Horsepower is a rate of doing work, not work itself.

It depends on the pressure on the otherside of the piston. In a rational setso if x amount of air pressure pushes down on a y size piston how much energy can be transfered through the piston?

of units the work done is the nett force times the distance moved by the point of

application.

Unfortunatly horse power is not part of a rational set of units and you will

have to work in Watts, metres and Newtons which are the relevant SI units

if you want to make this simple.

Watts are Joules per second which are Newton metres per second, and a

Newton is a kg metre per second per second.

RonL