The invention relates to a radial split diffuser (3) with an inner casing (13) and an outer casing (14) joined together along a cylindrical joint (15). Each diffuser passage is intersected by and extends transversely across the joint. The advantages of this design include: the elimination of a transition within the initial portion of the passages where air flow speeds are supersonic and minute surface discontinuities can significantly effect performance; and the simplification of manufacturing through use of metal castings to replace sheet metal fabrications in the manufacture of diffusers. The joint (15) in the present invention can be located downstream from the diffuser inlet a sufficient distance in a lower velocity area. The joint is located to enable access for precise machining of the critical initial portion of the passages within the inner casing, and to minimise air flow disturbance in the initial portion.
