OK - since I've had bugger all to do this afternoon, here is an example of Torque V Power:
T(Torque)=(HP x 5252)/RPM
HP= (T x RPM)/5252
RPM = (HP x 5252)/T
1 mile = 5280 feet
Lets suppose 2 engines produce constant torque from ZERO rpm to their
redline (ok these engines are bullsh!t, but it's just an example
):
Engine 1: 250 lb/ft @ 4200 rpm redline (Diesel)
Engine 2: 150 lb/ft @ 8400 rpm redline (2 litre V-Tec)
The engines are fitted to similar cars, that both weigh the same, and have
the same size wheels (lets say 24 inch diameter)
Therefore 60 mph = 60x5280 ft per hour = 5280 ft per min = wheel turning at
(5280/2 ft/Pi) 840 rpm
Now the maximum torque of the engine is constant across it's rev range, so
you'd think the engine with more torque would pull harder?
If the car was travelling at 60 mph, the wheel needs to turn at 840 rpm. No
problem both engines rev happily at 840 rpm, so just have a 1 to 1 gear
ratio and drive the wheels directly:
Thus the torque delivered to the wheel from Diesel is 250 lb/ft, from V-Tec,
150 lb/ft. Since I made the wheels with 24 inch diameter, the radius is 12
inch i.e. 1 foot, so the thrust of the wheel is = to the torque.
So without gearing the thrust of the cars are:
250 lb on the Diesel
150 lb on the V-Tec
Since Force = Mass x Acceleration and I already said the cars weigh the
same, then the acceleration of the Diesel is 1.6666 times as good as the Petrol
V-Tec.
But, suppose we use the gears on the cars?
Lets gear each car so that it is at it's redline:
Well since the power from the engine is not lost, then if you half it's
output rpm, the torque must double for the equations above to remain true.
Diesel @ 4200 would need to be reduced by a 5 to 1 gear ratio: Output torque
would be 250x5 = 1250 lb/ft
V-Tec @ 8400 would need to be reduced by a 10 to 1 gear ratio: Output torque
would be 150x10 = 1500 lb/ft
Thus if you make use of gearing, the V-Tec has more thrust than the more
torquey Diesel, and you'll see that to make full use of the engine, you need
to keep it at high revs, and thus you actually need more gears for this
engine!