Gasoline turbocharged direct injection (GTDI) engines, such as EcoBoost™ from Ford, are becoming established as a high value technology solution to improve passenger car and light truck fuel economy. Due to their high specific performance and excellent low-speed torque, improved fuel economy can be realized due to downsizing and downspeeding without sacrificing performance and driveability while meeting the most stringent future emissions standards with an inexpensive three-way catalyst.A logical and synergistic extension of the EcoBoost™ strategy is the use of E85 (approximately 85% ethanol and 15% gasoline) for knock mitigation. Direct injection of E85 is very effective in suppressing knock due to ethanol's high heat of vaporization - which increases the charge cooling benefit of direct injection - and inherently high octane rating. As a result, higher boost levels can be achieved while maintaining optimal combustion phasing giving high thermal efficiency. However, due to their different properties, optimization of a combustion system for both regular grade (91 RON) gasoline and E85 is non-trivial.This paper describes the initial design and development of a new combustion system for a flexible fuel (gasoline and E85) turbocharged direct injection engine. The development process utilized a boosted single-cylinder engine in conjunction with a second boosted single-cylinder engine with optical access which was used to provide insight into fuel/air interactions and resultant combustion performance. Using this approach a robust combustion system was developed to meet targets for both gasoline and E85 operation before multi-cylinder hardware was available.