How Rear Diffusers Work

This Corvette's exhaust pipes sit in the middle of its diffuser cut-out (visible to the left of the pipes).

You see them under the rear bumpers on touring cars, Formula 1 cars and many high-dollar exotics--those strange vertical fins that fill the large gap between the bumper and the ground. Diffusers are a little-understood aero-enhancement that can make or break a car's handling and stability at high speed.

The Venturi Effect

Imagine a cardboard tube with one side bent in toward the middle. As air flows through the tube it compresses on the front of that kink and speeds up to squeeze through the gap between the kink and the rest of the tube. After it gets past the kink, the air slows down and decompresses. This decompression creates a little vacuum force on the back side of the kink in a phenomenon known as the Venturi Effect.

Under-Car Aero

Imagine the bottom of your car as the kink in that cardboard tube. Air flowing under the car's front bumper compresses and speeds up to squeeze between the car and the ground. A diffuser is essentially a tunnel or void in the floorpan that allows air to expand and create a vacuum under the chassis. This vacuum helps suck the car to the road at high speeds to enhance stability with little to no penalty in aerodynamic drag.

The Vertical Fins

The vertical fins under the rear of the car help to guide air outside. Without these fins, air exiting the underside of the car would tumble and swirl, reducing vacuum under the car (ground effect) and increasing drag. You may also notice that the rear bumper on many diffuser-equipped cars is set much higher than the front bumper; this is to allow air to leave as slowly and efficiently as possible.

References

• Recumbents: Car Aerodynamics 101
• McGraw-Hill's AccessScience: Race Car Aerodynamics
• Symscape: Secrets of Underbody Tunnels, Rear Diffusers and Venturis