This paper presents a numerical study of the combustion chamber design influence on the performance of racing engines. The analysis has been applied to the Ferrari 10 cylinder 3.0 liter S.I. engine adopted in Formula One racing.The numerical investigation aimed to asses the influence of stroke-to-bore ratio changes on engine performance within real life design constraints. The effects of the stroke-to-bore ratio on both the volumetric efficiency and the thermal conversion efficiency have been investigated. Flame front area maps, wall areas wetted by burned gases, mean flow field patterns and main turbulent parameters have been compared for two different S/B ratios. Since higher intake and exhaust valve areas per unit displaced volume result in a higher volume of piston bowls, a lower S/B ratio leads to a lower compression ratio, which strongly limits the indicated mean effective pressure. Therefore, in the second part of the paper, an analysis of the influence of the piston shape on combustion process has been performed in order to optimize the imep of the lower S/B ratio engine.