VTEC System

The VTEC system changes the one side of the intake side cam profile to correspond to the engine speed. It maximizes torque at low engine speed and output at high engine speed.
The low lift cam is used by one side of the intake valves at a low engine speed. At high engine speed, both intake valves uses the high lift cam.

Fig 1: VTEC System Graph For Low Speed Valve Timing
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Fig 2: VTEC System Graph For High Speed Valve Timing
Courtesy of AMERICAN HONDA MOTOR CO., INC.