Cast-iron turbo manifold with T3 flange/with wastegate connection for 1.8/2.0-16V VW engines — TA Technix

The turbo manifold supports the turbine and withstands temperatures of up to 900 °C: it is the most heavily stressed component in a turbocharged engine. A clear-flowing design builds boost earlier; a suitable flange allows you to install a larger turbine than the original, and therefore target a different power level.

The choice of material matters just as much as the design. Cast iron withstands heat cycles without cracking and dampens vibrations. Stainless steel is lighter and allows better designs, but requires well-designed supports: a cantilevered turbine will eventually open up a weld.