Sure, an original BMW M4 rear diffuser gives your sports car a sleeker look. However, rear diffusers in general also benefit sports cars tremendously by increasing downforce. This means both a better grip on the road and reduced air resistance, more commonly known as drag. In short, factory rear diffusers enhance the aerodynamics of a BMW M4 overall, resulting in a more exciting and enjoyable drive. How does a rear diffuser work? Its three parts -- a set of vertical fins, an expansion chamber, and a smooth underbody -- work together to draw high-speed, low-pressure air under the car and re-expand it. This lowers air speed and increases air pressure. The result: a vacuum effect. The process sucks air back out from under the car; high-pressure air remains above it and low-pressure air stays below. Downforce then increases, drag decreases, and your BMW M4 handles much better, with resistance at a minimum. When it's time to upgrade your rear diffuser, avoid the aftermarket. These types rarely work; they're usually for aesthetic purposes only. For best results, buy a genuine BMW M4 rear diffuser. Not only will it be guaranteed compatible with your coupe or convertible right off the line. It'll also deliver the results you're looking for both down the road and on the track. You can order one online today, too, right from our auto parts counter.