[engineer]Torque is a function of volumetric efficiency. VE is the ratio of air that you can supply the volume of a cylinder compared to an equal volume outside the combustion chamber. Supply more air and fuel to a cylinder and you will raise the cylinder pressure. Forced induction does this nicely. If I had a scanner, I could post a free body diagram of the forces involved, with the pressure acting on the area of the piston creating a force that travels through the con rod into the cranks radius and voila, torque. Peak Torque is reached at peak VE.

Now, obviously most engine's do not reach their peak HP/TQ numbers at the same RPM. HP is defined as work done over time. You may notice that the units for work are the same as the units for torque, force times a distance. A 4-stroke engine has half as many power strokes as complete turns. At low RPM (remember tq*RPM is HP too), we have fewer chances to achieve that high cylinder pressure per unit time, lets say 1 second, and because engines do not typically reach their highest VE low in the power band, HP numbers are low.

As RPM's are raised, more and more power strokes occur per this same second, and power loads are a function of the average cylinder pressure, not just peak. Peak HP can often be found therefore after the peak TQ number is reached.

As far as gearing is concerned, you may notice that you can half the speed of any shaft, and under the same input conditions double the torque. The HP figure however, will remain unchanged. Another free-body diagram would show this nicely. As an engine revs several times through the power band over its pass down a 1/4 mile, peak acceleration is found when the TQ numbers are highest (without accounting for traction, aero drag).[/engineer]