Cast-iron turbo manifold with T25 flange/with wastegate connection for 1.8T-20V Audi/Seat/Skoda/VW engines — TA Technix

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

Material choice 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 always eventually open up a weld.