Say for a minute you had a motor that had a perfectly flat torque curve from 1000 rpm to 6000 rpm and stays in the same gear. Discounting wind resistance the car will accelerate at a constant rate all though 1000-6000rpm because the torque is what is working. Yet the car is making 6 times as much HP at 6000 rpm it isn't accelerating any faster then it is at 1000 rpm.