The purpose of variable valve timing is to alter engine characteristics as needed, and to optimize performance and economy under all driving conditions. In an internal combustion engine, air and vaporized fuel is supplied to the combustion chamber by parts known as the cams. The cams open the valves for a certain amount of time during each intake and exhaust cycle. The height that the valves are opened is known as lift. The length they are opened is known as duration. The synchronization between intake and exhaust valves opening is known as timing.
In a non variable valve timing engine configuration, the position and shape, collectively known as the profile, of the cams and camshaft is static, and is set for a certain RPM. This is a balance between low RPM torque and high RPM power. Variable valve timing allows cam profiles to change to optimize torque at low RPMs and horsepower at high RPMs. Most variable valve timing systems work by advancing or retarding the timing of the intake or exhaust valves.