Scavenging for a Living
The engine exhaust manifold in your car is linked directly to the cylinders where gas and oxygen mix together before being ignited by a spark plug. Once the fuel has been burned, the cylinder's exhaust valve opens up to allow the waste gases to escape, sending them out into the manifold. While the pressure inside the cylinder is strong enough to push out most of the gas, the manifold itself has to be designed in such a way that it sucks as much exhaust from the cylinder as possible. This is called scavenging. Some exhaust manifolds are also engineered to concentrate the heat of the gases so that they burn off any remaining fuel that might linger after combustion.
Sensing, Then Sending
Most engine exhaust manifolds feature an oxygen sensor mounted near their exit. This sensor checks the level of oxygen in the exhaust gas, and then transmits that information to your vehicle's engine computer, where it's used to shape the air/fuel mixture in each cylinder.
Out Through the Tailpipe
The oxygen sensor works while the gases collected by the engine exhaust manifold are expelled to the rest of the exhaust system, where they shoot through the catalytic converter and muffler before leaving via the exhaust tip. Just as a manifold needs to scavenge as much gas as it can, it also must act as a funnel that concentrates and then uses exhaust pressure to expel gases with the force required to exit the system.
Some vehicles use an exhaust header in place of traditional manifolds. This is particularly true in high-performance automobiles, where maximizing horsepower is a priority over absolute efficiency. Headers are designed to offer low restrictions to gas flow, so they can rapidly extract exhaust through extremely effective scavenging. Although, depending on their design, they can impact torque at lower engine speeds. You can replace an exhaust manifold with a header, but it's rare for the opposite to be true without negatively affecting engine operation.
