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This paper describes how the use of a computer model, followed by rig and engine testing, were employed to investigate the application of port throttles to a 4-valve SI engine. The results suggested that the throttling should be split between port and plenum throttles as this gave a faster bum rate than port throttling alone. It was argued that port throttling is most applicable to controlling the level of charge dilution on high-performance engines with large valve overlap periods. Port de-activation was also investigated, first separately, and then in combination with port throttles. Alone it increased the tolerance of the engine to EGR very significantly, and in combination it had the ability both to increase the EGR tolerance and to allow the use of a high-overlap camshaft.