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      08-22-2010, 06:41 PM   #81
Dr Stig 2
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2010 BMW Z4 35is  [10.00]
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Torque Vs Horsepower

A Physics lesson for the debate:

If you want to accelerate, you need a force.
Force = mass x acceleration.

Torque is a measure of force, ie the force that can turn the motor or wheels.
(There is a different measure of the torque at the wheels than at the flywheel, due to the gearing of the gearbox and differential)

There is a reason that Torque doesn't vary anywhere as much across the rev range, and that is because it is a measure of the true engine ability to turn energy (petrol/fuel/gas) into a force.
For heavier cars you need more force, ie more torque.
Thats why big torque motors like modern direct injection diesels make great cars for towing. An M3 would be useless for towing a boat, you would have to rev the sh*t out of it to keep it moving. An M3 has less torque in comaprison and it is generate much higher in the rev range. It is a motor that relies on operating in a high rpm band)

Horsepower is almost always quoted as a peak figure. This is meaningless unless you drive constantly at that particular rpm.
Horsepower comes from the energy created each cycle (a single revolution of the engine has 3 cycles in a 6 cylinder engine).
Cars that can rev to higher rpms usually means the cars have higher power output, as you are converting more energy in a higher number of cycles in the same amount of time. (ie revs per minute)
Therefore almost all cars have a fairly linear power curve from about 1000 rpm to the peak.
Its more useful to compare the HP per 1000rpm, as this gives you a measure of the engines ability to rev quickly.
Car engines can fall off in efficiency at too high an rpm, where you really dont want to drive anyway.
Depending on efficency of the engine, higher power outputs generally means higher fuel consumption. Fuel is the sole source of the energy that creates the horsepower.

TO sum up, when you drive at low rpm and are accelerating, the Torque is the king. You may be accelerating wildly for the first second in a gear, from say 2000rpm up to 5000rpm. This is all torque, as you are only getting say 100-180 kw over this rev range (in a 35i).
When the Torque peak starts to fall off, you get a boost from the higher power levels in the driving acceleration, as you are burning an incredible amount of fuel from 5000rpm to 6500rpm which is converted to kinetic energy.

Once you reach the power peak, you may as well change down a gear, you are flogging a dead horse in terms of better acceleration performance.

Many race drivers will ignore the power peak in a good turbo car, where the torque figures are huge in comparison to the normally asperated cars.
They will drive in the 2000-5000rpm band and use the torque for everything.
They miss out on the high screaming noise of the top rpm in the engine range, but they use less fuel for the endurance aspect of a race and miss out very little on accelerative force and acceleration times.
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