I can't find a good physics explanation of what the difference is.

From what I have gathered torque is the amount of force being applied to the wheels. If that is the case what's horsepower measuring?

 

torque is the measure of force generated by an engine. power is the amount of force generated over a certain amount of time... in this instance rpms. For all intents and purposes, torque spins tires, hp pulls top end.

 

iirc, and someone correct me if I'm wrong, but one I heard in physics class: Torque is how large a trailer you can pull, HP is how fast you can pull it.

 

Torque is a rotational force. How much weight you can bench press, for example.

Horsepower or power, is how much energy is expelled per unit time. Multiply power x time and you have energy. You can get more power by either a) increasing the torque or b) increasing the speed (of a given torque).

All of the Torque vs Power debates are for knuckelheads. Power, since it is expression of energy/time, makes a car accelerate.

 

In an internal combustion engine...

Torque is the amount of force a pistion applies to the crankshaft. Horsepower is a calculation of the amount of force (torque) at an rpm to determine how much "work" can be done.

IIRC, 1 HP = the amount of power required to lift 1lb, 1 foot, in 1 minute. When engines (steam and gas) started replacing horses as the energy for farm machines, they needed a way to relate the work these "new" engines could do in comparison to the horses they replaced. It's simply a measure of force over time and distance.

HP = torque x rpm / 5252.

5252 is a constant. That is why ALL dyno curves cross at 5252 rpm, from chainsaws to top fuel dragsters.

 

The great HP vs Torque debate is better stated:

Higher RPM Lower Torque vs Lower RPM Higher Torque.

Constant HP
6575 rpm X 400 tq = 500 HP
5252 rpm X 500 tq = 500 HP
4375 rpm X 600 tq = 500 HP

Constant RPM
5252 rpm X 400 tq = 400 HP
5252 rpm X 500 tq = 500 HP
5252 rpm X 600 tq = 600 HP

Constant Torque
6575 rpm X 500 tq = 625 HP
5252 rpm X 500 tq = 500 HP
4375 rpm X 500 tq = 416 HP

The debate is which is quicker in the 1/4 mile acceleration.

People talk as if HP/TQ are a this or that type of thing. You can't have one without the other.

A better way to look at the difference is, Torque is like the weight of a hammer head and HP is that weight moving at speed.

You can have a lite head (lo tq) long handle (hi rpm) hammer (Hi HP) vs a heavy head (hi tq) short handle (lo rpm) hammer (Hi TQ). Then swing at different speeds.

Which is better/faster at driving in a nail?

Each person will have their preference.

The quickest is a one hit, driven in nail. Think nail gun.

 

The easiest way to think of torque and horsepower is:

Torque is the force

Horsepower is the work done

An example is such: find a large heavy brick or stone wall. Put both hands on the wall and push. Didn't move? Push on it harder. Lots of force, lots of torque.

But since the wall didn't move there is no horsepower.

Once the wall starts moving then there is horsepower. The higher the torque, or the faster the wall moves, the greater the horsepower.

RBob.

 

Ok so horsepower is the amount of work an engine is capbable of exerting.

How is it helping a weak motor to spin at really high rpms to get as much work done as a larger motor? Is the inertia of the rotating mass working with the current availble torque to give it more force?

Figuring this out has truely help me understand there is no replacement for displacement. Now how to get that displacement spining at 9k is the question.

 

indeed, which is why most people just talk about horsepower, because most people are just concerned with the amount of work done by the engine (and the work done by the wheels), and where in the rpm range that work occurs the most.

though, if we want to get pedantic, hp is a unit of power, not work. So it's a measurement of work done per second. Directly related to watts, rather than joules. Though, unlike what Rbob said, there can be considered HP even before the wall moves. heat output is still an expenditure of energy per second, and is thus definable as HP. You can expend huge amounts of heat trying to move the wall, and that can all be considered HP used, but quite hopelessly. It all depends on where you meausure, that's why we measure at the wheels, or in the walls case, we'd measure at the wall, becuase like a person, the engine will produce a lot of waste heat that does not relate to moving the wheels.

