Two-into-one exhaust (2-1)

A two-into-one (2-1) exhaust collects the header pipes from two cylinders into a single collector and one silencer. Similar layouts exist for other engines, such as 4-1 or 4-2-1 on inline fours.

Compared with two separate pipes (2-2), a 2-1 system is usually lighter and cheaper, and a well-designed collector uses exhaust pulses to help scavenge the cylinders, which can improve midrange torque. Many aftermarket systems for twins are 2-1 for this reason.

When fitting an aftermarket exhaust, check that it is road-legal and E-marked, that noise levels meet the rules and that the engine's fuelling is adjusted if the manufacturer recommends it. Also make sure the pipe does not reduce ground clearance or get too close to luggage.