rjstractor wrote:
RoyJ wrote:
If you read his post again, it's flat *power* curve, not torque curve.
A falling torque curve is desirable in a heavy duty motor, because as you lug down the revs in a gear, you get torque rise, thus fairly constant power.
The Cummins loses less power at lower rpm than the Duramax, and thus won't need 10 speeds nearly as much.
Exactly, the motor with the flat power curve maintains horsepower when shifting to the next higher gear, whereas the motor with a flat torque curve loses horsepower with every shift. The extra gear ratios help minimize this.
Roy is pointing out that an engine with a high torque rise is nice for pulling because as it pulls down the torque increases. It's more about downshifting than upshifting. If you think of a heavy truck climbing a hill most of the rear wheel torque demand is determined by the weight of the load and the incline of the hill. To a much less degree the wind and rolling resistance come into play. Because of this fact, as the hill gets steeper and the truck slows down there is only a slight decrease in the torque demand. If as the engine pulls down the torque does not increase the hill wins. The only way the truck can get the needed torque to the rear wheels is if it slows enough to grab the next gear. On the other hand, under the same scenario with an engine which posses a high degree of torque rise, as the truck slows the engine pulls down delivering more torque to the rear wheels to meet the increased demand of the hill.
If you think of a car accelerating down a track you see an increase in rear wheel torque demand the faster the car goes to meet the increasing resistance of drag. Keeping the torque curve high as the rpms increase is important for a dragster.
