What Are the Functions of an Exhaust Manifold?by Richard Rowe
Although your engine's moving parts makes combustion possible, one of the most important simply sits there moving gases. The exhaust manifold (or "header" if made of separately welded tubes) moves spent combustion gases from the engine's exhaust ports to a central collection point ("collector"). From there, the gases go through the car's emissions system, mufflers and out the tail-pipe.
Almost any car can run without exhaust pipes of any kind, but no engine will run for long without a manifold. The exhaust valve is still slightly open when the engine's intake valve opens and the piston goes down (sucking in fresh air and fuel from the intake valve). Without a manifold, all of the cylinder's combustion gases would rush out in a hurry, making the exhaust valve (very briefly) the path of least resistance for airflow into the cylinder. Along with air and fuel from the intake, the engine would suck cold air in through the exhaust valve, exponentially increasing combustion chamber temperature and quickly melting the exhaust valve, valve seat and piston top. This phenomena is known as reversion and is a known engine killer.
Promote Exhaust Scavenging
Exhaust manifolds contain exhaust gas temperature and heat, which increases pressure inside of the contained area. The high pressures in the exhaust manifold force the gas to "squirt" out through the collector and into the exhaust pipe. Because the exhaust gases have mass, they also have inertia, meaning that they will create a vacuum in their wake as they exit the manifold. This effect is called "scavenging," and acts to suck the remaining gases out of the engine. Standard cast iron and "log" type manifolds typically exhibit little of this power-building scavenging; the effect is generally most pronounced in tubular headers, which are designed to enhance scavenging.
Speed Exhaust Flow
Like putting your thumb over the outlet of a hose to increase the water's range (speed and pressure), so to do headers/manifolds use somewhat narrow "primary" tubes to speed exhaust flow at low RPM. Very low exhaust gas velocity will cause much of the gases to remain in the engine at low RPM, resulting in a loss of torque. Exhaust manifold designers must strike a careful balance between creating primaries that are long and narrow enough to help speed exhaust gases through at low volume (low RPM) but are large enough so as not to restrict flow at high RPM. Exhaust gas velocity and scavenging are two sides of the same coin in manifold design; one will inevitably affect the other.
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