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The flow characteristics such as flow coefficient and swirl ratio in cylinder were measured at steady-state condition by using a steady-state flow measuring system which consists of an impulse swirl meter and a viscous flow air meter. Effects of the in-cylinder swirl on the emissions (NOx, THC), BSFC, MBT, and lean mixture limit (LML) were investigated by using single cylinder research engine with a helical intake port. The combustion pressure in the cylinder was measured to analyze the combustion duration (such as the initial burn duration and the main burn duration) and COV of IMEP which have an effect on LML. By generating the in-cylinder swirl, the combustion duration is remarkably decreased, while the emissions and BSFC are increased. The lean mixture limit is increased with the in-cylinder swirl ratio. In the case of high swirl ratio (Rs=2.48),the lean mixture limit is increased by 3.3 with optimized fuel inject ion timing.