Honda Civic Bumper Diffuser Chrome Single Exhaust – Model 2016-2017.
The aft part of a car underbody is where a diffuser is usually located. It works by accelerating the velocity of the airflow underneath the car. The pressure under the car is affected by the diffuser so that it can expand back to ambient in the diffuser, as the car moves through the air. It uses Bernoulli’s principle, such that the pressure decreases while the velocity increases. Since the pressure below the car is lower than on the side and above the car, downforce is produced if implemented correctly. The diffuser “drives” the underbody, which produces the downforce. Front diffusers also exist (especially on leMans Prototypes or similar cars); however, they generate downforce purely from momentum exchange with the air, as there is nothing ahead of them to drive. A poorly designed front diffuser can create a low pressure region toward the front of the car which slows the air behind it down and reduces the effectiveness of the rest of the underbody. Front diffusers usually route air away from the car so that it doesn’t affect the rest of the underbody. The air can be vented through a channel or expelled near the front wheels.
Injecting the exhuast into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This fast-moving air helps evacuate the diffuser more quickly, which helps drop the pressure at the underbody. However, this makes the diffuser rather sensitive to engine speed. When the driver lifts off the throttle, the exhaust flow is greatly reduced, which makes the diffuser less effective, robbing the vehicle of downforce. Thus, handling is negatively affected.