For many years now, injection has been of great utility in the wind tunnel activity. Sometimes injection is the sole source of power. Other times it is used in an auxiliary fashion in conjunction with the main compressor. This paper focuses on this last instance. In a transonic facility, some supersonic injectors are installed at the entrance of the transition section, with the ultimate aim of expanding the Reynolds number envelope without demanding extra power of the main compressor. The objective of this work is the numerical investigation of the mixing process between the supersonic streams coming out of the injectors, and the tunnel main subsonic current. In a first part of this reporting, the present authors have advanced the basic information, and discussed the results for the physical situation that was designated as “the design point”. This corresponds to a setting such that, exactly at the entrance of the mixing section — where the two flows start to interact —, the static pressures of the two mixing streams are equal. To obtain such state, one acts upon the stagnation pressure of the supersonic nozzles. The design point corresponds to a working condition, for which, in principle, losses are minimized. The aim of the present paper is to report upon a set of conditions that we refer to as the “off-design points”; here, the initial static pressures of the two currents are not equal. Besides investigating these new settings, many comparisons with the operation at the design point are also performed. Initially, an overview of the flow field for all cases is presented, and attention is called to the expansion and compression domes, that now, for the off-design points, are much more illustrative. After that, a thorough study is undertaken comprising the mixing layers growth behavior, and the performance of the mixing chamber in terms of pressure losses, overall gain and efficiency. As a consequence of these analyses, the nozzles stagnation pressure for a zero gain was obtained. This is an important result, because it represents the threshold for the efficient use of the injection process.