NOx emissions control is one of the key factors to meet future emission regulations for internal combustion engines. NOx is controlled better by dosing urea, providing ammonia to Selective Reduction Catalyst (SCR). This study investigates ammonia distribution across SCR and introduces a new method for evaluating the ammonia uniformity index (UI) based on ammonia slip in a hot air rig, rather than traditional NOx conversion efficiency. Three silencer configurations were tested, each under four distinct operating points. For every operating point, an individual ammonia UI was calculated using the proposed method and then compared to the corresponding UI derived from NOx conversion.