Valve performance plays a significant role in the overall power production of an engine. If the valves are too small, the air flow will be restricted, and the engine torque will decrease. The airflow through the inlet and the exhaust ports of an engine is unsteady, due to the periodic opening and closing of the inlet and the exhaust valves.

There are several types of valves that are used: a poppet, rotary, disc and a sleeve. The most common valve is the poppet valve. The poppet valve is inexpensive and has good sealing properties, making it such a popular choice. The following evaluation will assume poppet valves are used for both the inlet and exhaust ports.