Porsche calls it 'VarioCam', meaning that while the engine is running, the intake camshaft, in relation to the driving gear, is adjustable If a 'plus' is added to the description, then two cams per intake valve are in action. In the above figure 1, the two red marked partial cams press down on the outside edge of the bucket-tappet. Although it is pressed down, it does not however, work on the valve. This is opened by the center cam through a small pin with very little stroke. Only when the outside bucket-tappet rim opposite to the pin is arrested by a bolt (figure 3), can both red marked partial cams together move the complete bucket-tappet and the valve can open considerably more and for a longer time. One can, through oil-pressure, reach different stroke lengths for each transmission element. (figures 4-6)
The Honda system (figure 7) has been around much longer. In the above simple illustration can be seen, only one valve, two cam followers and two different cams, the front cam for shorter, the rear for longer valve timing. Any possible springs have been left out. Imagine please, that the engine operates this intake valve exclusively through the front cam in a certain (lower) RPM range. Additionally, this smaller opening also positively affects the (cold) starting.
It begins to get interesting when the engine revs are higher and one would like to have longer opening times. During all this time the rear cam has also been working. It moves the rear cam follower, however, without effect on the valve. Now you must imagine only a small bolt on the right of the camshaft, which can be shifted through a magnetic valve by oil-pressure in a few thousands of a second, connecting both cam followers with each other. The rear cam now opens the valve earlier and closes it later.
In the described mechanism, a motor control unit which provides, in the lower RPM area (through the front cam) enough torque, and in the upper RPM area (through the rear cam) for unhampered respiration, for both everyday- and sports engines. A newer system is, e.g., the Valvetronic by Audi . The mechanism shifts a sliding block on the camshaft and also regulates the valve lift in two stages. Thus, the system is not as variable as, eg, the Valvetronic valvetronic by BMW . Toyota (WTL) also has a comparable system.
The intake camshaft is provided with a gearing. On it are bushes whereby two different cam heights and cam forms are axially movable. These are shifted through two magnetic valves which drive the bushes in, depending on the demands made on the engine, to the left or to the right. The sleeves must be now somehow be held in their position, which occurs after the principle of gearchanging, with a slight indentation and a spring-loaded ball-bearing.
Both intake valves of a cylinder are even opened, by the way, in the partial load area, at different times which is possible with offset cams. This system is called 'asymmetric valve opening'. With this special arrangement of the cams, and a special design of the intake parts, swirl-chambers or swirl-tumble-flaps can be dispensed with. Now, in order to bring the camshaft sleeve back into position, a second pin is required which enters the sleeve and resets the starting position. 07/08