This paper provides insight into abnormal combustion events observed during engine development of a highly turbocharged downsized engine configuration. The results and findings in this paper may contribute to the understanding of problems in small downsized engines which are becoming more common in the search for reduced fuel consumption. These problems are combustion limitations at high manifold pressures and compression ratios as designers and engineers endeavor to further reduce engine capacities.Abnormal combustion effects, analysis and development efforts are described for the 0.43 liter test engine, which was fitted with a port fuel injection fuel delivery system. The inline two cylinder engine used in experiments was specifically designed, constructed and developed to enable 25 bar BMEP and 60 kW of brake power to be reliably achieved while operating on pump gasoline. Producing this specific output is one way forward in emerging passenger vehicle engines to reduce fuel consumption whilst maintaining engine power at European intermediate class requirements. Previous investigations with this powertrain highlighted potential drive cycle fuel consumption improvements in excess of 20% when compared to a 60% larger 1.2 liter normally aspirated configuration.Engine development explored combustion limitations through a number of parametric studies, including a range of manifold absolute pressures up to 270 kPa and compression ratios ranging from 9 to 13. Two forms of abnormal combustion were observed under high load conditions. These included partial burns/misfires and varying levels of end-gas knock, including the most destructive form commonly termed mega knock, with knock amplitudes reaching in excess of 60 bar.Experiments highlighted that abnormal combustion, specifically knock in the end-gas region, was the dominant factor in limiting the performance of this small engine. Highest knock intensities were observed when knock was deduced to occur on the intake side of the pent roof combustion chamber. Hence, the extent to which the larger engines can be downsized while still maintaining equal performance is combustion limited.