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4-2-1 engine exhaust systems for automobiles are aftermarket equipment 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.〔http://auto.howstuffworks.com/421-exhaust-system.html〕 ==Design criteria== A 4-2-1 system 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. Since four-cylinder engines have a 180 degree crank angle, cylinders 1 and 4 exhaust their gases at different times than cylinders 2 and 3. These gas pulses must not interfere with each other when they meet, and this consideration dictates the lengths of the pipes necessary. Longer pipes will help produce more power at lower engine rpm, and vice versa, thereby altering the power curve.〔http://www.billzilla.org/engexhaust.html〕 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 larger internal diameters. 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.〔http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「4-2-1 engine exhaust systems」の詳細全文を読む スポンサード リンク
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