After treatment (AT) system has evolved over the period of time with ever changing stringent emission norms. Systems are still developing to meet new evolving challenges of diesel engine to meet fuel economy & necessary power to drive the end application. Times have changed when the purpose of AT system was to take care of not only treating engine exhaust but also responsible for attenuation of engine propagated noise. The systems today have become sophisticated and smart enough to work wide range of test conditions & duty cycle to meet the emission norms. Current trend is to meet the performance targets by making these designs compact & less restrictive in terms of backpressure. This creates tradeoff within acoustics attenuation, performance parameters & backpressure offered by these devices.One of the major constraint in development of AT, is available customer packaging space & time to develop these designs in shortest period. Integration of AT system with OEM, presence of mixers, perforated region and substrates in the design are critical parameters for increase in pressure drop in the overall system. Pressure drop prediction inside AT system is an important analysis carried out to predict the impact on the overall engine performance.The objective of this work is to evaluate the pressure drop modeling strategy with actual test data. In this study investigation has been done to evaluate the impact of mounting of elbows and swirling motion on pressure drop in the domain. Based on the study, strategies were found to check convergence and turbulence models (e.g. k- ϵ model) which can be used for swirling flows. This study has also helped in understanding the impact of clocking on velocity and pressure drop inside AT system.