The paper discusses investigations into improving the full-load and transient performance of the Ultraboost extreme downsizing engine by the application of the SuperGen variable-speed centrifugal supercharger.Since its output stage speed is decoupled from that of the crankshaft, SuperGen is potentially especially attractive in a compound pressure-charging system. Such systems typically comprise a turbocharger, which is used as the main charging device, compounded at lower charge mass flow rates by a supercharger used as a second boosting stage. Because of its variable drive ratio, SuperGen can be blended in and out continuously to provide seamless driveability, as opposed to the alternative of a clutched, single-drive-ratio positive-displacement device. In this respect its operation is very similar to that of an electrically-driven compressor, although it is voltage agnostic and can supply other hybrid functionality, too.In the work reported here a prototype SuperGen unit was tested on the Ultraboost extreme downsizing demonstrator engine and the performance compared to that of the originally-specified positive-displacement device. This engine has previously been described in detail and represents a 60% downsizing factor versus a 5.0 litre naturally-aspirated V8, although the ‘standard’ baseline combination of supercharger and turbocharger was found in earlier work to be a limitation on achieving the full downsizing factor at low engine speed.The improvement in full-load performance in the area where the turbocharger cannot generate the required boost by itself is reported. The transient response of the combined system at low engine speed is also presented, together with part-load fuel economy data at several engine speed and load points. Finally, this part-load data is used for vehicle modelling work showing that a more-efficient high-pressure stage can bring further fuel economy benefits to extremely-downsized vehicle applications.