  Header (Exhaust Manifold) and Up Pipe
The factory manifold and up pipe are another source of restriction in the flow of exhaust out of the car. The manifold is the stuff that gets exhaust gas out of the heads, and most of the way to the turbo. The up pipe is the last piece of tube that consolidates all the exhaust gas from the four manifold tubes and carries it into the turbine housing.
The Prosport header pictured above combines the header and up pipe into one package, packaged and sold together.
Tubular header manifolds are very expensive to build. There are many bends, and the tolerances are tight. For this reason, the header upgrade is a pretty expensive way to make power.
The up pipe is not very long, but on the 2002-03 WRX, and the Legacy GT through 2006, it does contain yet another catalytic converter. If one were (not legally, of course) to remove this cat, one could gain some more horsepower. Theoretically. ;)
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