This study examined a high-speed, high-powered diesel engine featuring a pent-roof combustion chamber and straight ports, with the objective of improving the specific power of the engine while minimizing any increase in the maximum cylinder pressure (Pmax). The market and contemporary society expect improvements in the driving performance of diesel-powered automobiles, and increased specific power so that engine displacement can be reduced, which will lessen CO2 emissions. When specific power is increased through conventional methods accompanied with a considerable increase in Pmax, the engine weight is increased and friction worsens. Therefore, the authors examined new technologies that would allow to minimize any increase in Pmax by raising the rated speed from the 4000 rpm of the baseline engine to 5000 rpm, while maintaining the BMEP of the baseline engine. A cycle simulation of the engine was initially investigated to examine the specifications of the intake and exhaust systems when the rated speed of the engine was raised by 1000 rpm. On the basis of those findings, a single-cylinder engine was constructed with a pent-roof combustion chamber and straight ports. This revision of the specifications worsened combustion, however, and the gross IMEP was found to be lower than that of the baseline engine. The causes for the worsening of combustion in experiments were analyzed, and the shape of a pent-roof combustion chamber and specifications of a fuel injection system that would provide better combustion were identified. A 4-cylinder engine was then constructed under those specifications. At the rated speed of 5000 rpm, its specific power was measured at 76 kW/L, which is over 25% better than that of the baseline engine, while increases in Pmax and exhaust temperature were minimized. It was also found that in addition to the raised rated speed, the increased intake air mass flow was a factor contributing to suppression of both the Pmax and of the exhaust temperature.