So...if the torque rating does not even enter the calculations when determining how fast an engine will pull a truck and trailer up a hill, why bother talking about it? The reason the max torque value is valuable is that it will help a thinking person visualize the power band of an engine. Notice that in order for torque to have any significance an rpm needs to be specified. As soon as the rpm is specified you are no longer talking torque.....now you are talking power. Torque@rpm = power. Assume two 400 HP diesel engines (engines A & B) make 400 HP at 2900 rpm. Now assume engine A has a perfectly flat torque curve over an operating range of 1500 rpm and its max torque is 725 lb ft of torque at 1400 rpm. As soon as we mention rpm we are no longer talking torque. Torque at rpm is power. So 725 lb ft of torque is 193 HP. So engine A has an pulling range of 1500 rpm during which time its power will build from 193 HP @ 1400 rpm to 400 HP at 2900 rpm. Now let's assume engine B has a maximum torque of 1500 lb ft of torque also at 1400 rpm. So engine B will have a torque curve which drops off sharply as the engine rpm goes up bu it will have a perfectly flat power curve over its 1500 rpm operating range. Engine B will be very easy to keep in its power band because it will be making 400 HP from 1400 rpm all the way to 2900 rpm. Gearing will be far less significant for engine B to be able to make use of its 400 HP.
So even though engine B is capable of producing over twice the torque of engine A it cannot pull that 24000 lb truck and trailer up the 12% grade at a speed any faster than 52 mph.
