SP5103,

A bit of clarity concerning the EMD "gear driven" turbocharger:

1) The external over-running clutch provides a positive gear drive arrangement through throttles idle/1 to about #6 at sea level. The higher the altitude, i.e. "thinner" the air, the lower the throttle position that the turning wheel will "come off the gear train".

2) The EMD turbocharger is a thermal-reactive design, which means that it is the temperature of the exhaust gases that impart the necessary energy to drive the turbine wheel off the gear train. The "pressure" of the exhaust gas is NOT a driving factor. Thus, at full load, and at VERY high altitude the turbine wheel could possibly exceed maximum RPM design limits, and as a result there is a built in HP limit in the electrical control system (there is a magnetic pick-up screwed into the side of the turbocharger housing to send the turbine wheel RPM signal to the load control software).