 

that quote, I believe, has more to do with the cost effectiveness of producing large amounts of hp compared to making smaller engines produce equivilant amounts of hp. Want 300HP? it's much simpler (and probably cheaper) to just go get a bigger engine, than to try and force feed a 2.8l engine enough air and fuel to do it.

stroking the engine out extends the range the engine makes usable power. It's not a matter of inertia, inertia cant produce an increase in acceleration (accel of accel). If anything, rotating mass is one limiting factor of high rpms. there's other factors that move someone to stroke an engine rather than bore it out to increase displacement, like piston head sizes and such. It's give and take.

 

Actually if you're pushing on the wall and the wall is not moving than no work is being done. The physics definition of work is force multiplied by distance which, ironically can be expressed in units of lb-ft (same units as torque, even though it represents energy in this case). You can push on the wall, but all you're doing is burning chemical energy in your body as it's converting it to mechanical energy and wasting it as heat.

As far as horsepower, you can look at this way. Let's say you have a car running at it's absolute top speed at WOT.

From a thermodynamics perspective there's an energy balance that must be maintained per the First Law of Thermodynamics (law of conservation of energy). Since hp is a measure of energy per unit time, you can look at a particular period of time (say 1 hour) do do the energy balance. Which means you can take the hp of the vehicle over the 1 hour and get the energy produced in that 1 hour time period and then compare it to the overall energy of the system. If we look at the total amount of energy in this particular system over the 1 hour, we have the following (starting at the output of the crankshaft):

1. The kinetic energy of the vehicle (1/2*mass*velocity^2) related to the forward velocity.

2.) The energy being dissipated as heat between the tires and the road as well as the energy lost due to wind resistance.

3.) The energy being dissipated as heat in the drive train. There are drivetrain losses where mechanical energy from the engine is being converted to heat in the transmission, differential, etc.

If you add all of this energy up, it should equal the energy output at the crankshaft. Or take that total energy, divide it by the 1 hour and you should get the amount of horsepower at the crankshaft.

 

Or back it on up to the combustion chamber, then you can figure in the power consumed rotating the engine assembly!

Nice discussion.

 

Yeah, but I wanted to take combustion efficiency out of the equation. Then you have to start looking at energy thrown away into the cooling water and mechanical losses in the engine, etc. You have to be careful on defining system boundaries otherwise the numbers won't add up properly.

 

So how does more rpm make more horsepower? 150lbs of torque at 1000rpm doesn't make as much horsepower as 150lbs or torque at 8500rpm, why?

 

The amount of work done over time has increased.

I should quote the definition of 1 HP: "the force required to raise 33,000 pounds at the rate of one foot per minute" (Websters New World Dictionary, 2nd edition, 1984).

If the rate is increased to 2 feet per minute, then the horsepower is doubled.

By increasing the RPM, the rate at which the work is done has also increased.

RBob.

 

Typical GM 350 engine is only about 25% efficient, when comparing total fuel consumed vs total actual work performed.

The thermodynamic/frictional losses are great.

 

hp, the term, is work over time, aka power. Work, however, is equivilant to energy. So while indeed, no work is done over time, there is still energy being used over time. So the term hp can be applied to both useful work and wasted work, aka heat. It's much easier to not get hung up on kinetic and radiant outputs by simply using the SI units.

hp = 746 watts. So a hp is the same unit as a watt. You dont need to move any where to output watts, that seems fairly obvious. Watts are joules / sec. Ok so a joule is work, which is energy expended. Here's where everyone gets hung up. A joule is both equal to a Newton Meter and a Watt Second. Meaning, as far as the units are concerned, there is no difference between energy expended moving something, and energy expended producing radiant energy. And when you're talking about engine output, you need both.


you just repeated what i said originally. That the energy expended is part of the equation when you talk about hp, regardless if it is energy related to movement or the attempt to move. So long as it's energy expended trying to move, it's relevant to us.

