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The exhaust system is often the greatest area of design inefficiency related to engine performance. Exhaust gases leaving the engine need to be expelled, yet there is always a need to slow the exit of these exhaust gases to provide "back pressure": if the exhaust gases leave too quickly, there is insufficient back pressure and the engine power output drops; if the exhaust gases leave too slowly, the engine needs to push the exhaust out using some of its power for that purpose. A typical exhaust system will collect exhaust from a cylinder in what is often called an "exhaust manifold": a big shared chamber in which a cylinder pushes its exhaust gases. As each cylinder is firing in sequence, it is trying to push its exhaust into the manifold where there is still high back pressure from the exhaust of the previous cylinder.
What an exhaust header does to alleviate this problem is provide an individual exhaust tube for each cylinder and merge these tubes together much later in the exhaust pipe where the combined exhaust back pressure would be much lower1. Since the engine if pushing against less back pressure to expel the exhaust gases, it is using less power for that purpose2. Exhaust headers can be very pricey, but they can radically change the personality of an engine!
Last update: January 17 2011
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