terminology, the first component of an exhaust
system; the pipe
that leads immediately off the manifold
An important part of performance tuning consists of matching the header to the engine configuration and vice versa, because the header's geometry determines the flow of exhaust gases from the cylinder. The maximum throughput of a header helps determine how freely the engine can breathe, which directly affects maximum power.
At the same time the header has to supply a certain amount of back pressure or the exhaust valves will not cycle properly, causing power loss and/or valve damage.