The diffuser is an expansion device which works by speeding up the air flowing underneath a vehicle using the venturi effect. This creates a low pressure region below the vehicle which results in downforce. The diffuser enhances the transition or equalizes the high speed air flowing underneath the car with the slower freestream air by providing it space to expand and thus slow down. When designed properly, it can lead to a massive gain in downforce and even a reduction in drag. A majority of the research and development is restricted to motorsports 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 fins, strakes, flaps and multiple chambers. This paper investigates the effects of addition of fins/strakes and flaps, their geometry as well as location, on diffuser performance. Numerical investigations have been carried out using three dimensional, steady state RANS equations with the k-ω turbulence 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. Three strake geometries are considered with varying lengths, positions and number (of fins). Three flap designs with different sizes are studied at different locations. The fins/strakes increase the downforce by up to 35% and even increase the efficiency. They increase the effectiveness of the diffuser (especially in situations of yaw) by separating the different sections, preventing tyre squirt and aiding flow attachment by shedding vortices. 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.