A study into the effect of two methods of changing the geometry of a vaneless diffuser on the performance of the compressor of a road haulage diesel engine turbocharger is described. The development of compressor variable geometry will enable the full potential of variable geometry turbines to be realized. This will give a more flexible power unit which will provide, for example, better low-speed torque and hence a smaller gearbox, and shorter journey times or larger payloads than are currently the practice. The disadvantages are added complexity and cost in the relatively simple turbocharger, and the need for an engine management system. The latter is currently being implemented on many vehicles to meet tight emissions regulations in Europe and elsewhere, and is thus not a drawback limited to variable geometry turbocharging. A compressor test facility, including appropriate instrumentation and a computer-based data-acquisition system, was constructed with the specific aim of investigating the unstable flow regime prior to and including surge. Alternative fixed vaneless diffuser geometries were designed to simulate a variable geometry diffuser which could be achieved through a flexing diffuser wall and a sliding throttle ring. Both the converging wall and throttle ring arrangement moved the peak pressure ratio to lower flowrates, and at the near surge flowrates (where the device would be introduced, when operating in a variable geometry mode) improvements in both pressure ratio and efficiency are shown. While the converging wall concept exhibited slightly better aerodynamic performance than the throttle ring, it has implementation difficulties with respect to material integrity under continuous flexing when developed to a fully variable geometry device. The simplicity of the sliding throttle ring makes it a more viable proposition. Prototype variable geometry (VG) devices have been constructed with a view to further rig and engine testing.