Over the past few years, car design has evolved significantly with more focus on aerodynamics, airflow is more important than ever. With the advancements  in aerodynamic design, aero elements that originated in Racing have found their way onto today’s street cars. Today’s BMWs feature factory rear diffuser panels that can easily be swapped or replaced, and aftermarket companies offer rear diffuser treatments for a wide variety of makes and models. So what exactly is a rear diffuser, and what is it for?



The diffuser is usually located at the back of the car, and its aerodynamic purpose is to increase the airspeed of the air flow underneath the car. The air pressure beneath the car is affected by the diffuser, so that the air pressure can normalize to ambient pressure in the diffuser as the car moves through the air. Much like a rear spoiler and an airplane wing make use of Bernoulli’s Principle, so that pressure decreases as airspeed increases. Because the air pressure below the car is lower on than it is on the sides and top of the car, a properly implemented diffuser will create downforce. Forward diffusers are also present on LeMans racecars, but these generated downforce purely from the “momentum exchange” from the air.


As Brawn GP proved to the rest of Formula One a few years back with Jenson Button, Injecting the exhaust flow into the rear diffuser can also help extract air from below the car. Today’s cars often feature diffusers integrated right into the rear bumper fascia – some functional, some purely aesthetic. Either way, adding a new rear diffuser to your BMW, Scion FR-S or other sports car will give your car a more aggressive, track-ready look.


In addition to the aerodynamic function and styling of a rear diffuser, a different style diffuser can also allow you to run a different exhaust configuration than stock, for example converting from single side exit to dual exit exhaust or quad tip exhaust setups.


Interested in a rear diffuser for your BMW? We offer diffusers for most late model vehicles- check out our selection at or call us at 714-582-3330 to speak with a Modification Expert about what diffuser setups are available for your vehicle.

Story by Nicholas Gregson
Photos by Doug Johnson, The Driven Lifestyle