Specifically for the EJ25 its built with an unequal length header famous for the sound, but its inefficient where equal length have better aerodynamics increasing flow and allowing more power. Equal length Headers may flow better but unequal length headers save weight from shorter exhaust runners and allows you too hit boost faster by about 200-300 rpms due to a shorter distance between cylinder 1 and 3 that the exhaust needs to travel to the turbo. Equal length headers also allow you too run a twin scroll turbo letting you build boost immediately like in the newer FA motors. Over all equal length headers is your best option they may not sound like your traditional Subaru but it will allow you to squeeze out all power you can get.
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