WRX Rear Diffuser, STI Rear DiffuserModel:  2015-2021 SUBARU WRX/STI REAR DIFFUSERFitment: 2015-2021 SUBARU WRX/STI - Type: Subaru WRX body kit, STI Body Kit - Made from Aerospace Lightweight ALUMINUM no ABS PLASTIC!!!! - Made in the USA - Comes fully CNC'd and professionally powder coated. - Comes pre-drilled - Fitment Guaranteed - Hardware included - Professional install highly recommended A rear diffuser is an automotive aerodynamic modification typically found mounted on the vehicle’s underbody located at the rear of the car, underneath the rear bumper. As a vehicle is in motion, air flowing in the underbody is typically at high-speed and has low-pressure. The design and purpose of a diffuser are to help reduce drag and to create downforce by facilitating the airflow underneath the vehicle’s underbody. A diffuser is used to increase the air volume along its full length, thus creating a void that must be filled by air. This in turn creates a Venturi effect, meaning the air flow is accelerated at the beginning of the diffuser creating a low pressure, thus creating downforce. As the air travels along the full length of the diffuser, the high-velocity air is eased back into standard velocity matching the ambient speed at the rear of the vehicle reducing overall drag. With the addition of vertical “fins” attached to the diffuser, this helps to optimize the diffusers efficiency by ensuring that all air drawn into the diffuser is from the underbody and not from the upper body surfaces. A diffuser can be further tune-up with the addition of a front splitter to help mediate the underbody’s airflow.