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 is a measure of the work that can be done in a unit of time.
Torque is a measure of the amount of force that can be applied at a given RPM.
For cars and trucks, more horsepower correlated to a greater amount of work being done over time, so in either situation horsepower would be the measure of productivity of that vehicle.
The formula for Horsepower is
HP = (Torque x RPM)/5252
So say a truck produces 300hp at 4000RPM. A car might produce 300hp at 7000RPM. The car’s engine will be producing less torque than the truck, but both are able to produce the same amount of power.
Torque in this situation determines at what RPM an engine must run to produce a certain power output.
For towing this becomes important because typically the higher you rev an engine, the more energy is lost to heat due to friction. So an engine that can produce a lot of torque at a low RPM will be more efficient and more reliable than an engine that produces less torque at a high RPM.
So if low RPM high torque is more efficient, why do cars use small engines that produce less torque and rev higher? Well it comes down to weight! A car is focused on keeping weight low while having the most possible power output. A truck is focused on maintaining the most efficient operation while still being useful.
The acceleration of a vehicle at low speeds comes down to its power to weight ratio and available traction. So a car will accelerate quicker if it is light and can produce a high power output.
Trucks are less focused about fast acceleration and trade weight for reliable efficient engines that can do lots of work for an extended period of time.
It’s also worth noting that a trucks towing capacity is more a measure of the quality of its suspension, drivetrain, gearing, and ability to slow down a heavy load. Not so much it’s power output. A truck with only 5 horsepower could tow a 50,000 pound load, it would just take a very long time to get anywhere!
Horsepower is a product of speed and force (it also depends on the direction of these things aswell). Or of rotational speed (rpm) and torque (Nm, lbft, etc.)
Let’s look at them separately first.
rotational speed is how fast something is spinning, how many times it rotates every minute.
Torque is “how much force is behind that turning motion”. A Nm (newton-meter) is just 1 newton of force with a lever that’s 1 m long. (or 2 Newtons of force with 0.5 meter lever).
And one lbft (foot-pound) is the torque that 1 lb for force makes with a 1 foot long lever (or 2 foot long lever with half pound force).
These 2 units are essentially the same thing, only one is bigger than the other, like seconds to minutes.
There’s this thing called a gearbox that can speed up rotation at the cost of torque; or increase the torque of something, but at the cost of speed.
Doubling the torque means halfing the rotational speed.
Because we have this, it makes sense to talk about the product of speed and torque; something that stays constant no matter what kind of gearbox you put it through (ignoring the losses of course). That’s called power. And horsepower or kW (kilowatt) is a unit of power.
If you have an engine with a lot of torque and low speed you can put a gearbox behind it to make it “seem like” it has more speed and less torque.
Similarly, if you have a fast engine with low torque, you can still put a different gearbox behind it so it seems to have more torque, but less speed.
If you want a fast car then power (in horsepowers or kW) is the most important thing, since you need to put down a lot of force (or torque) at high speed. And that means a lot of power.
If you want to tow, then horsepower is still the most important factor of the engine. Yes, you need more torque at the wheels, but that’s exactly why the gearbox is there. It takes the fast speed of the engine, slows it down and increases the torque by a lot, so you can tow stuff.
However, with towing the mass (or weight) of your car is also very important, because a light car can have trouble pulling up a heavy trailer up a hill. Even though it has enough power (and torqe) to do the job, but the wheels would just slip.
Let’s draw an analogy.
If Torque and horsepower were people, Torque would be Arnold Schwarzenegger while HP would be Usain Bolt.
Now both of them are tasked with carrying a 100 bricks across a bridge.
Now Arnold, given his sheer power, can carry 20 bricks at a time, making 5 trips in total, which take him 50 mins in total, or 10 min per trip.
Now Bolt, though not as strong as Arnold, can run like a cheetah. So though he only carries 5 bricks at a time, he only takes 2.5 mins per trip, doing the task in 50 mins.
