Remark Axle Back Exhaust System - 3.5" TipsIf you are looking for a way to make your 2015 21 WRX STi sound and look more aggressive, an axleback exhaust is a great alternative to a full exhaust system. The most restrictive component of the exhaust system are the muffler(s). By eliminating the mufflers, the engine will be able to expel exhaust gases more easily and reduce backpressure. The exhaust note will come alive as will the beloved boxer rumble. Completely bolt on, no modifications