Downpipe with flex pipe for Audi A6 type 4G, Q5 type 8R, A7 type 4GA 3.0 TDI Biturbo — TA Technix

The downpipe is the pipe that runs down from the turbocharger. This is where back pressure builds up: the turbine has to push the exhaust gases through a narrowed 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 up, and the pull lasts higher in the rev range. On a remapped engine, this is the part that allows the ECU tune to deliver what it promises; without a remap, the ECU compensates and much of the potential gain remains untapped.