2015-2018 MITSUBISHI G4 DIFFUSER
A rear diffuser is to efficiently increase the downforce of a vehicle. This helps increase grip and reduce aerodynamic drag.
How does a rear diffuser work ?
As a vehicle is driven down the road, high-speed air is passing underneath the vehicle at low pressure. As the air passes to the rear of the vehicle through the diffuser, the air expands through an expansion chamber. Through this expansion, air speed is reduced and pressure is increased (through what is called the Venturi effect). This pressure differential between the low-pressure air under the car and the high pressure at the rear creates a vacuum sucking air out from under the car. With high-pressure air above the car and low pressure under the car, high downforce is created while reducing aerodynamic drag.