4-2-1 engine exhaust systems

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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.[1]

Design criteria[edit]

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.

The gas pulses emitted from each cylinder can interfere with each other when they meet, and this consideration dictates the lengths of the pipes necessary. Clearly, the selection of cylinder pairing impacts the timing between pulses that communicate through the chosen tube lengths. Generally, longer pipes will help produce more power at lower engine rpm, and shorter pipes favour high-rpm torque, thereby altering the power curve.[2] However, the gases tend to cool as they pass through longer pipes, thus delaying catalytic converter activation. This can be overcome by coating the pipes with ceramic to retain the heat.

In contrast to a naturally-aspirated engine, a turbocharged engine would warrant pipes with different internal diameters, due to the higher exhaust gas mass flow rates and higher pressures arising from the boosted cylinder filling and the need to drive the turbine.

The primary lengths in the 4-2-1 system are not nearly as critical as the lengths of the secondary pipes. The engines are very sensitive to changes in the length of secondary sections, and most of the development effort is focused on secondary merge, length, diameter and step issues.[3]


Generally, engines on road-going cars are designed to meet high power requirements only at higher ranges of rpm. This gives the car better performance on highways and low- traffic roads. But for local commuting - which includes driving on busy streets and narrow roads - involving operating in low-to-medium rpm for significant periods of time, car performance would get slightly degraded. 4-2-1 engine exhaust systems have been specifically designed to help in such scenarios. The 4-2-1 design of the exhaust header shifts the power band slightly to the left of the normal engine power band, thereby providing higher output at middle-range rpm.[4] This can be realised by conducting experimental tests on an engine dynamometer.[5]

This type of exhaust system provides a significant mechanical advantage by creating low back pressure at the engine’s exhaust valves. Lower back pressure can improve the engine performance by optimising the exhaust gas flow characteristics, decreasing turbulence and mixing, thereby transmitting higher torque at mid-range rpm.[6][7]

Since 4-2-1 headers merge into two pipes before linking up to form the collector, there is usually more ground clearance.


As the engine rpm increases, back pressure rises abruptly, generating higher turbulence and degrading exhaust gas flow characteristics. Exhaust gases face restriction in being expelled, and contaminate the fresh air/charge drawn into the cylinder on the next stroke. Hence, there is a reduction in power and engine performance at higher rpm.

A poorly designed 4-2-1 exhaust system can cause peaks and troughs in the power curve. Naturally, this is undesirable, because shifting gears to match rpm and power requirements would be a challenge.

Short 4-1 vs. Long 4-2-1 headers[edit]

Short 4-1 headers are conventionally used to obtain high torque at high rpm. However, they face such problems during scavenging as internal mixing of exhaust gases with the incoming fresh air/charge.

Consider a four-stroke engine, with a 1-3-4-2 firing order. When the exhaust manifold is short, the high pressure wave from the gas emerging immediately after cylinder No. 3’s exhaust valves open, for example, arrives at cylinder No.1 as it finishes its exhaust stroke and enters its intake stroke. As a result, exhaust gas which has just moved out of the cylinder is forced back inside the combustion chamber, increasing the amount of hot residual gas. With a short exhaust manifold, the high pressure wave arrives at the next cylinder within a short amount of time, causing this adverse effect to continue from low to high engine speed. Hence, shorter length causes contamination and produces lesser power.

While in long 4-2-1 headers, exhaust gases get cooled on their way, because of the huge heat loss due to radiation, eventually delays the catalyst activation. Exhaust temperature can be increased by delaying ignition timing, but unstable combustion will result if delayed too much.

See also[edit]