Catless downpipe with flex pipe for BMW 5 Series F10, F11, F07, 6 Series F12, F13, F06, 7 Series F01, F02, X5 with EWG F15, X6 with EWG F16 - engine code N55 — TA Technix

The downpipe is the tube that runs down from the turbocharger. This is where back pressure builds up: the turbine has to push the exhaust gases through a restricted section and often a catalytic converter. Enlarging it frees up the turbine, allowing it to build boost earlier and more decisively - it is the exhaust modification you feel most on a turbocharged engine.

What changes behind the wheel: response. The turbo takes less time to spool, and the power holds higher in the rev range. On a remapped engine, this is the part that allows the ECU tune to deliver what it promises; without remapping, the ECU compensates and much of the potential gain remains untapped.