System Description: VTEC/VTC: Notes

The i-VTEC has a variable valve timing control (VTC) mechanism on the intake camshaft in addition to the usual VTEC.
This mechanism improves fuel efficiency and reduces exhaust emissions at all levels of engine speed, vehicle speed, and engine load.
The VTEC mechanism changes the valve lift and timing by using more than one cam profile.
The VTC changes the phase of the intake camshaft via oil pressure. It changes the intake valve timing continuously.

Fig 1: Fuel Efficiency And Engine Load Graph

Courtesy of AMERICAN HONDA MOTOR CO., INC.

FUEL EFFICIENCY AND ENGINE LOAD DESCRIPTION CHART

Driving Condition	VTC Control	Description
(1) Light-load	Base Position	For stable combustion the cam angle is retarded, and it reduces the entry of exhaust gas into the cylinder.
(2) Medium/high-load	Advance Control	Cam phase angle is controlled to optimize valve timing, improving fuel efficiency, and reduce emissions.
(3) High speed	Advance-Base Position	To reduce pumping loss, the intake valve is closed quickly. This gives a charging effect to the air/fuel mixture, maximizing engine power.