Power and torque - AlanGowdy
I drive a turbo diesel and really appreciate the 'ooomph' that its high torque level provides but.... could someone explain the difference between power and torque in layman's terms? They each seem to be important but are obviously not the same thing.
Power and torque - Epic 80
torque is the measure of \"twisting force\" and power is the rate at which this happens. Power=torque x speed. Power is what gives u performance. Therefore, to increase the power at any given time, u could increase torque, or increase the speed, or both.

I\'ll illustrate with some examples:

A formula one car develops low torque (around 175Nm or 129lb-ft), which is less than the peak torque of a lot of modern turbo diesels, but generates its vast power by the engine spinning at 17000 rpm.

A large truck generates maybe 2-3000 Nm (1500-2500lb-ft), but does it at low rpm (500-1500), to get similar power.

The key is gearing of your car. your turbodiesel produces power much earlier than an equivalent petrol, because its peak torque is at a lower engine speed, so in everyday driving, you get power at low revs. The power is more accessible in everyday driving. The petrols power will catch up at higher revs so is less noticeable unless you rev say above 4000.

I did a calculation once comparing a Golf TDI 90bhp diesel and a Golf GTI 115bhp petrol, at 2000 rpm, the diesel produced 20 bhp more than the petrol, so would accelerate faster from 2000rpm. By 4500 rpm, both engines were nearly identical. The TDI runs out of puff at 4700rpm, but the petrol goes on to produce more power at 5000rpm . So if you drove the petrol, you would accelerate faster than the diesel by keeping your revs above 4000.

There is much wrong information on this subject in circulation!

Power and torque - Ben {P}
I agree with Epic 80. Horsepower accelerates cars, not torque. Take a peak at the following web page for a more scientific look at the subject: www.pumaracing.co.uk/power1.htm

Its the area under the curve in what i call the power band that shows you how much useable power a car has. If you want to go as fast as you can, one changes up a gear where the power output of the engine has fallen to the same level as the power output would be in the next highest gear. This rpm range is the power band. The area under the curve on the graph represents the total useable max power. Diesel now can be realy quite quick, but we must remember high peak power figures, or torque figures, do not necessarily represent lots of useable power. That goes for petrol and diesel engines.

Power and torque - Tony N
A formula one car develops low torque (around 175Nm or 129lb-ft),
which is less than the peak torque of a lot of
modern turbo diesels, but generates its vast power by the engine
spinning at 17000 rpm.

I think you'll find an F1 engine makes a bit more than 129lb-ft!!!!!!
Power and torque - Ben {P}
I expect they make more than 129lb/ft. Maybe that is just the torque figure at max power. Does anyone on here have any inside knowledge about how much power the f1 engines of today actually make on a good day. I have a friend who worked somewhere which worked on f1 engines, and he has no idea. I think its a well kept secret.
Power and torque - Altea Ego
Exclent descriptions above. This is what i use to describe torque. Put your diesel car in first gear at the bottom of the hill, let ou the clutch and it will climb very steep hills, at tickover in first. This is torque, its the amount of twisting force that can keep a car moving up the hill, can be measured by the force required to stop it turning.

I cant think of a similar way to describe power.
Power and torque - Mike200
The new Mercedes-Benz engine that will be put in the McLaren MP4-18 when it is released is reputed to make 920BHP @ 19700 RPM. Working this out, Torque = 5252xBHP/RPM, torque should be about 245 lb/ft
Power and torque - Mike200
Just found a fairly good website on this subject, click here www.carkeys.co.uk/features/FE000373.htm
Power and torque - eMBe {P}
AlanGowdy, in simple terms, Think of it this way (if you are familiar with the old fashioned water wells:

Turn the handle to lift the water. The turning force you apply is the torque.

Power: This is where the concept of revolutions and time comes in. If you raise the water quickly by turning the handle very fast (i.e. fast revs), you are inputting more power. If you do it at a leisurely pace (i.e. low revs), you are using less power.

Energy: This factors in the total time you take in using the power you are applying.
Power and torque - Epic 80
Energy does NOT take time into account.

Power=Energy / Time

1Nm of torque = 1 Joule of Energy : 1 joule is required to lift 1 N by 1 m.

1 Watt = 1 N moved 1 m in 1 second.

So time only comes into play when talking about power.

e.g. A chocolate bar has energy stored in it, but this is not related to time. If you ate it, the rate at which you used the energy would determine the rate at which the energy you gained was used up; The power. YOu could still move only the same amount the same distance, but you need more power to do it more quickly.

A good analogy for power is as follows:

An adult may lift a 30kg sack of potatos from the floor to a table. A child may not be able to lift the sack, but lifting the potatos out of the sack and putting them on the table individually he/she could do it. The same energy would be used, but the adult would be more powerful because they could do it in one move (say 5 seconds), not several (say 2 minutes)

Hope this helps
Power and torque - John S

I'd agree with most of it, since power = work done in a period of time. I'm confused about the description of torque though.

Torque cannot equal joules - a joule is an amount of work, and work is done only when a body is moved against a resistence. Now torque is purely a measure of turning effort (think of its units, lb ft or Nm, and there is no time element in there) so it does not measure work. Watts measure work - a watt being a joule/second) The classic example here is a steam engine, which develops torque when steam is admitted but before it starts moving anything.

I'm always bemused when torque and power for engines are quoted as though they were separate items. The two can't be separated.

I'm still imperial on this and the relationship is:

HP = 2 x pi x N x T/ 33,000

Where N = engine speeed in revs/min and T = Torque in lb ft

So an engine described as 'torquey' is one which, yes, develops plenty of torque at lower revolutions, and so has a flatter power curve. However, it's power that moves vehicles, not torque.


