@SebAvi Well, ISA temperature at FL180 is -21°C, so we know the density altitude is higher by approximately 2,000 ft. That means performance started to degrade at a density altitude of more like 16,000 ft, which I think is reasonable for a turbocharger installation of this nature. However, in my own testing the other day, I didn't see any loss of performance until between 17,000 and 18,000 ft.