What is VTEC?

The Honda VTEC variable valve timing system explained in the 1991 CRX

Fig. 1
The 1.6 liter DOHC engine with VTEC used in the 1991 CRX. 

The VTEC (Variable Valve Timing and lift Electronic Control) system is explained here by example of the 1.6 liter DOHC engine used in the CRX, which was in 1991 the first car equipped with such an engine. It has four valves/cylinder and two overhead camshafts. 


Fig. 2
The outside cams operate on the valves by outside rockers.

Fig. 3
The three rockers are interconnected by the two hydraulically operated pins A and B.
The cam in the middle operates on the valves by the interconnected rockers.


For every cylinder there are three cams on the camshaft and three rockers instead of the usual two. At low to middle revs the outside in- and output cams are used. They have their own rockers which operate on accompanying valves. These cams are designed for moderate lift and timing. (Fig. 2).
At high revs the middle cam is used. It has a high lift and different timing. This cam operates on the middle rocker. The indirect selection of the two different type of cams is done hydraulically by shifting a pin, consisting of two parts A and B, in the rockers by which the middle rocker takes along the two outside ones, which as a result, also receive the fast middle cam profile. The valves are opened wide and for a longer period for more power. (Fig.3).

The two cam types have their own power distribution curve. At one point in both curves the engine delivers the same power. At this point the motor management switches from one state to the other. As a result the transition occurs smooth and without notice, except for some more push of course ...