Abstract: A rotary expansible chamber device capable of functioning as a compressor or fluid motor is made up of a housing containing two or more parallel working chambers wherein each chamber contains an impeller. Adjacent impellers mate with each other and the housing in a fluid tight relationship with each impeller being made up of a multiplicity of risers of uniform cross section wherein each riser has one or more lobes and one or more adjacent wells. The risers along each impeller are axially offset from each other in a spiraling relationship with each impeller containing risers of at least two different heights. As the impellers rotate, a compressible gas is forced to travel axially along the working chambers through the riser wells. As the compressible gas travels through the working chambers it becomes sealed within adjacent wells along an impeller in the form of a stepped spiral gas volume.