Ever wonder why HP/Torque fall off after a cetain RPM?

You're not the only one...
In short, from the first reply of that thread...

The relation between torque, power and angular speed is



Power falls off below a peak located at high engine speed, because less fuel/air (energy source) is brought in and burned per second. If that were the only factor, then power output would be proportional to angular speed



and torque would remain constant down to idle. In fact passenger car engines have a fairly broad and flat torque curve. See the curves at, e.g.
http://en.wikipedia.org/wiki/Power_band

However engine efficiency drops with at low speeds since combustion chamber shape, bore/stroke ratio, manifold runner shape and length, valve lift and intake/exhaust valve overlap, to name just a few factors, are tuned for best performance at higher engine speeds. Thus torque eventually falls. In racing cars, the tuning is "peakier," that is, they produce far more peak power but only over a narrow RPM range. As you might expect, the torque curve isn't as flat in this case, and it falls off more rapidly. See Fig. 3 here
http://www.corvetteactioncenter.com/tech/hp_torque.html

Vehicles that are optimized for very high torque at very low vehicle speed either have no high end to speak of (road graders, bulldozers) or, if they need both, use different systems (diesel-electric locomotives).

Update 12/13/10: revised picture links