How does the increase in expansion/compression ratio of an internal combustion engine change its efficiency when compression ratio is kept constant?

The main purpose of this experiment is to investigate the effect of expansion/compression ratio on efficiency of an internal combustion engine. Efficiency of internal combustion engines are affected by compression ratio, expansion/ compression ratio, internal friction and many other factors. Compression ratio is the factor affecting on the fuel combustion process mainly affects the combustion pressure, as compression ratio increases, fuel solubility therefore released energy increases. Expansion is the process of converting pressure produced by heat, into kinetic energy. With enough expansion ratio pressure inside the combustion chamber can equalized to the pressure outside pressure. With optimization of these two ratios, useful energy produced from unit amount of fuel can be increased. To conserve fuel burning efficiency and increasing expansion ratio, first I determined what expansion/compression ratio to test and then calculated the volume need to be reduced in the combustion chamber to fix compression ratio at eight and increase expansion ratio. To reduce volume, metal pieces were bolted on the cylinder head, graduated cylinder was mounted on the fuel tank. To make results closer to real life usage, 2 kilos of cylindrical mass was bolted on the crankshaft. In each test same amount of fuel was used, RPM was kept constant in each run, times were recorded for each setup. Each configuration of expansion/compression ratio is tested for four times. Four different time data for each expansion/compression ratio configuration summed and divided by four to get average time. To calculate average percentage efficiency gain, average time of running for each expansion/compression ratio configuration was divided by the average time of configuration with expansion/compression ratio of 1 and 1 was subtracted and divided by 100. Graphs show that efficiency increases as expansion/compression ratio increases