check wikipediaVTEC (standing for Variable valve Timing and lift Electronic Control) is a system developed by Honda to improve the combustion efficiency of its internal combustion engines throughout the RPM range. This was the first system of its kind and eventually led to different types of variable valve timing and lift control systems that were later designed by other manufacturers (VVTL-i from Toyota, VarioCam Plus from Porche, and so on). It was invented by Honda's chief engine designer Kenichi Nagahiro.Single overhead camshaft is a design in which one camshaft is placed within the cylinder head. In an inline engine this means there is one camshaft in the head, while in a V engine there are two camshafts: one per cylinder bank.The SOHC design is inherently mechanically more efficient than a comparable pushrod design. This allows for higher engine speeds, which in turn will by definition increase power output for a given torque. The cams operate the valves directly or by a short rocker as opposed to overhead valve pushrod engines, which have tappets and long pushrods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.SOHC designs offer reduced complexity compared to pushrod designs when used for multivalve heads, in which each cylinder has more than two valves. double overhead camshaft (also called double overhead cam, dual overhead cam or twincam) valvetrain layout is characterized by two camshafts being located within the cylinder head, where there are separate camshafts for inlet and exhaust valves. In engines with more than one cylinder bank (V engines) this designation means two camshafts per bank, for a total of four.Double overhead camshafts are not required in order to have multiple inlet or exhaust valves, but are necessary for more than 2 valves that are directly actuated (though still usually via tappets). Not all DOHC engines are multivalve engines — DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared, however today DOHC is synonymous with multivalve heads, since almost all DOHC engines have between three and five valves per cylinder.i-VTECi-VTEC introduced continuously variable camshaft phasing on the intake cam of DOHC VTEC engines. The technology first appeared on Honda's K-series four cylinder engine family in 2001 (2002 in the U.S.). Valve lift and duration are still limited to distinct low and high rpm profiles, but the intake camshaft is now capable of advancing between 25 and 50 degrees (depending upon engine configuration) during operation. Phase changes are implemented by a computer controlled, oil driven adjustable cam gear. Phasing is determined by a combination of engine load and rpm, ranging from fully retarded at idle to maximum advance at full throttle and low rpms. The effect is further optimization of torque output, especially at low and midrange RPMs.