An exhaust treatment system (19, 219, 319, 419, 519, 619, 719) comprising: an exhaust conduit for conveying exhaust gases from an engine of a vehicle; a housing assembly having a first axial end and a second axial end, the first axial end supports an inlet tube, which is generally aligned with an end _of the exhaust conduit, the second axial end of the housing assembly supporting an outlet tube, which is generally aligned with an end of the exhaust conduit; an aftertreatment device; a flow device (23, 223, 323, 423, 523, 623, 723, 1001, 1101, 1413) disposed upstream of the aftertreatment device, the flow device including an inlet (43, 53) and an outlet (45, 55) and defining an exhaust flow redirection angle β measured between the inlet (43, 53) and the outlet (45, 55) for redirecting exhaust flow through the flow device (23, 223, 323, 423, 523, 623, 723, 1001, 1101, 1413) such that the exhaust flow swirls about a longitudinal axis of the flow device; and a diameter restriction (24, 424, 524) disposed between the flow device and the aftertreatment device such that after the exhaust exits the flow device, the exhaust flows through a passage in a baffle, the passage in the baffle being sized such that an inner diameter of the passage is less than a diameter of a housing assembly, characterized in that the housing assembly defines a first section disposed upstream of the baffle and a second section disposed downstream of the baffle, the flow device being disposed in the first section of the housing assembly while the aftertreatment device is disposed in the second section, and in that an inner diameter of the passage of the diameter restriction per the diameter of the housing assembly is in a range of about 0.55 to about 0.85 such that the diameter restriction (24, 424, 524) is adapted to reduce the amount of unvaporized or unhydrolyzed reactants at the aftertreatment device. an exhaust conduit for conveying exhaust gases from an engine of a vehicle; a housing assembly having a first axial end and a second axial end, the first axial end supports an inlet tube, which is generally aligned with an end _of the exhaust conduit, the second axial end of the housing assembly supporting an outlet tube, which is generally aligned with an end of the exhaust conduit; an aftertreatment device; a flow device (23, 223, 323, 423, 523, 623, 723, 1001, 1101, 1413) disposed upstream of the aftertreatment device, the flow device including an inlet (43, 53) and an outlet (45, 55) and defining an exhaust flow redirection angle β measured between the inlet (43, 53) and the outlet (45, 55) for redirecting exhaust flow through the flow device (23, 223, 323, 423, 523, 623, 723, 1001, 1101, 1413) such that the exhaust flow swirls about a longitudinal axis of the flow device; and a diameter restriction (24, 424, 524) disposed between the flow device and the aftertreatment device such that after the exhaust exits the flow device, the exhaust flows through a passage in a baffle, the passage in the baffle being sized such that an inner diameter of the passage is less than a diameter of a housing assembly, characterized in that the housing assembly defines a first section disposed upstream of the baffle and a second section disposed downstream of the baffle, the flow device being disposed in the first section of the housing assembly while the aftertreatment device is disposed in the second section, and in that an inner diameter of the passage of the diameter restriction per the diameter of the housing assembly is in a range of about 0.55 to about 0.85 such that the diameter restriction (24, 424, 524) is adapted to reduce the amount of unvaporized or unhydrolyzed reactants at the aftertreatment device.