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Left side of a Ford Cologne V6 engine, clearly showing a (rusty) cast iron exhaust manifold - three exhaust ports into one pipe.

In automotive engineering, an intake manifold or inlet manifold is a part of an engine that supplies the fuel/air mixture to the cylinders. An exhaust manifold or header collects the exhaust gases from multiple cylinders into one pipe.

The word manifold may come from the Old English word manigfeald (from the Anglo-Saxon manig [many] and feald [fold]) and refers to the folding together of multiple inputs and outputs. Alternatively, it may come from the Latin mano (hand) and simply mean "hand-shaped".

The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine. It may also serve as a mount for the carburetor, throttle body, fuel injectors and other accessories to the engine.

Due to the suction effect of the downward movement of the pistons and the restriction caused by the throttle valve, in a reciprocating spark ignition piston engine, a partial vacuum (lower than atmospheric pressure) exists in the intake manifold. This manifold vacuum can be substantial, and can be used as a source of automobile ancillary power to drive auxiliary systems: (ignition advance, power assisted brakes, cruise control, windshield wipers, power windows, ventilation system valves, etc). This vacuum can also be used to draw any piston blow-by gases from the engine's crankcase. This is known as a closed crankcase ventilation or positive crankcase ventilation (PCV) system. This way the gases are burned with the fuel/air mixture.

The intake manifold has historically been manufactured from aluminum or cast iron but use of composite plastic materials is gaining popularity (e.g. most Chrysler 4 cylinders, Ford Duratec 2.0 and 2.3, and GM's Ecotec series).

Exhaust manifolds are generally and traditionally simple cast iron units which collect engine exhaust and deliver it to the exhaust pipe. However, when greater performance is required, this restrictive tube is often replaced with individual headers (also known as extractors in Australia) which are tuned for low restriction and improved performance. There are two types of headers. Collector style headers use pipes that merge into a collector, and can be utilized with mufflers. Collector headers can be used for the street or for a race car. Zoomie headers have no collectors, and are used exclusively on race cars. Headers have been widely available from aftermarket sources for decades, and some manufacturers have begun using them as original equipment. The Honda J30A2 engine does away with exhaust manifolds altogether, using an integral engine block passage to route gases directly to the catalytic converter.

• engine tuning