Improving the thermal efficiency of spark ignition (SI) engines is strongly required due to its widespread use but considerably less efficiency than that of compression ignition (CI) engines. Although lean SI engine operation can offer substantial improvements of the thermal efficiency relative to that of traditional stoichiometric SI operation, the cycle-to-cycle variations of combustion increase with the level of air dilution, and become unacceptable. For improving the thermal efficiency by extending the lean-stability limit, this study examines the effects of spark discharge energy and in-cylinder turbulence level on lean limits and cycle-to-cycle variations of combustion for SI engine operation. The spark discharge energy was increased by a high-energy inductive ignition system using ten spark coils and the in-cylinder turbulence level was enhanced by a custom adapter installed in the intake port. The results show that increased spark discharge energy by ten spark coils is effective at shifting the lean stability limit to leaner operation, compared to that of a single spark coil. With shift of the lean-stability limit, significant improvement of thermal efficiency is observed, relative to that of stoichiometric operation. Furthermore, a combination of increased spark discharge energy and enhanced in-cylinder turbulence level makes it possible to allow stable operation at more extended lean-stability limit. This is mainly attributed to shortening the durations of spark timing-to-CA5 and CA10-to-CA90 by both increased spark discharge energy and enhanced in-cylinder turbulence level. However, the cycle-to-cycle variations of SI combustion increase with increasing excess-air ratio even for operation by ten spark coils with the intake port adapter. Finally, the relationship between the spark discharge energy and the SI combustion is examined and compared for ultra-lean operation without and with the intake port adapter. Although indicated thermal efficiency is improved' by increased spark discharge energy, the variations of the spark discharge energy do not relate to the variations of the combustion, since total spark discharge energy does not affect both durations of the spark timing-to-CA5 and the CA10-to-CA90, and eventually the heat-release efficiency.