# Please explain Horsepower and Torque [duplicate]

I have been trying to understand both of these for a while. While I kind of understand them for some reason I feel like I'm missing the point. Simple explanations leave me confused and longer explanation leave me feeling dumb, as I don't study physic or calculus.

So can you give a decently vast but at the same time simple explanation? My understand standing is horsepower ( which is 33,000 ft-lb ) is the amount of work your car can do in a minute so if my car have a max of 690 horsepower, that's how much force my car can lift and keep moving, and torque is the same but with how much force is needed to get all the compounds moving inside the engine so if its 400 torque then that's how much force is needed to get the vehicle moving and after it moves the horsepower pick up.

I don't know if I explained it right or not but either way I'm still not fully understand, I have done research for a few day but still not understand.I know they may be simple and I should get it but I don't know if my idea is right or not

• Power = Torque * RPM, so low torque at high RPMs can give the same power output as high torque at low RPMs. Motorcycles, for instance, usually have less torque (smaller engine) than cars but compensate with high max. revs. (e.g. 10000rpm) to achieve the same power level. – JimmyB May 9 '16 at 11:47
• Contrary to the the accepted answer, mechanics.stackexchange.com/a/25595/6794 explains it correctly. – JimmyB May 9 '16 at 12:01

What you are missing is the concept of rotational speed. This is required to fully understand the relationship between horsepower and torque.

We'll start with the concept of torque. Torque is simply defined as the rotational force of an object on an axis. This is easily calculated as T = R x F, where:

T = Torque

R = Position Vector (Essentially the distance from origin point to where force is being applied)

F = Force Vector

Angular velocity is simply the rotational speed of of an object on its axis and is measured by rotational distance over time, such as radians or degrees per second. RPM (Revolutions per minute) is a unit of measurement of how many full revolutions (360 degrees) an object makes over time.

Horsepower is an imperial unit of measurement regarding power. It is described as 550 ft-lbs per second, or 33,000 per minute to equal 1 horsepower.

Now, let's talk about horsepower and it's relationship to torque and angular velocity(in the form of revolutions per minute), relative to automotives. Given that power is constant, torque and revolutions per minute will always be inversely proportional. You either have high torque and low RPMs, or low torque and high RPMs.

This can be defined as P = (T * RPM) / 5252, where:

P = Power

T = Torque

RPM = Revolutions per minute

Let's test this out with a few examples. Say your car outputs 100 horsepower at 4000 rpm. Let's to a little bit of basic algebra.

100 = (T * 4000) / 5252

100 * 5252 = T * 4000

(100 * 5252) / 4000 = T

Torque = 131.3ft-lbs

Now, let's say that the vehicle is creating 100 horsepower at 200 rpm.

100 = (T * 200) / 5252

100 * 5252 - T * 200

(100 * 5252) / 200 = T

Torque = 2,626ft-lbs

Notice how between both equations, when the RPM figure is low, the torque figure is quite large, while when the RPM figure is high, the torque is quite low, even though both instances, 100 horsepower is produced.

The reason we divide by 5252 in this equation is because at 5252 RPM, 1ft-lb of torque is equal to 1 HP. In a more technical term, it is the rounded value of 33,000ft-lb/min divided by 2π rad/rev.

With all this in mind, let's talk about the real world, where power is not a constant value in a vehicle.

At a standstill, your vehicle requires relatively massive amounts of torque to get it moving. This is due to your RPM being at idle-speed (roughly 600-1000, depending on the car) and requiring more turning force to begin moving the wheels. Remember, low RPM means proportionately high torque. The more torque your vehicle can produce, the faster you will start accelerating. if you look over the previous equations, you will understand why this is the case.

As vehicle speed picks up and so does rotational speed, torque gains become proportionately low. This does not mean that as you go faster, your torque figure drops or that once you produce more torque, you move slower. Both these figures will rise higher and higher until max output is reached. It just means that when looking at torque, RPMs and horsepower, more effort is required to move from standstill than to maintain a high vehicle speed.

• Thank you for explaining. The number and science kind of throw me off but I guess its a must to fully understand. – user271165 May 9 '16 at 9:57
• I would upvote you but it says my rep is to low – user271165 May 9 '16 at 9:57
• Nice answer +1 even though it's closed as a dupe. – DucatiKiller May 9 '16 at 20:19