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The 4 cylinder engine has the advantage of sequential induction pulses. There is always another induction pulse for every 180 degrees of crankshaft revolution. This reduces the pressure waves in the intake manifold and makes intake flow much smoother. More than 4 cylinders will enhance the effect but it takes at least 4 to make the induction flow fairly consistent.
Do you want power or torque? A 600 CC motorcycle engine at 15,000 RPM makes ungodly power. A 2.4 liter long stroke engine makes gobs of torque. The latter will give you more power for the same amount of fuel.
Fewer cylinders (but more than 4) will give you less surface area of the cylinder head and cylinder walls for heat to dissipate. The 3 foot bore and 9 foot stroke of the most efficient engines on the planet give you and idea of how to build them more efficient but they run at 100 RPM. The also have a scotch yoke to eliminate the side loads on the piston.
regards
Mech
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