Catless downpipe for BMW X5 xDrive 30d/40d Series type E70 - engine code N57 — 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. Widening 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 and the power holds higher in the rev range. On a remapped engine, this is the part that allows the ECU tune to deliver its advertised performance; without remapping, the ECU compensates and much of the potential gain remains untapped.