I posted this on Eng-tips and no replies yet:( I'm trying to design an as-simple-and-cheap-as-possible supersonic wind tunnel using an air compressor (this even possible? given the energy losses from shock waves...). I have access to a Porter-Cable 150psi, .8hp air compressor. Questions: 1. Just the air compressor itself: If outlet exposed to atmospheric pressure Patm, choked flow achieved when tank pressure P0 = Patm / .528 = 28psi. BUT, the specs of my air compressor say 2.6CFM at P0=90psi. If outlet is .25" diameter, then Vout=2.6CFM / A = 38m/s, way lower than Mach 1 for choked flow. What's going on here? 2. If outlet is indeed choked, then P0=28ksi minimum. Say I increased P0 to potentially 150psi. I understand that mass flow and density increase linearly with P0 in choked flow. But, if I add a diverging duct to outlet, would the beginning portion of the duct experience choked flow? I just want to understand if "over"-choked flow (P0>28psi) gives me any leeway to increase area and still keep flow choked. I may be thinking too hard/wrong. Any help appreciated!