Increasingly stringent regulations and rising fuel costs require that automotive manufacturers reduce their fleet CO2 emissions. Gasoline engine downsizing is one such technology at the forefront of improvements in fuel economy. As engine downsizing becomes more aggressive, normal engine operating points are moving into higher load regions, typically requiring over-fuelling to maintain exhaust gas temperatures within component protection limits and retarded ignition timings in order to mitigate knock and pre-ignition events. These two mechanisms are counterproductive, since the retarded ignition timing delays combustion, in turn raising exhaust gas temperature.A key process being used to inhibit the occurrence of these knock and pre-ignition phenomena is cooled exhaust gas recirculation (EGR). Cooled EGR lowers temperatures during the combustion process, reducing the possibility of knock, and can thus reduce or eliminate the need for over-fuelling. It has also been shown to reduce exhaust gas temperature and improve combustion efficiency through improved combustion phasing. This paper reviews data collected on ‘ULTRABOOST’, a collaborative research project which is co-funded by the Technology Strategy Board, the UK's innovation agency. The project is based on a prototype, heavily downsized 2.0L DISI engine running under controlled boost conditions, featuring low pressure or ‘long route’ EGR. This allows for the EGR feed gas to be fed from either pre- or post-catalyst sampling points.This paper will present data collected at equivalent operating conditions using both catalyzed and un-catalyzed exhaust gas to feed the EGR system. Some comparison of equivalent BMEP levels will be made but this is not the focus of the investigation. Current standards utilize a CO2 ratio for EGR measurement, but the results presented highlight inaccuracies in this measurement method when repeatable test conditions for pre- and post-catalyst EGR routes are required due to the significant changes in gas composition pre- and post-catalyst. The test engine was artificially boosted utilizing a Combustion Air Handling Unit to simulate commercially available turbo-machinery, which allowed for the catalyst effects to be assessed in isolation. Results from the engine are presented for constant intake manifold pressure and constant EGR mass flow to highlight the effects of the EGR feed gas composition on a range of engine performance metrics.