An intake manifold is the part of an engine that supplies the fuel and air mixture to the cylinders. Air doesn’t just enter your car’s engine on its own, it goes through a series of engine components before it reaches the cylinders where the air is burned with fuel. Air first enters a car’s engine system through the air box where it moves to the throttle body and then the intake manifold which passes it through to the engine’s cylinders.
Older engines had cast-iron intake manifolds. These manifolds were either wet or dry types.
Had coolant passages within the manifold that allowed for coolant to run through the manifold.
Had exhaust passages in them that allowed for the hot exhaust fumes to run through and heat the floor of the manifold, this vaporized the fuel before it reached the cylinders. Vaporized fuel and air mixture is much more reactive than liquid and burns cleaner. Heating the floor of the manifold stopped the fuel from condensing in the plenum area.
Today’s intake manifolds are made of die-cast aluminium or plastic. They do not have to function such that fuel vaporization and fuel condensation are a concern. They simply have to add equal amounts of air to each cylinder. These type of intake manifolds has 2 basic components; the plenum area and runners. The plenum area acts as an air reservoir and is used to distribute intake charge to enhance engine breathing. Air enters the intake manifold and moves to the plenum area which directs it through the runners and into the cylinders.
The runners are flaps within the intake manifold that open and close as air bounces around between the valve and plenum area. Air-only runners have smooth finishes and a minimum number of bends. The length of the runners is designed to achieve the best performance during a particular range of engine speeds and changes when the engine develops its peak torque.
If these runners become stuck in some way it can stop the flow of air into the engine and compromise engine breathing. Most of the time if this happens your car will start idle irregularly or roughly; start to stall; backfire etc. which can be mistaken for engine problems when it’s the runners that have collapsed.
We hope that this has been informative and helpful, do drop a comment if you have any questions.