit its max torque sooner than the first car. 

A more realistic scenario is that of a drag race between two vehicles. One delivers a whopping 774 lb-ft of torque, and because it is an electric engine, it does so at 0rpm, which means it can out-accelerate, a powerful gasoline-powered drag vehicle capable of delivering 1100 lb/ft of torque because for its to reach max torque, it needs to hit 1180 RPM. 

Why Do They Even Call It Horsepower and Not Donkey or Cat Power?

Well, that’s a question many people ask, and they assume that it has something to do with our love of the stallion. In reality, the term horsepower is thanks in part to a person who changed the way we measure power. Back then, James Watt was comparing the output of a steam engine and draft horses to mainly sell people on the idea of this new steam-powered service. He observed that a single horse could turn a mill wheel exactly 144 times an hour, the radius being 3.7 meters. So, he figured that a horse should be able to travel 2.4 × 2π × 12 feet in sixty seconds, aka a minute. 

Watt then applied the formula to his new steam engine to provide an estimate of how much more effective they (steam engines) were compared to a horse. Back then, everyone used horses, so it was a comparison that everyone could understand. That’s when horsepower was born, and all other animals never stood a chance!

Engine Torque

Applying Torque and Horsepower 

Applying both horsepower and torque in a machine all depends on what the machine does. If you want to pull a large tree stump out of the ground, horsepower will not work unless you add over a hundred meters of rope. Though if you have something more sporty, horsepower pushes it forward. Car manufacturers have been manufacturing vehicles for now well over a hundred years, have found the right balance, and adjust those figures ever so slightly. 

We know from experience that a car engine, let’s say engine A, is torquey, but very few people can clearly distinguish the characteristics of a regular road car. The consensus is that you will want to have at least a 10% difference to tell the difference and feel it, more importantly. In other words, having an engine with 5% more torque can’t be felt unless it’s on the low end, which is a whole different discussion. 

How Does Torque Apply to Electric Vehicles? 

Electric vehicles like Teslas are becoming increasingly popular and hence more common. We see an increasing number of them almost every day. But do these electric vehicles spell the death of torque measurements in the automotive industry? 

In the future, there will be no mention of torque or horsepower. The industry as it moves eventually to entirely electric vehicles will only mention kW. 

Anyone who has driven an electric vehicle knows that it can deliver an immense amount of torque. It can out torque most gas guzzlers, regardless of how much they guzzle. An electric vehicle can go from standstill to 60mph in a matter of 2-3 seconds, despite being weighed down by its batteries. 

Speaking of torque, the Tesla Model S P90D just broke the record for an all-electric currently production car ¼ mile sprint in just 10.9 seconds. Compare that to the Bugatti Veyron, which does it in 10.175 seconds. So, it’s not hard to see that electric power is undoubtedly on its way up as the gap in performance at least is closing. Within a few years, we may never have to talk about horsepower or torque, except for trucks and other forms of commercial transport that will continue to rely on gas and diesel engines. 

Why do Some Manufacturers choose to Use Different Terms for Torque?

Today, we live in a truly globalized world. Regardless of what people say, we still use different labels and measurements, which mean essentially the same thing. But they also confuse most average Joes. It’s the same imperial vs. metric difference. So, the representation of torque here isn’t any different. Some automotive manufacturers may express it as pound-feet (lb-ft), others as foot-pounds (ft-lbs), and then there is Newton Meters (NM). 

Pound-Feet (lb-ft)

A single pound-foot is the moment of inertia on a single axis that applies to a single pound of force within a foot’s radius. So, the TL: DR, pound-feet is mainly an assertion of the force applied or exerted by a vehicle’s wheels as it hits the pavement. 

Foot-Pounds (ft-lbs)

It is the UK’s version of the pound-feet, but a little backward, which is indicative of how they drive on the wrong side of the road. Foot-pound may also refer to a unit of work but has been co-opted by mainly automotive manufacturers. 

Newton-meters (NM) 

A single newton-meter is a single unit of torque of the metric system. So, a foot-pound equates to 0.73756-newton meters. 

Why Horsepower is Expressed Differently by Vehicle Manufacturers?

The same principle applies with torque. Generally, there are four main terms used, i.e. brake horsepower, horsepower, metric horsepower and kilowatt. Each of these terms main the same thing but in slightly different ways. 

Horsepower (HP)

You probably already know that horsepower is a very familiar figure or term used to denote a vehicle’s power. It relates to the amount of power the car’s engine can develop when you crank it. If anything, just about everyone is familiar with the term, and it looks excellent on brochures. 

Brake-Horsepower (BHP)

Now, this term is often used in countries like the UK and other parts of Europe. It denotes the vehicle’s power at the tires but not at the crank. It is the actual output of the vehicle’s horsepower, its ability to produce from the rear, front or all four of its tires. 

Metric Horsepower 

Now, this is a strange one, but bear with us, it’s easy once you understand it. It is often written as cheval-vapeur (CH), cavallo vapore (CV) or pferdestärke (PS). It significantly differs from the standard or what’s referred to as the imperial measurement of horsepower, owing to the way it is calculated. To calculate metric horsepower, they would in the old days attach a horse to a 75-kilogram weight, over a pully, and then the horse would be timed as to see how quickly it could lift the weight by one meter. Now for those who were trying to figure it out, the result is a single second. They then drafted an equation equating one metric horsepower as 98.6 percent of the standard horsepower in comparison. 

KW

Kilowatt is used mainly in Australia and New Zealand but happens to be a very precise measurement of power. It is a measurement used by just about every engineer out there. The measurement means the amount of energy transfer for a period of time, resulting in 1 kilowatt or KW being equal to 1.341 standard horsepower. 

How Does Torque and Horsepower Apply To Electric Motors?

We alluded to the fact that electric vehicles tend to be more torque happy, and that’s because of how electric motors work. The torque of an electric motor can be increased by roughly raising the voltage input. However, to determine the horsepower of an electric motor, you have to know the input voltage or current draw in amps and the rated efficiency of the device. Also, keep in mind that 746 watts equate to 1 HP. The current draw and input voltage are taken from Ohm’s Law, which equates to volts x amps = watts; yes, we know this is again graded school stuff. 

Calculating the horsepower of an electric motor requires multiplying volts with amps and rated efficiency, then divided by 746 watts. 

A couple of electric motors, like the ones that are used in shop air compressors, have their horsepower rated as “peak” horsepower. That usually indicates the amount of power the motor has to start, which can often be up to seven times its rated continuous horses. The higher power is owing to the motor’s windings, which take a lot of current to engage. Once fired up, it does not require as much current to continue working. 

The full torque output of an electric motor is at 0 RPM! Compared to an internal combustion engine, the latter requires a given operating speed to achieve that peak torque. However, you need to be mindful of an electric motor’s efficiency rating. Ones with a higher rating tend to be more powerful, cost less and will run cooler compared to a far less efficient motor. 

Conclusion 

Whether you’re building an electric car of your own, buying a pump for some reason or maybe in the market for a new car, understanding the relationship between horsepower and torque is very important. While they are two sides of the same coin, so to speak, they have a relationship that requires understanding both individually and then collectively.