In this paper a study to analyse the influence of exhaust gases thermal energy saving and reduction of pressure pulses interference on engine dynamic performance during the load transient of high speed direct injection turbocharged diesel engines is described. The analysis has been performed thanks to the use of a dual walled air gap exhaust manifold and a 4-2-1 pulse exhaust manifold, which have not only been modeled but also manufactured and tested on an engine test bench able to carry out constant speed load transient tests. Finally, in order to estimate maximum improvement in engine performance, an optimum design of a synthesis exhaust manifold has been proposed and modeled showing that exist a potential not only to improve engine transient performance at low engine speeds but also the gas temperature at the catalyst inlet and the steady effective torque.