Take your car, now use the hand of god to stop it. Wheels aren't moving, crank is not moving. Yet, until it explodes, that engine is still putting out hp trying to move. There's 0 hp at the crank, and at the wheels, but there's plenty hp and torque acting On the crank. It's just not enough to move it. In a person, our engine is much harder to visualize, there's no rotating pieces. It's all electric, the hand is your "crank". no power at the crank as far as the wall is concerned, but lots of power acting on the crank. Even if the net force is 0, if you are exerting a force, you need energy to do that, and energy expended is work.


as for the torque at 1000 and the same at 8500... Like Rbob said, if we assume it's the same car and the engine has no load then the hp would have to increase because the torque is moving the crank 8.5 times the distance in the same time. hp at 8500rpm should be roughly 8.5 times that at 1000rpm, assuming the torque is the same. You get weird curves when you start adding transmissions and final gears into the mix.

 

If you take a "stock" 350 motor and raise the 350 ft# peak torque range by doubling its rpm 3000 to 6000, the "HP" will double, 200 to 400.

I you use a bigger displacement motor, same "HP" 400 but 700 Ft # at 3000 rpm

 

OK, I read ALL of this and now I have a headache. I didn't know about the 5252 thing. I have always built my engine with torque in mind over hoursepower because I feel that without torque there is no horsepower.

Anyway, I'm off to take some Advil now.

 

this is too complicated & now my head hurts too.
i saw this on another site & found it funny.
the last 2 apply here,

Understeer: when you hit the wall with the front of the car.
Oversteer: when you hit the wall with the back of the car.
Horsepower: how fast you hit the wall.
Torque: how far you take the wall with you.

 

best answer yet, I love it!!!!!!!!!!!!!!!!!

 

When you build an engine, HP and Torque are limited by the physical limits of the engine. The bore/stroke, heads/cam, intake/exhaust, and the strength/cost of the parts. All of these place a restriction on the build.

After the most is obtained from the engine then this "fuel/air to Torque/HP" converter has to be attached to the "TQ/HP to Motion converter" the car.

The car coverts engine torque to "car usable torque" by using the transmission/rearend gears.

The Torque restrictions on the car are wheel spin and broken parts. More torque then can be converted is not used. At wheel spin or the parts breaking point, torque drops to very little. It would be like your feet sliding on the floor while pushing on the wall or the wall braking not moving.

To lift the front wheels (of my car) it takes 14400 ft# of torque (1800# X 8 ft). If the motor makes 500 ft#, then to lift the front wheels, I need 29:1 gears. 2:1 in the TC, 3.06 first gear, and a 4.75:1 rear gear. 2 X 3.06 X 4.75 = 29. This is with a 24" tire (for easy math), that won't spin.

This is my cars torque limit, anything more and it will "wheelie" and try to flip on its back.

The type of motion desired is 3 dimensional, velocity (direction on 2 planes, a flat surface) and time (Acceleration and Deceleration).

Some peoples wish is to narrow this velocity to just one direction (forward) and to maximize it, as to distance and time (acceleration). This is done two ways, on a distance fixed track with electronic timing devices (time and mph measured), or on public roads using traffic control devices to mark the start (winner is ahead of loser).

 

I needs more than Advill now. I understand tire spin and breaking point. with my old engine, it would spin like crazy untill I got my hands on some M/T et streets and broke the rearend.

Anyway, I think all this is more info than I need because I do not plan on power testing my engine against a wall and we all know our cars are not the best for picking the front tires up off the ground, at least not with my low power engine.

 

If you put the "right gears" in your car, even a "low power" engine can lift the front wheels. It may only be good for "popping wheelies", but it can be done. Your launch would be like a frog leap. Not sure about the 60 ft time.

A Pinto or Chevette may beat you in a 1/4 mile race, it just depends on what you want to spend your money on.

 

Also a lot of people will describe a car as being "torquey" or "lots of horsepower". They use these incorrectly, sort of. One just makes it peak torque longer or lower in the powerband(think diesel), while the other makes it higher in the power band(think vtec honda). That should clear it up a little when you hear tv personalities and other people describing it.