the core flow and because the diffuser can accept a greater mass flow without the expected proportional increase in frictional losses the benefits of the invention are depicted in fig3 where u03b7 is the efficiency of the diffuser and where m is the mass flow lbs sec as depicted the performance of a diffuser of the present invention as shown by mr movable rib offers a greater range than a similar diffuser with either fixed full vanes as shown by fv fixed ribbed vanes as shown by fr or vaneless as shown by v efficiency of the present invention is only bettered by the fixed vane design fv but then only over a relatively narrow range several modifications to the present invention are suggested in fig4 and 8 it is to be understood that the various modifications may be practiced together or alone as required one modification is the provision of partial radius ribbed vanes 23 a ribbed vane of partial radius does not extend all the way to the diffuser outlet 15 in addition or in the alternative a pinched portion 24 having a pre determined radius r can be incorporated into the shroud wall 11 it will be understood that in designs incorporating a movable hub wall the pinched portion or pre determined radius will be incorporated into the hub wall a pinched portion is useful when a reverse flow situation exists at.