Scramjet engine combustion chamber is normally ground tested in connected pipe mode and has limitations in simulating all combustor entry parameters as in flight engine. Effect of variation in combustor entry pressure, test gas medium due to vitiation and combustor entry flow profile on combustor performance has been brought out through CFD simulations. Computations are carried out using in-house CFD software “PARAS-3D” which solves the Reynolds Averaged Navier-Stokes equations and uses 7 species 7 reaction model for combustion. Entry pressure is having substantial influence on combustor performance at pressures lower than a certain value and this pressure limit reduces for higher fuel equivalence ratios. In the case of vitiated medium, presence of CO₂ and the excess H₂O result in lower total temperatures leading to reduced pressures in the combustion chamber. Non-uniform flow profile at combustor entry is also found to lower the scramjet combustor performance due to reduced mixing.