What is the turbine efficiency of a modern jet engine? Just the turbine efficiency only. What percentage of fuel energy the turbine converts into mechanical energy.
Slide 7 of this presentation (shown above) states the latest technology for a turbine with cooling flow is 89% efficient, while an uncooled turbine is about 90% efficient. The table from this presentation comes from the well renowned book by Jack Mattingly, "Elements of Propulsion: Gas Turbines and Rockets".