What is horsepower and torque and how do they relate to a car being fast or a truck having a towing capacity?
In: Engineering
Horsepower = top speed
Torque = top pulling power
Internal combustion engines can maximize one or the other, but not both at the same time. Typical car engines are tuned to have average of both, while big trucks will usually favor torque. Race cars can vary quite a bit, for long races usually horsepower is best because you will have long tracks over which to move as quickly as possible. Drag racers maximize torque because the race is only a short track and the winner is typically whoever gets off the start-line quickest.
Electric motors don’t have the same limitation, their horsepower and torque both increase linearly with the amount of input electrical power. Street racing is already starting to favor electric cars like the Tesla for this reason. You’d see a lot more electric race cars if the batteries could supply a large amount of power for a long race, and maybe that will happen in the future.
Fun fact: Horsepower is *not* the power of 1 horse. On average a horse actually generates somewhere about 15HP max.
It’s a metric coined by the engineer James Watt as a deliberately misleading marketing technique to make engines sound more powerful than they actually were – at a time when horse and carriage was known and trusted, not to mention actually faster and more efficient.
Torque is rotational force. What keeps a wheel spinning is a force, and that force is called torque.
When you spin something, you are applying a torque to that object.
Of course, where there’s motion, there’s energy, so we know that where there’s a torque, there’s also some energy. Energy in a specific time frame is called power.
Horsepower, as the name suggests, is a unit of power.
Obviously, the more torque you give, the more power you get in return.
With that being said, there’s another factor that’s part of this – revolutions per minute (RPM). That is, how fast the object spins.
When horsepower stays the same, torque decreases at the same rate at which RPM increases. You can switch horsepower, torque and RPM in this statement in any way you want and it will still be true.
In other words, torque, horsepower and RPM are directly proportional to one another.
The car’s engine produces torque. Horsepower is then calculated based on the torque produced and the RPM at which it is produced.
However, as far as combustion engines go, the horsepower/torque figures you hear about cars are not fixed values. These are peak numbers. That is, at a certain RPM, the car will reach its peak torque and at a certain RPM, it will reach its peak horsepower. Google “(car name) dyno sheet” and you’ll likely find a sheet that shows this.
Now, how does torque affect how fast a car goes?
When traveling at a certain speed, there are forces trying to stop you, like air and rolling resistance. With that in mind, we know you need at least a little bit of torque to keep your current speed, or else you’ll eventually stop.
The more torque you have, the more resistance you can fight against.
If you have only the torque needed to maintain, not gain speed, you have reached your top speed. Giving the car more gas will have no effect.
The more torque you have than is necessary, the more potential you have to move mass. So if your car is lightweight, you will take less time to reach higher speeds. If you’re towing, having more torque means you can gain more acceleration with more weight being towed.
And last but not least, you might be thinking: how come a 700-horsepower sports car can’t tow like a 200-horsepower pickup truck?
Simple: refer back to horsepower figures not being a fixed value. The truck actually has MORE power at low RPM, which obviously means more torque. The sports car cannot start moving if there’s too much weight to tow because its torque is less than what is needed to start moving when towing X kilograms. It cannot reach 700 horsepower because the load being towed stops it before it reaches optimal RPM.
Imagine a lever centered on a fulcrum. When you push down the left side, the right side rises an equal distance, with equal force.
Move the lever to the left, so you are pushing on the longer end: the right side rises relatively little, but with greater force. You have traded horsepower for torque.
Move the lever to the right, so you are pushing on the shorter end: the right side moves relatively further, but with less force. You have traded torque for horsepower.
This tradeoff of leverage between force (torque) and speed (horsepower) is exactly what happens with the gears in a car’s transmission – gears are just continuously rotating levers.
Truck drivers will tell you that torque gets you up a hill. Horsepower gets you speeding tickets. That is, increasing torque gives you more power. Increasing horsepower makes you go faster.
I was told horsepower is how fast you can get to the wall, torque is how far you can go though it. Or something like that
Energy = Power * Time. So Power is how quickly an engine generates useful energy. 1 horsepower was originally defined to be the power to lift 550 lb at a rate of 1 foot/second. Nowadays, it is much more convenient to think of 1 HP as being about 750 Watts.
Torque is rotational equivalent of force. If you push something, you exert force. If you twist something, you exert torque. The two are related: Torque = force * distance from the center of rotation. if you put 20 pounds of force on a one foot wrench, you are exerting 20 foot-pounds of torque on the nut. If you use a cheater bar to extend the wrench to 3feet, that becomes 60 foot-pounds of torque.
The power output by an engine is the product of the torque * RPM. This is really simple and really important so it bears restating:
Power = Torque * RPM.
Gearboxes can freely change torque to RPM and vice versa. A big engine with lots of torque has no more pulling power than a small high revving engine with the same power.
The only thing useful about torque as a metric is that it tells you something about the general nature of the engine: high driveshaft torque for a given power generally implies a big, heavy engine meant for long running in applications where weight is not an issue (ships, generators, locomotives) while low driveshaft torque for a given power generally implies a small, light engine meant for applications where power-to-weight is more important than the ability run for weeks on end (aircraft, race cars, motorcycles).
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