This paper presents a comprehensive multiscale study of the turbulence inside of an automotive monolith using a porous medium approach and a representative group of single channels. A series of RANS simulations of an axisymmetric model of the whole converter and a 3D model of a representative group of single channels is combined to study the turbulence at different scales. Results of simulations are validated against experimental data published in the literature. Good agreement is achieved. Results of simulations reveal that although the continuum porous media model produces good agreement with experimental velocity profiles after the monolith, it does not describe accurately the effects of turbulence inside the monolith. Literature reports that a transition from turbulent to laminar at the beginning of the monolith channels impacts significantly the performance of the whole converter, but, at the same time, it is usually neglected, due the complexity that it adds to the problem. According to the results from the single channels, there is a smooth decay of the turbulence viscosity inside the monolith, that does not appear using the traditional models of porous zones. This decay can be achieved at the converter scale, via the addition of a damping term for the turbulence to the kappa-equation of the RANS model. (C) 2017 Elsevier Ltd. All rights reserved.