Improving engine efficiency and reducing pollutant emissions are the everlasting pursuit for engine researchers. While increasing tumble ratio of the engine intake airflow was shown to benefit the engine performance, rarely any systematic study has been carried out on the effect of tumble flow on engine in-cylinder performance and its mechanism. To fill in this gap, we designed tumble deflectors with desired tumble ratio using computer-aided design and computational fluid dynamics in this work. Afterwards, tumble deflectors with a tumble ratio of 1.5 and 2.2 were 3D printed. Finally firing tests of the optical engine installed with different tumble deflectors were performed. A high-speed color camera and pressure transducer were used to record crank angle-resolved flame natural luminosity and cylinder pressure. Results show that higher tumble ratio leads to faster blue flame development and less yellow flame generation, which indicates an increase in flame burning rate and a decrease of the soot formation. This is closely related to the enhanced turbulence kinetic energy before ignition as was shown in the computational fluid dynamics simulation. Besides, the engine effective pressure is increased by higher tumble ratio, but the cycle to cycle variation of flame characteristics first decreases then increases with tumble ratio.