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The Ahmed body is one of the most studied 3D bluff bodies used for automotive research and was first proposed by Ahmed in 1984. The variation of the slant angle of the rear upper surface on this body generates different flow behaviours, similar to a standard road vehicles. In this study we extend the geometrical variation to evaluate the influence of a rear underbody diffuser which are commonly applied in high performance and race cars to improve downforce. We perform parametric studies on the rear diffuser angle of two baseline configurations of the Ahmed body: the first with a 0 degree upper slant angle and the second with a 25 degrees slant angle. We employ a high-fidelity CFD simulation based on the spectral/hp element discretisation that combines classical mesh refinement with polynomial expansions in order to achieve both geometrical refinement and better accuracy. The diffuser length was fixed to the same length of 222 mm similar to the top slant angle that have previously been studies The diffuser angle was changed from 0 to 50 degrees in increments of 10 degrees and an additional case considering the angle of 5 degrees. For the case of a 0 degree slant angle on the upper surface the peak values for drag and negative lift (downforce) coefficient were achieved with a 30 degrees diffuser angle, where the flow is fully attached with two streamwise vortical structures, analogous to results obtained from Ahmed but with the body flipped upside down. For diffuser angles above 30 degrees, flow is fully separated from the diffuser. The Ahmed body with 25 degrees slant angle and a diffuser achieves a peak value for downforce at a 20 degrees diffuser angle, where the flow on the diffuser has two streamwise vortices combined with some flow separation. The peak drag value for this case is at 30 degrees diffuser angle, where the flow becomes fully separated.