Horsepower Vs Torque – The Differences and Why They Matter

November 24th, 2018 by

Guide to Horsepower and Torque

 

People throw around automotive terms all the time without really understanding what they are. One of the best examples is when horsepower and torque specs for a car are quoted and everyone shakes their head solemnly. While you might know that the horsepower or torque output for one car is higher than another, do you really understand what that means? What if one car engine produces more horsepower, but another claims superior torque?

 

Some car enthusiasts will even sit around and argue about whether horsepower or torque is more critical for performance. You’ll find there are passionate and seemingly valid arguments on both sides of this issue. But, if you don’t really understand how horsepower and torque differ, plus why it matters, you’ll feel lost.

 

It’s time to get your science on and learn what separates horsepower from torque for car engines.

 

Work

 

To understand horsepower and torque, you first must grasp the concept of work. No, this isn’t like having a job. Instead, this is work in the scientific sense, which has to do with force. When one body or object pushes or pulls against another, things might move. To move a car, the engine must apply more force than any resistance to the wheels turning. If you’ve ever been off-roading and been stuck in mud or sand, you know what happens when resistance becomes too great.

 

Work is when force has been applied by one object on another, and the other object has actually moved. This is expressed in an equation:

 

Work = D (distance moved) x F (force applied)

 

You should also know that work is always calculated as movement along a line or plane, making it a vector force.

 

Torque

 

If you know much about engines, you know the crankshaft and flywheel rotate around an axis. To measure this non-vector force, we use torque. Quite literally, torque is a twisting force. People normally think of engines producing smoking tires, but those tires rotate through the twisting force of the crankshaft, transmission, drive shaft, etc. You need to understand why torque is such an important element of what an engine does, because it greatly affects performance.

 

Even though different areas use different units to measure force, such as pound-feet (lb.-ft.) or Newton meters (Nm), they all are describing the rotational force multiplied by the distance from the axis where everything is rotating. This might seem confusing, but it really is a pretty simple concept, once you get how it works.

 

For example, if you have a wrench that’s three feet long and you put 20 pound-feet of force on it to turn a bolt, the torque applied would be 60 lb.-ft. If the wrench only measured one foot in length and you applied the same 20 pound-feet to it, the torque would be 20 pound-feet.

 

Things get a little more confusing when you learn that while torque in the US is measured in pound-feet or lb.-ft., work is measured in foot-pound or ft.-lb. Some people believe the two terms are interchangeable, but they’re not. Work only happens when there’s movement, because you can apply torque to something like a bolt, but it might not be enough to make it budge.

 

Power

 

If you want to measure how quickly work has been done, power is the way to do it. Literally, power is how much work is performed during a certain block of time. If two people were to tighten the same bolt by applying the exact same amount of torque, but one does it in half the time as the other, that would mean the quicker person was twice as powerful.

 

In the United States we use horsepower or hp when referring to power as it relates to cars. Most people don’t know that one unit of hp is equal to the power necessary to do 33,000 ft.-lb. of work in just one minute. Where did this calculation come from? James Watt, an engineer from the 1700s, was the one who created it. Watts observed how much work a strong horse could perform as it cranked a gear-driven mine pump, hence why it’s named after a horse. More simply, the Scottish inventor used the standard of one horsepower being the power to life 33,000 pounds exactly one foot in a minute’s time.

 

So, 1 hp is 33,000 ft.-lb. of energy produced by a car each minute. That means hp = ft.-lb./minute divided by 33,000.

 

Another way of mathematically expressing horsepower is to multiply torque by engine rpm.

 

Dynos

 

Tuners and other car people know that a dynamometer or dyno is an accurate way to measure output from an engine. Just because an engine feels or sounds powerful doesn’t necessarily mean it is. Dynamometer literally means a power measurement device. You put the car on rollers with the machine connected, then run it through the different gears to get a readout.

 

Dynamometers do not actually measure mechanical energy or ft.-lb., despite plenty of people thinking they do. Instead, they measure torque or lb.-ft. plus engine RPMs, since the car isn’t moving and so that’s the only distance measurement available. Using the formula for horsepower, the machine uses rpm and torque to calculate horsepower output.

 

Most people don’t realize that a dyno isn’t measuring mechanical energy produced by a car’s powertrain. Most modern dynos have a lever arm that receives torque from the engine’s crankshaft through a magnetic field. The lever arm bears against a load cell, or more specifically a static force gauge. It’s important to note the gauge is spaced a precise distance from the center of the engine crank.

 

There is another dyno setup that’s also quite popular. Instead of a magnetic field, there’s a water brake. The dyno has two sets of pump vanes, one being static and one spinning. Those convey the torque from the engine crankshaft through a level arm to a load cell, providing a measurement of torque.

