Diffusers work in conjunction with other aerodynamic components of a car to help produce downforce. Because a car displaces air as it moves forward, the air molecules in the rear of the car get disrupted causing a vacuum of low pressure air.
To help channel the high speed air flow coming from underneath the car, diffusers redirects the high speed air flow to transition smoothly transition into the low pressure vacuum in the rear. This encourages a smoother flowing air flow underneath the car and allows better performance from other aerodynamic components such as aerodynamic wings.
What is a diffuser: A diffuser is a shaped part of a car's body, usually found in the underside of the car. It can be located at the rear underside, or closer to the front underside near the wheel wells. A rear diffuser is typically sloped upward from front-to-back, and may have vertical fins attached to the bottom of the sloped surface. The rest of this article will apply, for the most part, to the rear diffuser.
The diffuser has two (2) purposes: 1. To reduce drag. 2. To reduce lift.
What goes on at the rear-end of a car: The area immediately behind the car tends to have turbulent, slow-moving air. The fast underbody air that exits the car ends up meeting with this slow-moving outside air. The greater the air speed difference is, the more turbulence is created. This turbulence causes unwanted drag and disrupts the airflow that exits from the underbody, which in turn decreases the underbody's ability to reduce lift.
How the diffuser works: The diffuser itself does not actually create downforce - it works in conjunction with other aerodynamic components to reduce drag and lift. The upward front-to-back angle of the rear diffuser causes the fast-moving underbody air to expand and slow down. This slower air is then better able to meet up with the slow outside air, thus reducing the amount of turbulence behind the car. The vertical fins are there to make sure that the air at the rear left and right sides do not disturb the function of the diffuser.
When is a diffuser effective: As a general guideline, the rear diffuser needs to be angled upward at least 10 degrees. In order for a rear diffuser to be effective, the air that travels under the car from the front needs to be as fast and smooth as possible. Slow-moving, turbulent air is not going to help a diffuser do its job.
In order to create the smooth air that the diffuser needs, it is recommended that the car have a flat undertray (a.k.a. underbody) that covers most of the uneven surfaces and components on the cars bottomside. However, production road cars typically do not have, or only partially have, an area that is covered by a flat undertray.