In this paper, the combustion chamber of a scramjet at Mach 2.2 inlet has been investigated in two dimensions using RANS simulation. The fuel injection distance is located upstream of the cavity and the focus is on finding the appropriate distance between the injection location and the front wall of the cavity. For this purpose, five injection locations have been investigated in two cavities with a back wall angle of 45 and 90 degrees, and the best distance range has been determined by comparing performance parameters such as total pressure loss and combustion efficiency. Also, in order to simultaneously weight these two important parameters, a new parameter definition called total performance has been used, which is obtained by dividing the combustion efficiency by the total pressure loss. A comparison of these three parameters in different investigated cases shows that a distance of 20 mm from the front wall of the cavity was the most suitable option in both types of cases investigated (back wall angle of 45 and 90 degrees). It should be noted that in this study, constant inlet flow conditions were considered and its effects on the injection distance were ignored.