As a camshaft spins, its lobes - - or cams - - open and close the valves on your engine in time with the piston to allow air in and exhaust out. The shape of the cam affects your engine's performance at different speeds. At 4,000 rpm, the valves open and close about 2,000 times per minute. The air enters the cylinder very quickly by the time the piston is at the bottom of its stroke; if the valve were to shut suddenly, the air flow into the cylinder would stop and it would lose momentum. The faster the engine goes, the more momentum it needs. So it's better to leave the valve open a bit longer to keep air flowing at higher speeds.
There are several different camshaft configurations. Two of the most common are single overhead cam and double overhead cam engines. Single overhead cam engines have one cam per head, whereas double overhead cam engines have two cams per head. Both types of cam are driven by a crankshaft that pulls a timing belt or timing chain. A double overhead cam engine is usually used when there are four or more valves in each cylinder; single overhead cam engines don't have enough lobes to activate that many valves. More valves equal more power, so a double overhead cam engine is typically more powerful than a single overhead cam engine. The ability to vary the timing of the valves is key to camshaft design. Variable valve timing was developed to allow the engine to adjust for optimal performance at different speeds. For example, Honda's Variable Valve Timing and Lift Electronic Control, or VTEC, system allows an engine to utilize multiple camshaft profiles. An extra intake cam keeps the cylinder's intake valve open longer than the regular cam at high speeds, and disconnects at low speeds. This allows more air in when the engine needs more power and less air in when the engine's needs are not as great. Other systems just change the timing of the cams so an intake valve opens and closes a little later at higher speeds, keeping the air flowing.
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