Downpipe with dummy catalytic converter for VW Transporter T4 - 1.9l TD + 2.5l TDI models — TA Technix

The downpipe is the pipe that runs down from the turbo. This is where back pressure builds up: the turbine has to push the gases through a narrowed section and, often, a catalytic converter. Enlarging it frees up the turbine, which builds 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 thrust continues higher up the rev range. On a remapped engine, this is the part that allows the ECU to deliver what it promises; without remapping, the ECU compensates and much of the gain remains untapped.