Downpipe for Audi A4 S4/RS4 type B5, A6 2.7 Biturbo/A6 Allroad type 4B — TA Technix

The downpipe is the tube that runs down from the turbocharger. This is where backpressure 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 - this 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 power delivery 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 unused.