Abstract: A casting technique is disclosed which improves efficiency and reduces fabrication and assembly cost for a centrifugal compressor. The volute is cast integrally with the impeller housing to allow closer radial tolerances to be used to improve efficiency. In the preferred embodiment, compressors for multi-stage compression are assembled with intercoolers and the integral volute is cast together with the impeller housing and the lower gearbox housing and the associated intercooler. Efficiency increases of 2% or more are achievable. In multistage applications, efficiency gains in the early stages are compounded in each subsequent stage.