Legislations on vehicular noise levels are becoming stringent day by day. Also the customer expectations on the vehicle noise levels has also become more refined in the India market. Intake and exhaust flow noise levels are one of the major contributors to this overall noise level of the vehicle. These drivers have led the Indian automobile manufacturers to focus on engine and vehicle noise optimization. The 3-wheeler and 4-wheeler Quadra-cycle vehicles which are powered by single cylinder engines are more prone to high noise levels due to pulsating intake and exhaust flow dynamics caused by single firing cylinder. The study focuses on the exhaust muffler design optimization of a single cylinder engine powertrain to meet the targeted noise levels.One - Dimensional (1D) Simulation tools give significant information at the system level and reduce the time required for analysis and optimization compared to the three - dimensional (3D) CFD tools. Also, 3D CFD tools provides detailed information of the complex zones of the flow and are highly accurate in predicting the behaviour of the same. But the 3D tools incur high computational cost and time compared to 1D tools. So, to balance between the pros and cons of the above two, a 1D - 3D coupled simulation approach is introduced. In 1D - 3D coupled simulation, complex flow domains are modelled in 3D whereas the rest of the domain is modelled with 1D approach.In this paper, muffler design of a single cylinder engine is optimized for lower noise levels and at the same time meeting the targets of back pressure and engine performance. A calibrated single cylinder 1D engine model is used where optimization of the muffler is first performed in 1D approach by running Design of Experiments (DoE) and then the optimized conditions are simulated in 1D-3D coupled simulation environment. Industry leading simulation tool GT-Suite along with Converge Lite is used for the simulation and analysis. Also, 1D simulation results are compared with the 1D-3D coupled simulation results to analyse the difference between the predicted noise levels.