The objective of this study is to find the effect of the dynamic pressure wave on the volumetric efficiency by analyzing the fluid flow in the intake and exhaust manifolds, and to get the optimal design criteria for the intake manifold.This study showed that five to ten percent of volumetric efficiency could be increased by the optimum design of the intake manifold without modification of the engine main body.A finite difference model was developed using two-step Lax-Wendroff method to solve the governing equations of air flow in the intake and exhaust manifolds.The results of this study are summarized as follows: 1. The optimum length of intake manifold for the maximum volumetric efficiency can exist for each specified engine speed and manifold diameter. 2. When the reflective pressure wave of expansion wave due to the opening of the intake valve reached the intake valve side with the positive maximum value just before the intake valve is closed, the volumetric efficiency becomes maximum. 3. A comparison was made between the numerical and the existing experimental results in order to evaluate the accuracy of the simulated model, and showed good agreements qualitatively. It is expected that the simulation program developed in this study could be applied for other engine development studies.