Air motion inside the intake manifold is one of the important factors, which govern the engine performance and emission of multi-cylinder diesel engines. Hence the flow phenomenon inside the intake manifold should be fully understood in order to consider the current requirement of higher engine efficiency and lower emission. In this paper, the internal flow characteristics in the intake manifold of a six-cylinder diesel engine is investigated computationally for the variation of spacer and chamber width under steady state. Also, this study is to examine its effects on engine performance such as volumetric efficiency, fuel consumption rate, smoke and NOx. For the computational study, grid generation was carried out. The governing equations for steady, three-dimensional, incompressible, turbulent flow are solved with the two equation k- ε model to consider the complexity of the geometry and fluid motion. The overall flow field inside the intake manifold and various quantities, such as pressure, velocity distribution were examined. In accordance with the design modification of chamber width and spacer, fuel consumption rate and emission material such as smoke, NOx were investigated under motored conditions. As a result of this study, we could identify the flow characteristics inside the intake manifold and suggest the improved design to increase the engine performance.