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How an Exhaust Manifold works

Combustion is a nasty business. An engine may ingest air to mix with fuel inside the cylinders, but it only consumes the oxygen. That leaves super hot gases that must exit the motor so that a fresh supply can enter to restart the whole process again within milliseconds. The path these exhaust gases must take starts with the exhaust manifold.



The exhaust manifold’s importance to combustion
To make horsepower the modern motor completes a four-stroke cycle, the last of which is the exhaust stroke. At this point, a piston is ready to force the burnt mixture out through an exhaust valve. This flow exits through ports in the head and into the exhaust manifold, where it’s collected and routed to the muffler. It’s the first part of a vehicle’s complete exhaust system, absorbing the shock of left over air/fuel mixture.



The exhaust manifold not only collects and directs the flow out of the engine, but it’s also responsible for the speed of the gases exiting. Exhaust gas velocity is a result of the manifold’s size and design, all due to the harmonics of combustion. Despite the manifold looking like a grouping of pipes, there isn’t a constant flow as we know it. In a motor, exhaust gases leave in pulses, or waves, that need harmonization to pull each other along. Called the scavenging effect, the exhaust manifold enhances this as to reduce potential power losses through back pressure, the situation where the engine must use a greater effort to push these gases down the exhaust system.



Secondary functions
Since exhaust gases can reach up to 1800°F, manifolds need to contain this high temperature flow within itself as not to damage its surroundings. Manufacturing it out of materials resistant to high temperature and fatigue, such as thick-walled cast iron, achieves this as well as additional heat shielding in many cases. Besides feeding the catalytic converter, exhaust manifolds help with emissions by housing the oxygen sensor. This is a very important job as the ECU monitors the amount of oxygen in the exhaust stream checking the air/fuel ratio within the motor and adjusting as necessary for optimal efficiency.

What might seem as a bunch of cast iron pipes hanging off the motor is an integral part of its operation. Exhaust manifolds keep the harmful effects of combustion, heat and unfriendly gases, away from the engine bay while allowing the ECU to monitor engine health.