The key for comparing the displacement between the 4-cycle engine and the rotary engine is in studying the degrees of rotation for a thermodynamic cycle to occur. For a 4-cycle engine to complete every thermodynamic cycle, the engine must rotate 720° or two complete revolutions of the crankshaft. The rotary engine is different. The engine rotor rotates at 1/3 the speed of the crankshaft. On two rotor engines, front and rear rotors are 180° offset from each other. Each rotation of the engine (360°) will bring two faces through the combustion cycle (the torque input to the eccentric shaft). This said, it takes 1080° or three complete revolutions of the crankshaft to complete the entire thermodynamic cycle. Obviously, there is a disparity. How can we get a relatable number to compare to a 4-stroke engine? The best way is to study 720° of rotation of the two-rotor engine. Every 360° of rotation, two faces of the engine complete a combustion cycle. 720° will have a total of four faces completing their cycle. per face times four faces equals . That's a well-reasoned number and now gives something that can be compared to other engines. In addition, since four faces passed by in the comparison, it's like a four-cylinder engine. The 13B therefore compares well to a 2.6L 4-cylinder 4-cycle engine.