What are the possible ways to increase the power output of a production engine (CI and SI)?  Brake torque initially (at lower speed) increases with engine speed and after certain engine speed starts...

What are the possible ways to increase the power output of a production engine

(CI and SI)?

 Brake torque initially (at lower speed) increases with engine speed and after

certain engine speed starts decreasing in engines. Suggest reasons for the given

observation.