Variable Induction System: Description and Operation

To achieve steady and high torque over a broad engine rpm band, the engine is equipped with a variable intake manifold.

Three different intake pipe lengths can be obtained by means of two butterflies or flaps in the intake manifold.

These two butterfly valves are controlled by the Motronic engine control module in such a way that the engine develops the highest possible torque over a broad rpm band.





The characteristic torque curve of a normally aspirated engine depends mainly of how the engine's average pressure changes over the rpm band.

The average pressure is proportional to the volume of the air mass present in the cylinder when the inlet valve is closed.

The design of the inlet system determines how large an air mass can be sucked into a cylinder at a given engine rpm.

From this it follows that the air intake system determines the engine's torque curve. In general, it can be said that a longer intake manifold results in higher torque at lower rpm and a shorter intake manifold results in higher torque at higher rpm.

The intake manifold on the B308 engine is fitted with two valves or flaps which can be set so that three different intake manifold lengths are obtained. The three manifold lengths obtainable result in different torque curves with maximum torque at different engine rpm.

The control module controls the two valves or flaps by means of solenoid valves and vacuum operated diaphragm units so that the torque curves overlap is such a way as to produce the most favourable results.

The solenoid valves are supplied with current via the main relay and are grounded by the control module.





The inner valve or flap is located in the intake manifold between the cylinder heads. When the control module grounds pin 35 the solenoid valve opens and the inner valve or flap separates the inlet systems of the two banks of cylinders from each other.

The outer flap is located between the two resonating pipes connected to the intake manifold. When the control module grounds pin 7, the solenoid valve opens and the outer flap opens a connection between the two resonating pipes, the active length of which is thus reduced.

During wide open throttle acceleration from low rpm the control module grounds pin 35, thereby separating the cylinder banks from each other and connecting each of them to its own resonating pipe, the full length of which is utilized since the outer flap is closed.

When engine speed reaches 3200 rpm the control module grounds pin 7, causing the outer flap to open the connection between the two resonating pipes so that their active length is reduced.

When engine speed reaches 4100 rpm the control module breaks the circuit to pin 35 so that the two inlet systems (one for each bank of cylinders) are connected to each other and the effective pipe length is the shortest possible.

At speeds below 4100 rpm and with a throttle valve angle less than 50° the inner flap is always open so that the idle air control valve's supplementary air will be distributed equally between the cylinders.