VTEC System - General Information

The VTEC system was developed by Honda engineers to enable dynamic control of valve timing and valve lift. Engines equipped with this system are marked with the VTEC lettering embossed on the cylinder head cover. The corresponding engine series number (F23A1 or F23A4) is stamped on the side of the block facing the radiator.

Engines equipped with VTEC differ from the base version only in the principle of valve timing control and the design of the valve train. The design of the short engine block assembly, the lubrication and cooling system layout remain the same, as do the list, design and location of the attached components.

The engine control module of VTEC-equipped models is able to adjust the timing and lift of the intake valves by using camshaft lobes of different size and shape. Depending on the data received from the information sensor, the processor either engages or disengages the system.

The operation of both types of VTEC system is determined by the following input parameters:

a) Engine speed (rpm);
b) Vehicle speed (mph);
c) Throttle Position (TP) sensor output signal;
d) Current engine load, determined by the Manifold Absolute Pressure (MAP) sensor readings;
e) Engine coolant temperature.

The camshaft is equipped with three lobes driving each intake valve, which differ in profile shape and lift height, determining the duration and amount of opening of the respective valves.

At low engine speeds, the secondary intake valves are actuated by their own camshaft lobes, which have very low lift and a pointed shape (i.e., compared with the primary valves, these valves open only slightly and very briefly), keeping the atomized fuel from pooling inside the cylinder head. This produces good low-end torque with fast response, which provides excellent pulling power and high acceleration. The primary lobes operate normally during this time, creating swirl of the air-fuel mixture in the combustion chambers.

When higher engine output is required, the secondary rocker arms are locked to the primary ones via special synchronizing pistons. The synchronizing pistons are hydraulically actuated and electronically controlled. When the rocker arms are locked, both intake valves begin to open in the same manner, and the degree and duration of their opening are determined by the shape of the intermediate rocker arm cam lobe and have increased values.

The secondary rocker arms stop contacting their own cam lobes until the system is disengaged. In this case both valves open to full lift and with maximum duration, providing increased engine speed and output.