Andy is correct, autostik is on the right track, and the other posts are confusing.
HP ~ torque x RPM.
Torque is what accelerates you. It translates directly to the force you wheels push your car forward with. Your gear ratios will multiply the engine torque up or down, and effectively change the force your car is being pushed forward with. Higher gears (4th eg = 0.82:1) are not multiplying the torque by the same ratio as lower gears (1st eg = 3.86:1), and therefore is pushing the car forward with proportionally less force.
So by using very low gearing you could get incredible amounts of torque at the wheels, and push the car forward very very hard. There is one prob, that low gearing will make the engines torque occur at very slow car speeds. So all that torque ain't much use if the engine revs out at 5kph.
Obviously there needed to be a way of determining what force the torque could push the car with, at equivalent car speeds. Somehow the speed at which that force was pushing needed to be included in the equation.
Therefore we have POWER. Power is essentially a measure of FORCE x VELOCITY. Which for some rotating can be rearranged to give torque x RPM. This means power is not effected by gear ratios.
So now you can directly compared engines with different torques and different gearbox ratios. eg
Take a car (1) with 50HP @ 4000rpm (with say 4000rpm redline), using the power eqn you will find that engine will have 66 ft.lbs of torque.
Take another car (2) with 50HP @ 8000rpm (with say 8000rpm redline), using the power eqn you will find that engine will have 33 ft.lbs of torque.
So they have the same HP, but different torque. Which one is fastest?
If you have the same gear ratio's in both cars, car 1 will accelerate much faster but then reach it's RPM limit. car 2 will accelerate half as fast, but will reach it's RPM limit at twice the speed of car 1. Obviously not a fair comparison.
So gear car 2 twice as low as car 1, so they both will reach their RPM limits at the same speed. So that since car 2 is geared twice as low, it's engine torque would be doubled at the wheels compared to car 1. So now they have the same torque at the wheels, and will do the same speed at their RPM limit. Obviously they will now have identical acceleration and speed.
So simply, torque doesn't really mean much for 1/4 mile times. If a car with high HP is geared correctly, it will always beat a car (of the same weight) with lower HP but higher torque (obviously at lower RPM).
However if the high torque and high HP cars are on the same gearing, the torquey car will out accelerate the high HP car in the same gears.
From there it gets complicated. When people say the engine is torquey, or another engine is all revs no torque, they are refering to the entire power curve, not just peak figures. And it is the entire curve that will effect everyday driving.
I'll use my engine options for the baja as an example. My two main options were the WRX engine (2.0lt turbo), and the KLZE (V6 2.5lt NA). Both have almost identical peak torque figures, but the WRX makes it peak torque at slightly higher RPM, so it makes slightly more peak HP.
What does it mean to acceleration? If you select either motor in my acceleration calc program
http://offroadvw.net/tech/calc.htm and set them to the same gearbox with high stall speeds, they will give about the same 0-100 and 1/4 mile acceleration.
However if you set the stall speed (take off RPM in first gear) to something low like 2000rpm, the KLZE is not effected because it has a really wide torque band. The WRX engine however is significantly slower when driven like this, because at 2000rpm it is well below it's torque band and will be slow until the turbo kicks in at higher RPM. Basically with the V6 you can flattened it in any gear at any RPM and it will go. If the flatten the WRX at low rpm not much will happen until the boost comes on, so it you want to go you'd have to drop it back a gear to get it in it's peak torque area.
SO you would describe the V6 as a torquey motor (due to it's wide torque band) and the WRX engine as a revvy HP motor (due to it's lack of low RPM torque) even though they both have almost identical peak torque and peak HP figures.
[Edited on 1-5-2003 by Baja Wes]
