In this study, an Otto cycle engine was converted to the Atkinson cycle engine using the method of late intake valve closing and increasing the compression ratio. Firstly, the thermodynamic analysis was conducted. In the analysis, the variation of specific heats with temperature, heat losses, pumping, and mechanical friction losses was taken into account. In-cylinder pressure variations, torque, power, thermal efficiency and brake-specific fuel consumption variations with speed were obtained. For the experimental study, a new camshaft was designed and manufactured using a circular arc curve. The IVC timing of the Atkinson engine was delayed by a 20 degrees crankshaft angle with respect to standard timing. The compression ratio was increased from 8.5 to 9.5. Tests were conducted at 1400-3400 rpm speed range at WOT. Experiments were conducted at three different operating conditions: Otto cycle for the standard engine, late closing of the intake valve at 20 degrees CA at 8.5 compression ratio and late closing of the intake valve at 20 degrees CA at 9.5 compression ratio. In the tests, the variation of torque, power, BSFC and thermal efficiency with speed were investigated. The results showed that torque, power, BSFC and thermal efficiency were improved with Atkinson CR9.5 operation compared to the standard Otto cycle operation at high speeds. It was obtained that the torque and power of the Atkinson CR9.5 cycle engine increased by 6.7% and the thermal efficiency increased by 12.8% at 3400 rpm speeds. In addition, BSFC of the Atkinson CR9.5 engine decreased by 12.7%.