Manifold (automotive engineering)

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" literally means "hand shaped", which arises from the "fingers" leading to or away from each cylinder.

Due to the suction effect of the downward movement of the pistons in a reciprocating piston engine, a partial vacuum (lower than atmospheric pressure) exists in the intake manifold. If the engine has a throttle valve (i.e. in spark ignition rather than diesel engines) 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, windscreen wipers, power windows, ventilation system valves, etc).This vacuum can also be used to 'suck' 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 is usually made of aluminium and located between the carburetor and the cylinder head. On multi point injected engines, the intake manifold holds the fuel injectors.

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 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.

See also: engine tuning