This page provides you with the fundamentals of engine science and will take some of the mystery out of making more horsepower with a four stroke engine. An engine is just a machine that converts fuel energy to a rotating motion, which is commonly measured as horsepower. This mixture is then burned in the cylinder, creating heat which creates pressure. This pressure pushes the piston down in the cylinder to turn the crank shaft. Now, to the science part.
Fuel has a chemical energy.
A typical pound of gasoline contains 19,000 to 20,000 Btu. Therefore, the power output is directly related to how much fuel the engine can effectively burn. Now, don't run out and put bigger jets in you carb trying to make more power. Also, super charging or turbo charging packs more fuel and air into the cylinders, so this would also make more power. On the intake stroke, the piston moves to the bottom of the cylinder and creates a volume that is 100 cubic inches.