For passenger vehicles with direct injection (DI) diesel engines, it is desirable for the engine to have high speed and high power output performance comparable to that of gasoline engines, yet maintaining its good fuel economy. Accordingly, we examine a dual inlet port system, which operates both valves simultaneously at high engine speed to improve the volumetric efficiency but operates only one valve at low engine speed to obtain the optimum swirl ratio (SR). The swirl ratio with two valve operation (SR1+2) is affected by SR and the mean effective valve area (Zm) of each valve. Therefore, with experimental technique, it is difficult to find port configurations which are optimum for both high and low engine speeds simultaneously. We have employed a predictive method to obtain the optimum configurations. This paper describes how to derive the predictive equation of SR1+2 from the SR and Zm of each valve for the optimum intake system design. To examine the accuracy of the equation, predicted values were compared with measured values. The results show good agreement for practical applications. Moreover, a single cylinder engine with this system shows a reasonable improvement on the power output and fuel consumption.