Aerodynamics has always been a driving force in motorsport and road vehicle design and development, and continues to play an important role. A significant advancement in race car aerodynamics was the development of the vehicle underbody to produce downforce, in particular the implementation of the diffuser. This thesis concentrates on the performance flow mechanisms found in simple rear diffusers commonly seen in motorsport applications. There is little published work on these mechanisms, especially the influence of the more commonly used multiple-channel diffusers.