A piston valve comes in two types. There is an intake piston valve, which is responsible for allowing the fuel and air charge into the combustion chamber. The other type of piston valve is the exhaust valve. This piston valve is charged with allowing the spent gasses and fumes from the combustion process to escape the cylinder. Both versions of the piston valve are actuated by the camshaft and the amount of time the piston valve is opened. The amount it is opened also is determined through the timing process and the camshaft profile.
Perhaps no single engine component affects the power-building potential as much as the piston valve. The opening and closing of the piston valve as well as the ability to seal the combustion chamber are power-building functions of the piston valve. The combustion chamber is the area in an engine where the power is made. By effectively sealing the combustion chamber, all of the power extracted from the ignition process can be projected toward the crankshaft via the pistons and connecting rods. The more complete the combustion process, the more power can be realized by the engine.
While the valves are tasked with entrance and exit of combustibles from the combustion chamber, the valve timing is the blueprint of how this entering and exiting will occur and at what point in the combustion process this will happen. Experienced engine builders understand that if the camshaft is the heart of an engine, the valves must be the main arteries. Power is made and lost in the profile of a valve pocket and even in the angles chosen to grind the valve and seat. The size, valve stem design and valve angle are all important factors in the horsepower equation.
When looking at a cylinder head from an internal combustion engine, the larger valve will be the exhaust valve. The reason is that the intake charge is aided into the cylinder head by gravity and the sucking of the piston as it travels downward through its intake stroke. The exhaust, on the other hand, must escape through the exhaust valve with only minimal help from the piston as it travels upward through the cylinder walls on the exhaust stroke. It is imperative that all of the exhaust gas be expelled from the cylinder in order to get the most power from the intake charge. Valve timing, configuration and design all complete the process in concert.