Engine Power vs Torque? Which is Better?
What is the Difference Between Power and Torque?
This is a question that seems to come up a lot, especially amongst car "experts" in pubs across England. I have found myself trying to explain twice in the last week, so here goes...
When I was a child all we cared about was a car's power, number of cylinders and size of the engine, the top-speed and how quickly it could get to 60 mph, but now with the advent of good, smooth diesel engines with huge amounts of torque and more modest amounts of power the pub-debate has become more heated.
The new Jaguar XF 3.0 litre diesel astonishingly has twice as much torque as my Ferrari 308 GTS QV almost 50% more than my 6.0 litre V12 Jaguar XJ12 and even 10% more than my supercharged 4.0 litre V8 Jaguar XKR although not as much as the last of the outrageous 6.75 litre Bentley Turbo Rs and the Bentley Arnage although all but the Ferrari have more power. So what is torque and is it important?
Power vs Torque
What is Power? - What is Brake Horsepower
Top Gear, the very popular British motoring show is obsessed with POWER but usually glosses over the torque figure for each of the cars they test, with Jeremy Clarkson pretending that he doesn't even know what it means. One explanation I have heard repeatedly is that torque gives you top-speed and power gives you acceleration. This is almost the opposite of the truth. The power of the engine is actually the maximum rate of conversion of petrol (or diesel) into energy. If that energy is converted to forward motion it is equal to the force on the car multiplied by the distance moved (the definition of work-done on an object which is the same as the energy gained or lost by the object) so if power is the rate of energy being transfered then that is force times distance per second or force X speed, so when the the maximum rate of energy released by burning the petrol is equal to force created by air resistance and rolling resistance etc. the car will stop accelerating.
Energy = Force X Distance
Power = Energy / Time
Power = Force X Distance / Time = Force X Speed
So a car's top speed is the point at which the power of the engine is just enough to overcome the power dissipated by the air and other forces on the car.
Top speed is determined by power and how slippery the car is (assuming the gear ratios are such that the top-speed corresponds to the peak of engine power)
At high speeds the air resistance gets far greater with every mph increase in speed, but at low speeds a powerful car will easily accelerate through the air with plenty of power to spare, so the rate at which the car gains kinetic energy (i.e. energy associated with speed) is at least partially dependent on the engine's maximum power.
The power of an engine is measured against a brake (actually it is also often measured with a rolling road these days, measuring the power at the wheels and extrapolating the power at the engine, but the principle is the same) If you imagine an engine turning a pulley with a rope wrapped round it and the tension (i.e. force) measured at each end of the rope, the difference in the forces is the force applied by friction to the pulley. using the equation above Power = force x distance per second the power can be calculated from the speed at which the engine is rotating and the size of the pulley: Hence Brake-Horsepower or BHP
So, what is torque?
So What is Torque Then?
What is Torque?
Torque should actually be easier to understand than power. Power actually has strange units (a 400 horsepower car will not be able to beat 400 horses in a tug-of-war, although would probably go rather faster) torque however has the easiest of units: pound-feet (or Newton-metres) One pound of weight on the end of a one foot spanner, wrench or crank gives one pound-foot (lb-ft) of torque of turning force. A 100 lb-ft engine could be replaced by either a man applying a force equal to one pound of weight to the end of a 100' wrench attached to the wheel or 100 men with one-foot wrenches. If you double the length of the wrench (or number of men) you double the turning effect, or torque. Both examples have equal torque, but one has 100 times more power (i.e. the man on his own would soon run out of power and need a rest) so both cars may get away from a standstill equally quickly the single man will quickly be unable to maintain the increase in speed of the car due to lack of power.
Unfortunately this is where things become more complicated, because the torque at the engine is not particularly relevant; it is the torque at the wheels that matter and that is determined, by the gearbox ratios and the final drive ratio. Off-road vehicles often have very low gearing and therefore huge amounts of torque at the wheels no matter what engine they have, but you may need to change gear as soon as you go more than a few miles per hour, whereas high performance sports cars may have relatively low torque because for them the important thing is power available through a wide range of engine speeds up to very high number of revs per minute.
Torque vs Power
Conclusion: Torque vs Power
The Pub Debate...
The pub debate will continue, but if you want to win a race on a track, lots of power and lots of revs is more important, whereas if you want to go off road or tow heavy trailers torque is probably more important.
Which is Better Torque or Power?
Which is Better Torque or Power?