Abstract: A two cycle, preferably diesel, engine has a turbocharger with an exhaust driven turbine and supplemental mechanical drive maintaining a fixed minimum turbine/engine speed ratio with fuel efficient freewheeling (fully exhaust sustained) turbine operation at higher speeds and loads. A turbo boost system provides freewheeling turbine operation at intermediate speeds where it might or will not otherwise occur. When an intermediate speed is reached at which freewheeling can be sustained but is not occurring, the system1) temporarily increases engine speed to a predetermined higher level to accelerate the turbine and increase engine air flow,2) increases engine fuel input to a higher level capable of sustaining freewheeling, and3) reduces engine speed to the selected intermediate speed while maintaining a fuel input level adequate to maintain turbine freewheeling. Methods and means for providing and controlling intermediate speed freewheeling operation are disclosed.