<P> VVT - i, or Variable Valve Timing with intelligence, is an automobile variable valve timing technology developed by Toyota . The Toyota VVT - i system replaces the Toyota VVT offered starting in 1991 on the 5 - valve per cylinder 4A - GE engine . The VVT system is a 2 - stage hydraulically controlled cam phasing system . </P> <P> VVT - i, introduced on the 1JZ - GTE / 2JZ - GTE engine in 1996, varies the timing of the intake valves by adjusting the relationship between the camshaft drive (belt or chain) and intake camshaft . Engine oil pressure is applied to an actuator to adjust the camshaft position . Adjustments in the overlap time between the exhaust valve closing and intake valve opening result in improved engine efficiency . Variants of the system, including VVTL - i, Dual VVT - i, VVT - iE, "VVT - iW" and Valvematic, have followed . </P> <P> VVTL - i (Variable Valve Timing and Lift intelligent system) (also sometimes denoted as VVT - iL or Variable Valve Timing and Intelligence with Lift) is an enhanced version of VVT - i that can alter valve lift (and duration) as well as valve timing . In the case of the 16 valve 2ZZ - GE, the engine head resembles a typical DOHC design, featuring separate cams for intake and exhaust and featuring two intake and two exhaust valves (four total) per cylinder . Unlike a conventional design, each camshaft has two lobes per cylinder, one optimized for lower rpm operation and one optimized for high rpm operation, with higher lift and longer duration . Each valve pair is controlled by one rocker arm, which is operated by the camshaft . Each rocker arm has a slipper follower mounted to the rocker arm with a spring, allowing the slipper - follower to freely move up and down with the high lobe without affecting the rocker arm . When the engine is operating below 6000 - 7000 rpm (dependent on year, car, and ECU installed), the lower lobe is operating the rocker arm and thus the valves, and the slipper - follower is freewheeling next to the rocker arm . When the engine is operating above the lift engagement point, the ECU activates an oil pressure switch which pushes a sliding pin under the slipper - follower on each rocker arm . The rocker arm is now locked into the slipper - follower's movements and thus follows the movement of the high rpm cam lobe and will operate with the high rpm cam profile until the pin is disengaged by the ECU . The lift system is similar in principle to Honda VTEC operation . </P>

What is the meaning of dual vvti engine