 

An engine dynamometer will display the engine’s torque curve, or how much torque is produced throughout the entire rpm range. Ideally you want loads of torque at a low rpm, plus peak torque as early as possible, with little to no torque loss all the way to redline. This would be a relatively flat torque curve, something of a holy grail in the engine building community. The reality usually isn’t quite like that. They’re called torque curves for a reason, instead of torque plateaus.

 

Tuning Applied

 

Any way you cut it, the magical number is 5,252 rpm. That’s always when the horsepower and torque values are equal to each other. It’s an interesting fact you can throw out at a cocktail party, but people might not believe you. The useful part of this tidbit is that you need to increase engine torque output to increase horsepower. The alternative is to push peak rpm higher, something many race cars and even a fair amount of performance vehicles use. Even better, increase engine torque and increase RPMs to extract more horsepower from an engine.

 

If you’re dealing with a naturally aspirated engine and you’re not increasing the bore or stroke, this knowledge of how horsepower and torque relate to each other means you have only one viable way to boost horsepower. You must raise the operating rpm range of the engine, which in turn will push peak horsepower to a higher level. The tradeoff is that your peak horsepower will come at the high end of the rpm range, but in racing that’s not a huge problem. For road cars, like the Ford Mustang, such an engine tune would be less useful since you spend much of your time in the lower end of the rpm range. This is why many racing engines, particularly those with relatively low displacements, have a high rpm limit. Honda has famously used this method to make vehicles like the Integra Type R or S2000 quite potent, despite those cars utilizing small four-cylinder engines. Honda’s VTec works at the upper end of the rpm range, boosting top-end power, even in real world driving scenarios. That extra rev allows the smaller engines to produce more torque and horsepower, but at the upper end of the rpm range. If the engine makes the same amount of torque but at a higher engine rpm, the math tells us that horsepower numbers will increase accordingly.

 

The old saying is there’s no replacement for displacement. To an extent that’s true. Bigger engines, in very general terms, post bigger torque and in turn horsepower figures. That’s why you’ll find that the classic muscle cars packed massive V-8 engines, because that kind of a setup resulted in a lot of twist. With more air coming into the cylinders each combustion cycle, the more torque is produced by the engine. So displacement is a big factor for engine power.

 

Of course, if you want to dramatically boost horsepower without increasing displacement, the best method is forced induction. Just how much of an uplift in torque and subsequently horsepower an engine would see really depends on the forced induction setup. People will argue all day long about if turbocharging or supercharging is better. Oh, and there are different types of superchargers, plus many varieties of turbo systems. Then you have nitrous oxide, another method of forced induction that provides a temporary increase in torque and horsepower output.

 

Once you start to clearly understand the relationship between torque and horsepower, you’ll see the best way to extract maximum output from any engine. For example, trying to get huge torque and horsepower figures from a naturally aspirated engine that tops out at 6,000 rpm just isn’t a realistic goal, especially if the engine isn’t a humongous V-8 or larger, unless you use a forced induction system.

 

This means if you want big horsepower figures lower in the rpm range, torque needs to peak at an even lower rpm. You then tune the engine according to how you’re going to use it, understanding how torque and horsepower relate to each other.

 

While you can use forced induction or increase displacement to pump more air through the engine, that’s not where tuning ends. That extra air must mix with extra fuel, and in this day and age of rising fuel efficiency standards, that means automakers must get even more creative. Oh, and not to mention every stricter emissions standards, which are directly affected by all this. As a tuner you can choose to just dump more fuel into the cylinders, upgrading the fuel delivery system accordingly.

 

Conclusion

 

After reading all this, you’re hopefully not too confused. The relationship between horsepower and torque is by nature quite technical and complex, but it’s something you must understand. The two things are related quite closely, but they’re actually not that similar. If that sounds like a contradiction, it’s because torque and horsepower are somewhat contradictory.

 

So, which is better: horsepower or torque? That’s really an erroneous question. Of course, plenty of car enthusiasts will waste their time arguing this out online, at car shows, and anywhere else they find the occasion. But by the simple fact that they really think one is better than the other shows they don’t entirely understand the relationship between torque and horsepower.

 

Torque is absolutely necessary for an engine to do its thing, and hopefully that’s to produce mountains of horsepower. That twisting force, torque, is essential for getting the wheels of any vehicle turning. But, loads of horsepower is what will make a car scoot in a hurry, something that becomes really important when you’re racing on a track or just want to impress your friends. So, horsepower and torque are of equal importance, but for different reasons.

 

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