What is the function of an intake pipe behind the throttle on Citroen C5 3.0 engine?

The manifold length and curvature of each cylinder should be as same as possible. Because the engine operates in four strokes, each cylinder of the engine sucks air by pulse. According to experience, a longer manifold is suitable for low-speed operation, while a shorter manifold is suitable for high-speed operation. Therefore, some models will adopt variable-length intake manifold or continuous variable-length intake manifold, so that the engine has better performance in all speed ranges.