Well we explained a little about the four strokes and gave some pros and cons on boring and stroking an engine. So to pull it together and recap,...

the size of the cylinders, pistons, and length of the stroke determines the combustion chamber size. The amount of combustion chamber space when the piston is at its lowest travel point compared to the amount of combustion chamber space when the piston is at its highest travel point is what designates the compression ratio.
 Now I guess we can talk about engine breathing. That’s right I said breathing. Engines are like you and me. In order for us to run faster or play harder we need to breathe more efficiently and get more air. Well the same is true for an engine. The more oxygen you get into a combustion chamber, the more fuel you can put in which will create a bigger bang. That bigger bang represents more power and torque. As we talked about boring and stroking we were discussing ways of allowing the engine to fit more air and fuel. Now we will talk about how to get an engine to take more air and fuel more efficiently.
Many things control the amount of air and fuel allowed to enter the engine. Some of which are the time the valve is open, the size of the valve opening and how far the valve opens. So the more the valve opens and the longer it stays open the more air and fuel has the opportunity to enter the cylinder. The shape of the combustion chamber, and the design of the intake and head porting also play a role in the ability of the air and fuel to enter the chamber. Air comes in an engine through a throttle opening for example a carburetor. Then it travels through an intake manifold, which just channels the air to the head of an engine. Once the air enters the head the intake valve is what allows the air to enter the combustion chamber of the cylinder. It is how we design the head and control the valves that make the biggest difference in the ability for and engine to breathe.
The head is mounted on top of the cylinders or barrels of an engine. This component houses the valves for the intake and exhaust as well as the passages for the intake air and exhaust air to travel in and out. The design and shaping of the head and chamber will allow for air to travel at different velocities. The higher the velocity with least restriction will enable more air to flow in. The valves block the hole in the head that allow the air to enter the chamber. As we stated before the bigger the valves and opening, the further open the valve is pushed and the longer it is held open the more air that can enter. The valves are pushed open by rocker fulcrums driven by a push rod pushed by a lifter, which gets lifted by the lobe on the cam. On overhead designs the cam lobe pushes a follower or lash adjuster into the valve. This eliminates many of the before mentioned components creating again more efficiency. A spring on the valve closes the valve as the pressure from these components is released.
So that being said brings us to the camshaft or cam. Besides head design and valve size this is the component that makes a big difference. It is also probably the easiest upgrade in engine performance that can give you a substantial change in power.  The camshaft has lobes shaped much like a teardrop if you will in which control the valve indirectly. The camshaft rotates one half rotation for every rotation of the crankshaft. This is because the valves only open one time in four strokes and the crank turns two times to complete the four strokes. The lobe is measured in lift and duration. Lift is the measurement from the centerline of the cam lobe (teardrop) to the top of the lobe (teardrop) if you will. The duration is the measurement of how long the valve is open from the time it begins to open until it is closed. If you picture the teardrop, the more egg shaped the teardrop becomes the longer the duration. Changing the lift and duration will alter the amount of air and fuel that can enter the engine.
Another item that helps is variations in valve timing and overlap. If the intake valve is open a littler early nearing the end of the exhaust stroke the exhaust leaving creates a vacuum pulling more air and fuel into the combustion chamber. It also allows for the complete removal of spent air and fuel at higher rpms. If the overlap is too great a thing called cylinder scavenging occurs causing you to loose precious clean air and fuel, which exits out of the exhaust valve. This as you imagine will cause you to loose power.
As you alter all of these items you will get different effects in horsepower and torque. Some variations will allow for a larger power curve as well as torque curve. Some will allow more power at low rpms and some at high rpms. Using the right configuration will give you what you need. So if it is power you are looking for, maybe you need to let your engine breathe a little. Generally these upgrades are the least compromising to the engines longevity and reliability. However just like any upgrade they aren’t cheap upgrades. Remember money is power and horsepower cost money. Safe riding!