The air required for combustion in the engine flows through the variable intake manifold to the intake valve. Nowadays, common fuel injection systems for petrol engines inject the fuel in the intake manifold immediately prior to the intake valve. The length of the intake manifold may be variable at different engine performance conditions, actually setting contradictory requirements, so the highest possible air inflow speed and proper cylinder filling can be guaranteed at all engine speeds.
While the priority at low revolutions is to deliver torque as rapidly as possible, best attained by means of a long intake manifold, the need for maximum power output at high revolutions requires short intake manifolds.
The variable intake manifold complies with an electronically controlled flap system that meets both those demands, channelling the air via the long intake manifold at low revolutions and through the short intake manifold at high revolutions.