A variable swirl intake port system for 4 valves/cylinder direct injection diesel engines was developed. This system combines two mutually independent intake ports, one of which is a helical port for generating an ultra-high swirl ratio and the other is a tangential port for generating a low swirl ratio. The tangential port incorporates a swirl control valve that controls the swirl ratio by varying the flow rate.To investigate the performance of the intake port system, steady-state flow tests were conducted in parallel with three-dimensional computations. In conducting the steady-state flow tests, it was found that a paddle wheel flow sensor was not suitable for evaluating the characteristics of the high-swirl port and that it was necessary to use an impulse swirl flow meter. An automatic mesh generator that can efficiently produce computational meshes for the helical port, the tangential port and various combinations of the two was newly developed for use in running numerical computations.First, a parametric study was made of the helical port geometry for the purpose of selecting a port shape that would generate a high swirl ratio efficiently. The results revealed that a key factor in generating a high swirl ratio was to suitably control the direction of the intake air flow through the valve seat.The characteristics of the swirl ratio and flow coefficient were then examined when the two ports were combined. The results indicated that the total performance of both ports can be estimated from the performance of a single port.As the last step in this research, the effects of the types of intake ports that are combined and the valve arrangement were examined under a broad range of conditions. The results revealed an effective port design for obtaining a variable, wide-ranging swirl ratio.This resulted in the development of a variable swirl intake port system capable of controlling the swirl ratio over a wide range from 3.5 to 10.