intake manifold

The intake manifold is a cast or fabricated assembly that sits between the carburetor or fuel injector and the cylinder head, routing a mixture of fuel and air into each cylinder. In four-stroke engines, it opens during the intake valve event to fill the cylinder with fresh charge at the start of each combustion cycle. The manifold must distribute flow evenly across all cylinders; uneven distribution means some cylinders run leaner or richer than others, causing power loss and rough running.

Intake manifolds vary greatly by engine design. Carbureted engines typically use a single carburetor feeding a symmetrical cast-iron manifold with individual runners to each cylinder. Port-injected engines have fuel injectors mounted in the manifold itself, one per cylinder, spraying fuel directly into the runner. Speed-density and mass air flow sensors located upstream measure the incoming air charge, allowing the engine computer to meter fuel accordingly. Modern manifolds are often plastic polymer composite with metal inserts where durability matters, reducing weight and cost compared to solid iron.

Flow, heat, and tuning

Manifold design critically affects cylinder filling and engine breathing. Runner length and diameter are tuned to exploit acoustic resonance: pressure waves reflect from the closed intake valve and return to the intake port, helping push additional charge in under certain rpm ranges. High-performance manifolds often feature stepped diameter runners that transition from smaller to larger bore, improving low-end filling without sacrificing high-rpm flow. Racing engines may use multiple carburetors with short, straight runners to maximize air velocity and turbulence in the combustion chamber.

Heat management is a secondary but real concern. Manifolds absorb waste heat from the cylinder head and warm incoming charge; some fuel condenses on cooler walls, affecting mixture distribution. Thermal barrier coatings or insulating manifold gaskets can reduce heat soak, keeping charge temperature down and combustion more consistent. Leaking gaskets between manifold and head are common failure points, causing air leaks that destabilize idle and create vacuum leaks detected by the engine computer.

The intake manifold sits at the boundary between cold-side and hot-side engine systems. Changes to manifold design, port volume, or runner geometry propagate through carburetor jetting, fuel injector sizing, and ignition timing maps. This is why intake manifold swaps on older engines require careful matching of fuel delivery and spark curve to avoid driveability problems.