Variable Valve Timing & Lift Electronic Control (VTEC) is a system developed by
Honda to improve the volumetric efficiency of a
four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two (or occasionally
three)
camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT (
variable valve timing) systems which change only the valve timings and do not change the camshaft profile or valve lift in any way.
Japan levies a
tax based on engine displacement, and Japanese auto manufacturers have correspondingly focused their
research and development efforts toward improving the performance of their smaller engine designs. One method for increasing performance into a static displacement includes
forced induction, as with models such as the
Toyota Supra and
Nissan 300ZX, which used
turbocharger applications, and the
Toyota MR2, which used a
supercharger for some model years. Another approach is the
rotary engine used in the
Mazda RX-7 and
RX-8. A third option is to change the cam timing profile, of which Honda’s VTEC system was the first production engine to use cam‑switching technology to vary both valve timing and lift.
The VTEC system provides the engine with valve timing optimized for both low- and high-RPM operations. In basic form, the single
cam lobe and
follower/rocker arm of a conventional engine is replaced with a locking multi-part rocker arm and two cam profiles: one optimized for low-RPM stability and
fuel efficiency, and the other designed to maximize high-RPM power output. The switching operation between the two cam lobes is controlled by the
ECU, which takes account of
engine oil pressure, engine temperature, vehicle speed, engine speed, and throttle position. Using these inputs, the ECU is programmed to switch from the low-lift to the high-lift cam lobes when certain conditions are met. At the switch point, a
solenoid is actuated that allows oil pressure from a
spool valve to operate a locking pin, which binds the high-RPM
rocker arm to the low-RPM ones. From this point on, the valves open and close according to the high-lift profile, which opens the valve further and for a longer time. The switch-over point is variable, between a minimum and maximum point, and is determined by engine load. The switch-down back from high- to low-RPM cams is set to occur at a lower engine speed than the switch-up (representing a
hysteretic cycle) to avoid a situation in which the engine is asked to operate continuously at or around the switch-over point.
The older approach to timing adjustments is to produce a camshaft with a
valve timing profile that is better suited to low-RPM operation. The improvements in low-RPM performance, which is where most street-driven automobiles operate a majority of the time, occur in trade for a power and efficiency loss at higher-RPM ranges. Correspondingly, VTEC attempts to combine low-RPM fuel efficiency and stability with high-RPM performance.