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Potential benefits of Variable Compression Ratio (VCR) spark ignition engines are presented, based on an examination of the relationship between Compression Ratio, BMEP and spark advance at light load and full load. Alternative methods of implementing VCR are illustrated and critically examined. System control strategies are presented. Potential manufacturing constraints are identified and their influence on system configuration is examined. Fuel economy benefits attainable from other technologies such as cylinder deactivation, camless valve operation and GDI are shown to be inferior to the use of downsized boosted engines. VCR is identified as the key enabling technology of such downsized engines.