Due to reciprocating nature of IC engine, flow physics in intake manifold is complex and has significant effect on volumetric efficiency. Variable length intake manifold technology offers potential for improving engine performance.This paper therefore investigated effect of intake length on volumetric efficiency for wider range of engine speeds. For this purpose 1-D thermodynamic engine model of a single cylinder 611cc standard CFR engine capable of predicting pressure waves in the intake was developed. For validation, pressure waves were predicted at two different locations on intake manifold and compared against test data. This model was used to predict volumetric efficiency for different intake lengths and engine speeds.Volumetric efficiency was found to be a function of both engine speed and intake length, more so at higher engine speeds. Frequency analysis of intake pressure waves during suction stroke and intake valve closed phase was carried out separately. Best volumetric efficiency was observed for intake runner length exhibiting pressure waves with 4th order fundamental frequency during intake valve closed phase and 1st order fundamental frequency during suction stroke. Based on Helmholtz resonator theory and acoustic theory (Kinsler equation) optimum intake length was calculated. Simulation predicted optimum intake length was in agreement with theoretically calculated optimum intake length.