What is Horsepower?
Power is the creation of mechanical energy. Horsepower was a term that first came about as a measurement for judging the relative power of various methods of lifting coal from a mine. An engineer called James Watt established a unit to compare work rates against that of a work horse. The unit was fixed at 33,000 foot pounds of work in one minute, whilst not the most scientific or useful measurement it stuck. Thus giving birth to man's striving to achieve more power in order to achieve greater propulsion and speed.
Measuring Horsepower in an engine
Standard methods of measurement use a Dynamometer (or Dyno). This works by placing an engine under load and measuring it's output and this can be done either directly at the crank or by methods such as a rolling road. Measuring the amount of load that an engine can handle at given engine speeds gives us a torque figure. Torque being a product of power (as below) and can be converted to give us the benchmark power figure.
What is Torque?
Torque is a force that turns, twist or rotates. The spinning crankshaft of the engine thus creates torque. Torque is defined as force multiplied by distance.
Torque and power are related by the following equation.
Power = Torque x RPM / 5252
The above formula varies if the measurement units are not BHP and lb/ft.
Torque is an awkward measurement not easily allowing a single point comparison where power is a better absolute. This hides the importance of the spread of torque and power across engine speeds and is the reason Dyno-plot is designed to show you power graphs in dynamic comparisons in order to give you the full picture of car modifications.
A greater area underneath the torque curve indicates an engine that makes greater use of power across more of the rev range to counteract loads. A "peaky" engine will show by a steep power graph and a bulge in the torque graph at higher engine speeds, these will typically require well matched (and often altered) gearing to ensure the engine can be kept within this power band to achieve desired performance.
There are other ways to improve a car's accelerative responses, principally reducing the forces acting upon it. This is why many motorsport applications reduce the cars weight and thereby the forces acting upon the car thus allowing it to accelerate faster.