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The biggest efficiency factor is compression ratio, the higher the better from a thermodynamic perspective. Next is the rate of flow of air/fuel mixture as in cc/min. A longer stroke, under square, tends to have more torque due in part to crank offset. Most 4 bangers are designed to get their power at high rpm so stroke and piston mass are kept minimal. Compare this to an industrial engine. Reduced aero drag and weight means less power is required so less fuel is burned. So build a full fairing, maybe lose a few pounds, and consider converting your ride to diesel power or a turbo system.
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