The Perrin drop-in air filter for your WRX and STI uses a 2-stage foam filtration element to improve airflow in your stock air box; providing improved power and airflow while removing harmful particles from the incoming air. Using a special oil coating, the foam filter catches dirt particles while allowing air to pass unimpeded through the high-flow foam. This filter can be cleaned and re-oiled using the Perrin cleaning kit, sold separately.