The compression ratio and expansion ratio are fundamental parameters that determine the thermal efficiency of an SI engine, and the potential of setting these ratios to arbitrary values was studied as a way of improving engine efficiency. First, the efficiency resulting from different compression and expansion ratios was calculated from a theoretical formula. As a result, it was verified that a 20% improvement in thermal efficiency could be expected by adopting a super-high expansion ratio of 20 or higher, which is an extremely large value for an SI engine, while keeping the compression ratio within a range that can ensure appropriate combustion. Subsequently, this research calculated the possibility of improving engine efficiency under a condition that constrains the swept volume to a constant value in consideration of practicability. The results indicated that in this case the expansion ratio is determined by load, and that ideal engine characteristics can be produced in which high efficiency is attained in practical load regions while producing maximum power under full load. Finally, based upon the above efficiency calculations, the thermal efficiency was verified using an actual engine in the load region with the greatest efficiency improvement effect. In tests, the evaluations utilized a high-efficiency commercially available compact 4-cylinder SI engine with a compression ratio of 13:1. By adopting a super-high expansion ratio, the test results showed an improvement in thermal efficiency of 80% with respect to the theoretically predicted value. In addition, issues related to the adoption of a super-high expansion ratio cycle (a reduction in the degree of constant volume and an increase in cooling loss) were analyzed theoretically and some strategies for minimizing these issues were studied. The overall results indicated that the adoption of the variable super-high expansion ratio cycle proposed in this paper is highly effective as a realistic means of improving the thermal efficiency of an engine.