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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.
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. This manifold vacuum 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 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. 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