So, horse power is the amount of work done in a given time, measured in HP or Kilowatts.
In order to do the same amount of task, i.e. achieve same amount of horsepower, you can either increase your Tourqe (strength/force) or revolutions per minute (RPM).
HP is a function of Torque and revolutions.
When judging a car, compare the HP, the work it gets done, whether by brute force (torque) or RPM, not just torque.
No point having Mark Henry in your team if he can’t see when Ray Mysterio kicks his ass because he’s too heavy.
The HP and torque numbers don’t matter for speed and towing as much as vehicle weight, transmission & final gearing, brakes, and, suspension set up. My V-8 car, my 4 cylinder car, and a big rig truck all have about 200hp.
My V-8 is rated to tow up to 5,000 pounds, the 4 cylinder isn’t rated for towing and it isn’t recommended, and the big rig can tow 45,000 pounds. This requires torque. The V-8 pumps out close to 200 ft pounds of torque, the 4 cyl pumps out 140, and the big rig pumps out closer to 1,200.
The 4 cyl is the fastest out the group, followed by the V-8, then, very far behind is the big rig.
Now for the transmissions. The V-8 has a simple 4 speed automatic, with one gear for overdrive. The 4 cyl has a six speed manual with 2 gears for overdrive. The big rig can have up to 21 or more forward gears, 2 or more reverse gears, and overdrive is based on build specs & final use, plus customer input.
The 4 cyl has a red line that lets it spin up to 8,000 RPM. It has lower final drive ratios than the standard model, and spools up quickly to those speeds. The V-8 has a red line of about 5,000 RPM and is geared for top speed and acceleration. It winds up a bit slower, but it’s no slouch. The big rig is tached out at 3,000 RPM, and accelerates so slow, you can time your shift without using the clutch between shifts.
Engine sizes: The 4 cyl is a whopping 2 liters, the V-8 is 4.6, and the 4 cylinder big rig starts off around 12 liters.
Edit: Hit send too soon.
Torque is twisting power, always, by that I mean the ability to do some level of “work”. Having more torque than is necessary for the application is wasteful. To get horespower, you multiply torque times engine speed (set by manufacturer) divided by 5252. That describes the speed with which you can get some amount of work (as defined by torque) done.
A car with 100 HP and 100 foot pounds of torque will be slower than a 110 HP car with 100 foot pounds of torque. However, you have to consider the weight of a vehicle and other load factors, like whether you are going up hill. So if you need to tow something and you really only need to have highway speeds then a diesel, which has far more torque than horsepower is a good idea. If you have a race car then you will invert that, favoring horsepower to torque. There are a few notable race cars (like the Audi R18) that run on diesel and they make up for the lower HP by making the car lighter, more aerodynamic, stable, and it controls how the torque gets put down so as not to shred out your tires.
In many ways, we don’t really care about horsepower, I can simply spin the motor faster (which is why redlines on Porsche’s and the GT 350R are far higher than a non-racing engine) to get more speed to the ground. Increasing torque is a lot harder, I need to find a way to get more air molecules and fuel into the cylinder, so I have to stroke it, bore it, add valves, add cylinders, or add forced induction. Forced induction is just any turbine that will compress a bunch of air and jam them into the cylinders to get more explosive power. That turbine comes in two general forms, a turbocharger which is spun by the exhaust gases or a supercharger which is spun by a belt connected to the motor. Both products are essentially just turbines, the difference is what feeds the turbine energy to compress the air.
Horsepower is a measure of power and torque is a measure of twisting force, but let me expand on that. A waterwheel can develop a tremendous amount of torque, like 2,000 ft-lb of torque. But it does this at a low speed, say 10 rotations per minute. A BMW M3 makes about 400 ft-lb of torque, but it does this at 5,000 rpm.
So it sounds like the water wheel would be better for towing, right? Well, the difference is gearing. If you took that BMW’s 400 ft-lb of torque and geared it down from 5000 to 10 rpm, it will be making 500 times as much torque, or 200,000 ft-lb of torque.
