Computational optimization of a high-speed diesel engine, combined with diesel engine size-scaling, is presented. A multi-objective genetic algorithm was employed to simultaneously optimize fuel consumption and engine-out emissions of the down-scaled version of a previously optimized baseline engine. By separating the design parameters into hardware parameters (e.g., the piston bowl geometry) and controllable parameters (e.g., injection pressure and timings), multiple operating conditions were optimized simultaneously. A new variable was introduced to evaluate the convergence of the optimization, defined as the ratio of the number of Pareto designs and the number of valid designs in each generation. Particular interest was placed on the effect of injection pressure on the optimization of the engine and whether the previously optimized baseline engine design holds for different engine sizes. For 32 generations, totaling 1024 designs, no better design than the initial optimum, which was generated for the baseline engine, was found. This indicates that the current engine size-scaling model works well.