Vtec

VTEC (Variable Valve Timing and Lift Electronic Control) is a valvetrain system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine. This system uses two camshaft profiles and electronically selects between the profiles. It was invented by Honda R&D engineer Ikuo Kajitani. It can be said that VTEC, the original Honda variable valve control system, originated from REV (Revolution-modulated valve control) introduced on the CBR400 in 1983 known as HYPER VTEC. VTEC was the first system of its kind, though other variable valve timing and lift control systems have been produced by other manufacturers (MIVEC from Mitsubishi, VVTL-i from Toyota, VarioCam Plus from Porsche, VANOS from BMW, VVL from Nissan, etc).

Vtec have the same idea with DVVT and VVT-i. It acts like a normal fossil combustion engine in low RPM, however when the user revs until certain limit (usuall 5000rpm). The valve timer would activate (this is what people usually calls Vtec change) and produce a noisy sound.

It sounds a little bit like a 2-stroke motorcycle engine revs high. You feel like the torque suddenly increases and you have more power to accelerate.

What actually happens is that the valve intake open longer than normal. Allowing more air to come into the combustion chamber. The more air in the chamber, the more efficient the fuel burns.