The paper presents 1D mathematical model and simulation program, which were used to determine the control strategy to change the intake pipes length continuously. The model comprises the unsteady 1D model of the entire intake and exhaust systems, with the submodel for engine cylinders. Governing equations for 1D flow were solved using the numerical method of characteristics. The model was validated by comparing the calculated results with test bed results for original engine characteristics. The paper presents sample results for an contemporary 1.8 litre, 100 kW, automotive spark ignition engine. The engine is naturally aspirated and comprises 4 cylinders in line. The analysis was performed for the original engine layout with two-step change of intake pipes length and for the continuously variable length of intake pipes. The results indicate the superiority of the continuously variable length of intake pipes when achieving the highest engine torque and power in the full range of engine operation speeds. The results show the possibilities for significant engine torque increase in the lower speed range. The analysis comprises not only the pipe length determination, than also the intake pipe diameter optimization. The results of calculations determine the control strategy to hold the optimum length of intake pipes to achieve the highest engine torque and power for the given engine speed.