Early History. Very early Cadillac and Rover examples. Designers' preference for increasing engine size rather than complexity. Racing applications limited by narrow power band and piston/valve clearance. Applications rare.

Events around 1970. Emissions legislation provokes much more interest. How changing cam timing on a twin cam engine reduced NOx by 20% in an emission test. A lot of proposals and experimental work but the industry stayed with simpler solutions.

Variable Valve Timing. Alfa Romeo first manufacturer to launch a production VVT system. Helical splines. Timing chain slack displacement. Hydraulic vane actuator. Borg Warner cam torque actuation.

Variable Duration. Unsteady rotation by eccentric mechanisms. Rover adopt an AE device from the 1970s.

Two State Cam Change. Lock pin devices. Honda V-Tec. Lotus derived concentric tappet system.

Variable Lift. GM valve throttled experiments. Fiat oscillating cam system. BMW Valvetronic.

Hydraulic Solutions. The leaky hydraulic tappet. A variable lift hydraulic tappet with controlled valve closure. Fiat Multiair hydraulic system - multi-lift capability.

Future. Electric methods. Not easy to achieve without high electric power demand.