Your Subaru air intake system has several functions, including filtering incoming air and reducing noise.  As enthusiasts we're willing to give up some of that noise-reducing in exchange for higher airflow.  Items in this section are meant to reduce pumping losses and increase flow for incoming air.  Note that Subaru ECUs are very sensitive to changes in plumbing in and around the Mass Air Flow (MAF) sensor housing.  We highly recommend using custom-developed ECU maps that go with your WRX or STI intake system, or custom dyno tuning to perfectly calibrate the ECU according the entire intake system.  See all WRX intakes, hoses, valves, and other intake parts below.