As the piston continues its downward motion, the volume keeps on increasing, and to compensate for the progressive decrease in manifold pressure, atmospheric pressure pushes ambient air into the manifold to restore the balance. Let us look at this process in some more detail-Īs, say, piston number one moves downward on its intake stroke while the intake valve is open, the volume of the intake manifold increases by virtue of the fact that cylinder number one is now connected to the manifold through the open intake valve(s). When an engine runs, the intake strokes of the pistons create a negative pressure (partial vacuum) in the manifold plenum, but basic physics demands that the air that is being “sucked” out of the plenum must be replaced by air being pushed into the plenum under atmospheric pressure. Which brings us to the role runner length plays in manifold design by asking this question- Why is runner length important?Įven though this article is intended to discuss various active intake runner control strategies, the length of the runners can be thought of as a sort of “passive” runner control strategy. In practical terms, runner length is defined as the distance between the manifold’s mating surface with the cylinder head to a point, or line drawn through the plenum that is shared by all the runners. Note that the runner on the extreme right has a pronounced “kink” in it to bring it to the same length as the runner on the extreme left. While it might appear that the runners on this example are of an unequal length, this is not the case. The image above shows a typical intake manifold, on which the runners connect to the plenum individually, as opposed to branching off from a single exit point as was usual on older manifolds. Intake runners are the actual tubes that channel intake air from a reservoir, known as a “plenum” to the intake ports. In this article, we will briefly discuss the importance of intake manifold design, as well as the various means and strategies engine designers have adopted to increase the efficiency of intake manifolds, starting with this question- What is an intake runner? Have you only ever thought of the intake manifold on an engine merely as a set of pipes that channels intake air into the engine? Well, an intake manifold does do that, and while it usually gets in the way when you want to work on something else, the fact is that an intake manifold on a modern engine is a highly engineered component that is critical to efficient engine operation.
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