at least one generally circular plate 28 upon which a first stage of diffuser vanes 30 is mounted at a radially inner position adjacent the discharge ends 18 of the impeller vanes 16 the plate 28 also mounts a second stage of diffuser vanes 32 which are located radially outward of the first stage 30 and which are aligned with corresponding ones of the vanes 30 of the first stage according to the invention the vanes 30 and preferably the vanes 32 as well have cross sections configured as supercritical air foils the term 34 supercritical airfoil 34 is used in a conventional sense and refers to an airfoil that is characterized by very little camber in the forward portion with a severe camber at the rear portion the vanes 30 have high pressure sides 34 while the vanes 32 have high pressure sides 36 in addition the vanes 30 have low pressure sides 38 while the vanes 32 have lower pressure sides 40 as used herein the low pressure side is that that would be subjected to the least pressure if the vane were employed as a wing stated another way if the vanes were employed as wings lift.