Editing Variable valve timing (section)

===Cam phasing versus variable duration===
The simplest form of VVT is ''cam-phasing'', whereby the phase angle of the [[camshaft]] is rotated forwards or backwards relative to the crankshaft. Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered solely with a cam-phasing system.

Achieving variable duration on a VVT system requires a complex system, such as multiple cam profiles or oscillating cams.