These recitations of simple mathematics are misleading. They imply that all you need to do to make mad horespower is zing your motor up to 10 grand. Not so. What'll actually happen before you start bending valves is that horsepower will drop off. It takes a lot more than RPM to boost horsepower. A motor is essentially a big pump; it moves air. But a pump can only move so much air. You can move the pistons faster and faster, but if no more air can make its way into the cylinders, you're not going to be able to make more horsepower.

It's not that hard to design a motor that generates x torque at y RPM. But just because it makes a certain amount of torque at one RPM is no guarantee that it can do it at another RPM. As RPM goes up, the motor eventually reaches the limit of its ability to breathe. Beyond that point, torque falls off as there is less and less time to move air in and out. Simple mathematics allows the horsepower number to go up briefly as torque falls, but since it's torque that makes a car go, the motor's performance is decreasing. And pretty soon the horsepower drops as well.