An assembly can include a first radial compressor wheel that has a rotational axis and that includes a hub surface that includes an annular ridge disposed at a radius measured from the rotational axis; a second radial compressor wheel that includes a hub surface; and an annular baffle disposed at least in part between the hub surfaces where the annular baffle includes an outer surface and an inner edge that defines an opening having a central axis where the outer surface includes a surface portion to one side of the inner edge that faces the hub surface of the first radial compressor wheel, where the surface portion includes an annular channel over a radial width measured from the central axis and where the radial width spans the radius of the annular ridge.