Horsepower takes this into account. A car’s horsepower = torque * rpm / 5252 (the 5252 is just a normalization factor, the important thing is that it depends on torque AND rpm).
So really, horsepower is the much more important number. A car that makes 400 ft-lb of torque at 2,000 rpm will accelerate just as fast as a car that makes 200 ft-lb of torque at 4,000 rpm, provided the proper gearing.
Unfortunately, that doesn’t tell the whole story either. That BMW M3 makes 450 horsepower, but not always. The “horsepower” number you see advertised is really PEAK horsepower, and it’s only true ta certain RPM. If a car makes 450 horsepower at 7500rpm, then it’s making less than 450 horsepower for the rest of the rpm range. This is where the torque number comes into play.
When you want to tow something, you want something with a broad torque curve… you need to be able to pull it from a standstill, and might not be able to wait for the engine to wind all the way up to 7500rpm. So you want a lot of torque all the way down to idle rpm’s, like 1,000rpm. That’s where towing capacity comes in.
Ooh I can help with Torque! Watch this video about the amazing TORQUEMAN
…that I made for high school physics like 11 years ago.
Torque is the twisting force of the wheels, and Horsepower is how much work is done.
When you wring a towel out, torque is how hard you can twist it, HP is kind of like much water comes out.
transmissions exist, which is like putting the towel in a mechanical twister that you spin by hand, every 2 turns of your arm is one turn on the twister. You can apply
twice the pressure.
The torque measurement is coming from your arm though.
The 2 turns is great, but you want more, so you upgraded it to 4 turns per, to double the pressure on the towel again, but the little wooden dowels that hold it in place snapped.
So you get metal dowels.
Upgrade again to 8 turns per, and something else breaks. And you replace it for something a little more durable.
You keep this up, and before you know it, your towel wringer is top notch. durable, plus really bulky.
^That’s the truck that can tow.
Another person says, what if Make a device that spins the towel really fast? One arm turn is 5 spins of the towel.
Still applying the same torque with your arm.
This design puts more wear on Bearings and anything that is slightly off balance.
You upgrade to smaller, more precise parts.
Spin faster. Upgrade, faster, upgrade.
^This is like the fast car.
Remember, you’re the engine here, same torque for both, only the design is different.
Both can produce the same horsepower, and can have the same torque, but you can get a different engine that is better at doing that task.
As well as computers making it so we can customize the timing of how much power you have at what speed.
A car’s engine would be designed to have the torque be low in the beginning, then highest when it’s going fast so it can go even faster.
A trucks engine would be designed to have high torque at the start so it can start moving a large load from a stop.
I’m going to give this one a try… perhaps you are familiar with a torque wrench, or at least of trying to loosen a really tight nut or bolt. If you have a regular old wrench, or a torque wrench, and you still can’t loosen the bolt, you would try to contrive a longer handle. Mechanics do this by putting a pipe or cheater bar over the wrench itself, which makes the handle longer. This will increase the torque, allowing you to loosen the bolt.
You could, instead, find someone stronger (literally more powerful). They might be able to loosen the bolt without using the cheater bar.
There are a lot of considerations when trying to go fast or tow a heavier weight. Generally speaking, in each case you can do better if you have more horsepower, period. When you don’t have unlimited power you can, with gears in a transmission for instance, get more starting torque OR more eventual speed, but not both.
Hot-rodders will literally change the gears in their car, usually in the differential, to be quicker off the line, which means they will be more likely to win a short race. That modified car will have a reduced top speed in consequence, unless they make other, dramatic, modifications.
Horsepower and power are the same thing, by the way, Power is measured in Watts, and 746 Watts equals one Horsepower. Also, for those who don’t know, a torque wrench is a wrench with an included scale… you can measure exactly how tight a bolt or nut is. This is a big deal when working on engines and boilers, that sort of thing.
This guy has a good video on the subject
And lots of other scientifically explained car terminology if you’re interested
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