A method and apparatus for reducing total pressure loss in the exhaust of a combustion device via an axial diffuser having no turning vanes, support struts, rods, or other obstructions to exhaust flow. The axial diffuser includes an inner wall and an outer wall each having a turn radius and forming an annular cross section having increasing cross-sectional area in the downstream direction of flow, the flow being transitioned from axial to radial direction thereby. The diffuser is supported in the plenum by rods, struts or a cradle; from a stand surrounding the axial diffuser via struts or rods; or from existing diffuser support structure by web stiffeners. The diffuser flow path turn radius of the outer wall ranges from about 50-95% of the radial distance from the inner wall to a wall of the exhaust plenum, and of the inner wall ranges from about 10-70% of this distance.