This VTEC system changes the exhaust valve lift timing and lift height depending on the engine speed and load in order to optimize the fuel efficiency and the engine output.
The rocker arm oil control solenoid switches the VTEC system on and off; the solenoid is controlled by the PCM.

VTEC Camshaft:
The exhaust camshaft is structured so that each cylinder has a separate low lift cam lobe and a high lift cam lobe.

VTEC Rocker Arm:
The exhaust rocker arm is composed of the primary, the mid, and the secondary rocker arms. The inner side of the exhaust primary rocker arm and the exhaust mid rocker arm have a VTEC switching piston in each rocker arm. The inner side of the exhaust secondary rocker arm has a stopper piston and a return spring.

Rocker Arm Oil Control Valve:
The rocker arm oil control valve is integrated with the rocker arm oil control solenoid and the spool valve, it is installed on the cylinder head.
When the rocker arm oil control solenoid turns on or off depending on a signal from the PCM, the hydraulic pressure subjects the spool valve to an ON/OFF control.
By switching the ON/OFF of the spool valve, the hydraulic pressure applied to the VTEC switching piston in the rocker arms is controlled.

Based on the vehicle speed signal, engine speed signal, engine load signal, throttle position signal, and engine coolant signal sent from various sensors, the PCM determines the lift amount and controls the ON/OFF of the rocker arm oil control solenoid.
The PCM generates and determines the control amount based on signals from various sensors to subject the rocker arm oil control solenoid to a feedback control.

VTEC LO:
The rocker arm oil control solenoid is turned off by the PCM. Oil pressure from the rocker arm oil control valve does not enter the exhaust rocker shaft. The primary and the secondary exhaust rocker arms are lifted by the low lift cam lobe.

VTEC HI:
The rocker arm oil control solenoid is turned on by the PCM. Oil pressure from the rocker arm oil control solenoid enters the exhaust primary rocker arm via the exhaust rocker shaft, and it moves the VTEC switching piston in the rocker arm. This causes the VTEC switching piston to slide into the exhaust medium rocker arm, locking the exhaust secondary and medium rocker arms together. Both exhaust rocker arms are lifted by the high lift cam lobe.

The VTC system makes continuous intake and exhaust valve timing changes depending on the engine speed, the engine load, and the vehicle speed.

VTC Actuator:
The VTC actuator is composed of the vane, the lock pin, the spring, and the housing with the timing gear. It is installed at the end of the camshaft.
When the VTC actuator vanes rotate to advance or retard position, the intake camshaft timing is changed.
When the engine stops, the vane is locked in the initial position by the lock pin so that the engine will start smoothly. After the engine starts, the vane turns freely because the lock pin is pushed back against the spring tension by oil pressure.

VTC Oil Control Solenoid Valve:
To control the camshaft angle continuously, the VTC oil control solenoid valve controls oil pressure to the advance chamber/retard chamber of the VTC actuator according to a signal from the PCM.
The solenoid valve has a strainer that to avoid foreign material contamination.

The VTC system diagram is as follows.
The PCM advances or retards the valve timing based on the data from various sensors, and performs feedback-control to the VTC oil control solenoid valves.

Advance Rotation Operation:
When the advance chamber is charged with pressured oil from the VTC oil control solenoid valve, the camshaft rotates to the advanced side.
In case of the oil non-pressurized state (engine is stopped), VTC actuator B is locked in the most advanced position by the return spring and the lock pin.

Retard Rotation Operation:
When the retard chamber is charged with pressured oil from the VTC oil control solenoid valve, the camshaft rotates to the retarded side.
In case of the oil non-pressurized state (engine is stopped), VTC actuator A is locked in the most retarded position by the lock pin.