An arrangement or collection of pipes or tubing with several inlet or outlet passages through which a gas or liquid is gathered or distributed. The manifold may be a casting or fabricated of relatively light material. Manifolds are usually identified by the service provided, such as the intake manifold and exhaust manifold on the internal combustion engine. Some types of manifolds for handling oil, water, and other fluids such as engine exhaust gas are often called headers. In the internal combustion engine, the intake and exhaust manifolds are an integral part of multicylinder engine construction and essential to its operation. See Internal combustion engine
The engine intake manifold is a casting or assembly of passages through which air or an air-fuel mixture flows from the air-intake or throttle valves to the intake valve ports in the cylinder head or cylinder block. In a spark-ignition engine with a carburetor or throttle-body fuel injection, the intake manifold carries an air-fuel mixture. In an engine with port fuel injection or in a diesel engine, the intake manifold carries only air. For the diesel engine, the air should be inducted with a minimum of pressure drop. The purpose of the intake manifold is to distribute the air or air‐fuel mixture uniformly to each of the cylinders and to assist in the vaporization of the fuel.
The engine exhaust manifold is a casting or assembly of passages through which the products of combustion leave the exhaust-valve ports in the cylinder head or cylinder block and enter the exhaust piping system. The purpose of the exhaust manifold is to collect and carry these exhaust gases away from the cylinders with a minimum of back pressure.
Some automative spark-ignition engines have an air-distribution manifold as part of the exhaust-emission control system. This manifold distributes and proportions air to the individual exhaust ports through external tubing or integral passageways.