John S
Power and torque - RichardW
Torque cannot equal joules - a joule is an amount of
work, and work is done only when a body is moved
against a resistence.

It does. Work done (energy) = Force (N) * distance (m)
Which is the same as torque measured in Nm

>>Now torque is purely a measure of
turning effort (think of its units, lb ft or Nm, and
there is no time element in there) so it does not
measure work.

Not so. Force (N) = mass (kg) * acceleration (m/s²) Hence Nm = kgm/s²

>>Watts measure work - a watt being a

No, watts measure power - the RATE at which work is done.

Not sure where this leaves us really. Power makes cars go along (you must do a given amount of work in a given time to overcome the air and frictional resitance trying to slow you down). However, if we consider the work being done at any given time (kgm/s²), and then differentiate that with respect time (eg the rate of of change of work done) we get (my calculus is a bit rusty but I think this is right) a function of the order kgm/s³, which is power. I THINK this means that the more torque available at a given time the greater the rate of change of work can be, eg acceleration. This is why diesels feel to pull better because without the revving the nuts off the engine they can do a greater rate of change of power. So maybe the key is not the absolute value of the torque, but the engine speed at which is occurs

As you pointed out John, the two are vitrutally indistinguishable, but people like to quote numbers.


Power and torque - edisdead {P}
Here's a very good analysis of the relationship between power and torque, from first principles:


I think the marketing boys are slowing coming round to realising that there's more to it than just quoting inflated bhp figures at us.
Power and torque - John S

Take your point, but I still don’t agree that torque = work. I realise that torque = force x distance. Yes, there’s a distance term in there, but it’s not the distance over which the force moves, it’s there because torque is a twisting force and can only be described by a term which includes a force and a distance from the point about which it’s acting. The fact remains that torque, where no movement occurs, does not equal work done, even though the units are Nm or ft lb. When it's measured on an engine test, the measurement is one of static force on the crankshaft dyno, coupled with a distance from the centre of the dyno - no movement is involved. Consider the steam engine, when steam is first admitted to the cylinder. The pressure acts on the piston, the piston attempts to turn the crankshaft. Until the steam pressure is sufficient to move the crankshaft against the load then no work is being done – the engine is stationary. Nothing’s moved so no work has been done. The Newton is a derived unit but then so are the kg and pound which both include ‘g’. What I’m saying is that the horse power is 550 ft lb per second, the crucial bit being ‘per second’. Yep, watts measure power, or the rate at which work’s being done – badly explained on my part. But, again that crucial element is there the RATE at which works being done.

Whatever, you’re right about the different ‘feel’ of engines. Diesels have flat torque curves and develop maximum torque much lower in their rev range than the average petrol engine. They therefore have a power curve which is fairly linear, and because of the low engine speed for peak torque, then they are developing more power at those lower engine speeds. Acceleration is related to excess power available over that required for constant speed, so they accelerate the car well at lower engine speeds. Alternatively, when you get a high revving Honda unit, for example, the maximum torque occurs at very high engine speeds. The engine has a rising torque curve, and so a peaky power curve – result, the engine doesn’t pull that well at low engine speeds, but the power output rises at a greater rate than the engine speed, so it appears to really fly when the revs are up. The figures neeed to be read with care.


John S

Power and torque - Epic 80
In your example above JohnS, the dyno has to apply a force at a distance to counter the torque of the engine, that is what is keeping the rpm of the engine constant at the point you are measuring. If you blip the throttle of your car in neutral, you can increase the revs to the limiter with hardly any throttle, because no torque is being applied to the shaft. you only see the twisting force when you oppose it with a twisting force in the opposite direction. This is when work is being done and it is when the shaft experiences a torque. If the dyno was not doing work resisting the torque of the engine, the revs would rise as in the neutral example. This really is basic physics.
Power and torque - Epic 80
The dyno is where the concept of "Brake horsepower" comes from. The dyno has to brake the engine, to apply the reaction force to the crankshaft of the engine in order to measure the torque, which is then used with the speed to calculate the brake horse power.

Power and torque - John S
Epic 80

I realise it's basic physics, and basic physis (or is it mechanics?) says work is done when a body is MOVED against a resistance. If it doesn't move no work is done. The blipping the throttle analogy is a red herring. It's quite clear that revving the engine with no load connected produces little power. What the dyno does is resist the motion of the crankshaft so giving it some resistance to move against. The torque measured isn't work, it's a load. The work is done in the dyno, pumping the fluid. You can then calculate the power output from the torque and the rotational speed. You can't calculate it from the torque alone.


John S
Power and torque - John S
Epic 80

Just read your second post, which says exactly what I've just said. Yes, the torque is necessary, but it isn't work. It's an essential component of calculating the work done though. That's why I've said you can't separate torque and work, as some try to.


John S
Power and torque - eMBe {P}
Epic80 said:
Energy does NOT take time into account.

Power=Energy / Time <<

Errrr! Contradictory statements! Which of your statements is NOT true?

A heater rated at 1kW has a POWER rating of 1kW.

If you use it for 1 hour, the ENERGY used is 1kWh.

Going back to my example of lifting water in a well.

The torque is the force you apply multiplied by radius of the handle.

Assuming you are turning the handle at a comfortable constant 30 revs/min, the Power is easily calculated using the equations already quoted.

The energy used will deppend on how long you go on drawing the water from the well. You may spend 10 minutes to fill a couple of buckets, or may carry on for 1 hour or more filling buckets and buckets if you have the energy to carry on.

Power and torque - AlanGowdy
Aha - thanks all. So I chose appropriately when I bought a turbodiesel, for I have too much mechanical sympathy to happily rev the knickers off a car. At 4000 RPM or so I reach for the next gear.

Value my car