The automotive underbody diffuser is an expansion device which works by speeding up the air flowing underneath a vehicle. This reduces the pressure below the vehicle thereby increasing downforce. When designed properly, it can lead to a massive gain in downforce and even a reduction in drag. However, a majority of the research and development is restricted to motorsport teams and supercar manufacturers and is highly secretive. Most of the publicly available research has been done for very simple shapes (bluff bodies) to study the effects of ground clearance and rake angle. Very little research has been done for complex geometries with vanes, flaps and vortex generators. This paper aims to investigate the effects of the addition of vanes/strakes and flaps, their location as well as angle, on diffuser performance. Computational Fluid Dynamics simulations have been carried out using three dimensional, steady state RANS equations with the k-ε turbulence model on STAR CCM+ V9.06. The simulation methodology has been verified using experimental data first. The diffuser geometry is based on the Formula SAE car developed at the University. Vanes and flaps have been simulated at various positions and angles. The flow features are studied and the performance is quantified in terms of downforce and downforce to drag ratio or efficiency. The vanes increase the downforce by up to 13% and even increase the efficiency. They increase the pumping action of the diffuser and isolate the different channels, minimizing the adverse effects of tire squirt. The addition of a flap above the trailing edge of the diffuser also has a marked effect as it evacuates more air from underneath the car. A maximum improvement in downforce of 25% is seen with the vane and flap used together.