An exhaust aftertreatment system includes a housing including first and second axial ends. An exhaust inlet is disposed adjacent the first axial end. An exhaust outlet is disposed adjacent the second axial end. A catalytic converter is disposed between the first and second axial ends. A diffuser is disposed inside the housing between the first axial end and the catalytic converter. The diffuser includes a shell in communication with the exhaust inlet. The shell includes proximal and distal portions and a front side facing the catalytic converter. The proximal and distal portions are substantially equal in frontal area. The proximal portion is adjacent the exhaust inlet and includes a first plurality of front side openings totaling a first open area and the distal portion includes a second plurality of front side openings totaling a second open area. The first open area is about 1.8-2.2 times the second.