4-2-1 engine exhaust systems

4-2-1 engine exhaust systems for automobiles are designed to ease the flow of exhaust gases of combustion leaving an internal combustion engine, thereby achieving optimal power output. Conventional exhaust systems, which are outfitted as standard components by automakers, are typically of the 4-1 type.

The system goes by the name "4-2-1" as a reference to the exhaust pipe layout for a 4-cylinder engine: four pipes (primary) come off the cylinder head, and merge into two pipes (secondary), which in turn finally link up to form one collector pipe. This collector then leads to the catalytic converter and muffler, before the exhaust gases finally leave the car through the tail pipe.

A 4-2-1 system often has the exhaust runners of cylinders 1 and 4 linked up, and cylinders 2 and 3 linked up, to form two separate secondary pipes of larger internal diameters than the primary pipes. Many systems, especially those on motorcycles, instead pair 1 with 2 and 3 with 4, the reasons for which are a matter of detailed exhaust design.

The pairings are defined by the intervals between firing events, that is the angle the crankshaft rotates in-between like events in each cylinder. In an even-firing, four-stroke four-cylinder engine, ignition intervals are 180° from cylinder to cylinder, following the designed firing order. Naturally, the same timing applies for exhaust valve openings (assuming the same cam profiles per cylinder), which dictates when each exhaust gas pulse escapes through the exhaust pipe.

In the case of inline fours, the pairing of cylinders 1 with 4 and 2 with 3 is considered "non-sequential", since a typical firing order would be something like 1-3-4-2 - anything else involves "sequential" pairings. Sequential implies a separation of 180-540 crank degrees between pulses in each merged section, non-sequential therefore being 360-360 degrees, which is also evenly spaced in this case. For individual banks of cross-plane V8s, where these 4-2-1 exhausts are often called "Tri-Y" exhausts, pairings can be 90-630 (sequential), 180-540 or 270-450 crank degrees, in numerous combinations - 360-360 is only achievable with cross-overs between the banks.

...
Wikipedia