I presume that the butterfly valve opperated by the govenor on thelve likes of a Stuart Turner S-9 has a similar effect to a needle valve, and this seemed to work ok, throttling the air supply while still maintaining adequate power on an inlet pressure of 30 to 60 psi. The engine would opperate down to a pressure that went off the gauge, probably less that , as long as the volume of air was enough, put the engine on a smaller compessor with insufficient air volume, and no way would the engine run and keep running.
Ian S C