[Flieger]

Torque causes movement over a distance which is work. Power is the time derivative of Work. Torque is like a force (here it is a force x distance so Newton-Meters or Pound-Feet). Work is that Torque being applied to a load, integrated over the whole distance (degrees or revolutions) that load travels. Think of pushing a rock up a hill.

Power is how fast the Work is done. It is dW/dt (time derivative). dW/ds (distance derivative) is Torque.

Basically, what you want to know is that:

Torque x engine speed (rpm) = Power

Torque x # of revolutions total = Work

Torque is measured on a dynomometer by putting a load on the engine (usually a water brake) and seeing how hard it can turn. Since they cannot stall the engine without stressing it too much, they apply a moderate load and measure how fast that work is done to get power.

When you multiply torque by the rpm it is achieved at, the graphs look like they do.

As with basic calculus, the derivative of x^2 is 2x and the derivative of 2x is 2 so a constant torque would yield a linear power slope...

basic calculus and physics.

By the way, in the USA, Horsepower = (lb-ft x rpm)/5252