In current investigation, an innovative variable-length intake manifold is proposed for a 4-cylinder spark ignition engine to raise volumetric efficiency. Unlike common variable-length manifolds that operate solely on the basis of adjusting the optimal length of the inlet duct, in this proposed design, in addition to changing the optimal length by using different throttles, a non-linear combination of different throttles at different engine speeds is used. In addition, the effect of connecting the runners of cylinders no.1 and no.2, and cylinders no.3 and no.4 to each other with three different diameters is also investigated, and the innovation of this study is considered. To prove the efficiency of the new design, firstly, a stock engine is modeled utilizing the GT-Power software in wide-open throttle mode and then validated with the experimental results and previous studies then with the direct coupling of the model to the 3D Converge Lite solver. The proposed variable-length intake manifold with different throttle bodies is analyzed and reviewed in a one-dimensional-three-dimensional way. The results of using the proposed manifolds illustrated that by using a non-linear combination of throttle bodies and variable connection of runners in the entire speed range of the engine, the volume efficiency, thermal efficiency, torque, and effective specific fuel consumption of the engine are improved compared to the stock model by 5.17%, 0.67%, 5.90%, and 0.66%, respectively.