The exhaust pipe immediately after the turbocharger. Aftermarket downpipes reduce restriction and boost lag, often used with tunes.
Downpipe is the exhaust pipe immediately after the turbocharger. Stock downpipes typically include a small catalytic converter (especially on European market cars) and have flow restrictions designed to meet emissions and noise requirements. Aftermarket downpipes replace the stock unit with a larger-diameter, less-restrictive design that improves exhaust flow from the turbo, reducing turbo lag and increasing power potential.
Downpipes are an essential modification for serious tuning of turbocharged cars. The stock downpipe restriction is one of the major bottlenecks limiting the engine power potential at higher boost levels. Replacing the downpipe with an aftermarket unit (typically 3 to 4 inches in diameter versus 2 to 2.5 inches for stock) allows the turbo to spool faster and the engine to flow more exhaust gas at high load.
Downpipe modifications often raise emissions concerns. Catless downpipes (with no catalytic converter) maximize flow but are illegal for street use in most jurisdictions due to emissions regulations. Catted downpipes (with high-flow catalytic converters) maintain emissions compliance but cost more. Some markets (particularly the European Union and California) have specific requirements for downpipe certification. Tunes specifically designed for downpipe modifications are typically required to extract the full power benefit.