| discussing a much broader spectrum of data than just a single psi point. Efficiency actually is more tied into airflow, which directly ties into engine RPM. Here's a map of a stock turbo compressor:
 Max efficiency is 73% between ~10#/min airflow and 15#/min and a pressure ratio of 1.35 to 1.7 (5.1psi to 10.2psi) One may think that the turbo will produce the most power through this airflow and pressure region of its operation, but this is neglecting several other factors. Although it will produce the most power per psi and #/min of airflow than in any other operating condition, this datapoint is not reflective of the system's performace as a whole. At the datapoint of 20#/min and a PR of 2.2 (17.4psi) there is a total CFM flowrate of 122CFM which equates to 4700RPM. The compressor is operating at a 68% efficiency at this point. It goes without doubt that although the compressor would be operating more efficiently at 10psi of boost and ~2200RPM, it isn't going to make more power than when it is operating at 17psi at 4700RPM. In addition, at the very top of the map is a point with the same efficiency, 2.5PR (21.75psi) and 21#/min airflow (4500RPM) - this would definately make more power than at 17psi and not produce any more "heat" per psi than at 17psi (although the total air temp would be higher by about 9%), given proper intercooling, you can bring that charge temp down and in effect, offset the efficiency of the turbo. FWIW, at that pressure/efficiency and a 70-degree ambient inlet temperature at sea level, the compressor outlet temp will be 301F. We saw the same compressor discharge temps on Dee Ridgeway's TT at 26psi with the GT28RS and after the intercooler it had dropped down to just a hair over 100F. Stock intercoolers would have long since choked the airflow, adding even more heat into the equation, however, they would also not have been able to handle the task of dissipating this amount of heat and you would begin fighting detonation issues long before as well. Long story short, compressor efficiency by itself is not the be-all end-all of power output determination. Otherwise, that "other logic" would tell us that we shouldn't run more than 10psi of boost or turn the motor faster than around 2300RPM (suprisingly enough, you couldn't even generate 10psi of boost at 2300RPM even if you wanted to. :)
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When talking about efficiency of a turbocharger, you are - 
