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Old 09-11-2004, 22:13   #1 (permalink)
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POWER vs TORQUE

hey guys,

ive had this discussion with my mates many times and they always tell me to come on here and ask u guys "they know everything else so ask them this"

what is the specific and actual difference between power and torque, what is their definition and what gives rise to each one?

cheers,
alex
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Old 09-11-2004, 23:22   #2 (permalink)
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Re: POWER vs TORQUE

Torque is measuring twisting force. You have a torque wrench you set it to 10nm, it will apply 10nm of torque. 1 nm, is one newton of force at 1 metre of leverage.

Power is a little more complicated. Power is kind of a description of its ability to do work.

In the early days, power was use to describe the ability of say a horse to carry a bag of coal up a shaft. Its basically torque measurement over time. Watt came up with this "horse power" measurement so he could sell his machines. It emphasised that while horses and humans could lift a lot of things, on average, they were pretty slow, machines would perform this work extremely steadily with no rest, and thus win out.

I can have a little 2 stroke engine that makes about 5 Hp, but lets say, 50nm of torque. 50Nm isn't a great deal of torque, I can make about that much torque with a crankshaft style screw driver with my bare hands. But a engine can do it about 10,000 rpm, which I certainly cannot do. You can gear that little 2 stroke down so at the same RPM I make say my 75nm, it would be making thousands of Nm of torque. I will get pretty tired after 15 minutes, but the engine will keep going strong for hours or days given the fuel. Power takes that into concideration, the speed it makes its torque.

James watt actually overstated his estimation of the ability of a unit of "horsepower" so that there would not be any complaints that these new steam machines were not able to live up to the rated power. Its one of the reasons why the industrial revolution happend so quickly.

Also a car with 1 HP that weighs the same a large horse does not mean that it will be as quick as horse over 200 metres. Thats not where that measurement of power came from.

You can calculate the power of a engine if you know the torque and RPM of that torque, but your calculation is the power produced at that RPM.

So power and torque are heavily related. Torque is the turning force of the engine. Power is basically the torque at a certain RPM point.

A great number of people get confused by what is power and what is torque and how they are related.
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Old 09-12-2004, 00:02   #3 (permalink)
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Re: POWER vs TORQUE

Man this is technical....for me on the street rwkw comes in at terminal speed only and torque is what accelerates you there...I prefer torque any day..fat torque...
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Old 09-12-2004, 00:03   #4 (permalink)
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Re: POWER vs TORQUE

cheers mate, thats been heaps helpful, so if they are greatly related, why can some engines have reasonably high power figures, but very little torque?

i knew that torque was the twisting force, but how is this actually translated into engine performance? is it the force at which it turns the crankshaft or something along those lines?


cheers,

alex
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Old 09-12-2004, 00:10   #5 (permalink)
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Re: POWER vs TORQUE

Quote:
Originally Posted by XMU2drescue
cheers mate, thats been heaps helpful, so if they are greatly related, why can some engines have reasonably high power figures, but very little torque?

i knew that torque was the twisting force, but how is this actually translated into engine performance? is it the force at which it turns the crankshaft or something along those lines?


cheers,

alex
Man i am a simple man....imagine a car with 350 rwkw and 500 nm of torque at high rpm...at low rpm it has 100nm..ok
Then you buy a 200 b datsun with 111 rwkw and theoretically 500n m of torque at 1000 rpm...the datsun will out accelerate the other car.Apart from having big torque..it has to get to the ground as quick as low as possible in the rpm range..500 nm at 1000rpm will give you scary acceleration and 500nm at 5000 rpm only would be like a dead dog.
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Old 09-12-2004, 00:42   #6 (permalink)
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Re: POWER vs TORQUE

Torque and Horsepower cannot be directly compared because they represent very different things, although they are intimitely related.

As IPhido said, in a automotive application, torque is the twisting force or put a better way, the amount of 'work' something is capable of. Power is how much work can be completed in a given period of time.

i.e. Consider this scenario; two men lift a 100kg mass 1 metre off the ground (this is the work). The first man takes 10 seconds to lift it, the second man takes 20 seconds (rate of work).

Because the first man was able to perform the same work in half the time, he has twice the power of the second (assuming all other factors are equal).

So you could say, Hp (power) = NM (work) x RPM (time). Hopefully that is not oversimplifying it..?
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Old 09-12-2004, 01:23   #7 (permalink)
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Re: POWER vs TORQUE

to calculate horse power the equation is HORSEPOWER = (TORQUE(ft,lbs) x RPM)/5252
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Old 09-12-2004, 01:36   #8 (permalink)
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Re: POWER vs TORQUE

Quote:
Originally Posted by Pieoter
to calculate horse power the equation is HORSEPOWER = (TORQUE(ft,lbs) x RPM)/5252
cheers, I knew someone would jump in with the exact formula
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Old 09-12-2004, 02:13   #9 (permalink)
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Re: POWER vs TORQUE

http://www.webster-dictionary.org/definition/horsepower <-- Horsepower

http://www.webster-dictionary.org/definition/torque <-- Torque
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Old 09-12-2004, 04:06   #10 (permalink)
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Re: POWER vs TORQUE

http://auto.howstuffworks.com/horsepower1.htm
http://science.howstuffworks.com/fpte.htm

hope these links help, they are very well writen so that almost anybody can understand them
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