Attached is a chart I made to compare between a turbo and non-turbo engine BSFC numbers. Both of these data sets are from freely available publications on military 6.5L engines, and are both done on JP-8 fuel. These are comparing BSFC numbers at specific torque outputs for two RPM levels. As you can see, the efficiency results are very similar at the 2000 RPM level.
Part of me wants to add a turbo to increase maximum torque at low RPMs. The other part of me wants to leave it N/A to reduce weight and complexity.
