Internal combustion engine comprising—a first pair of cylinders which accommodate, according to a rectilinear reciprocating motion, relative first pistons operatively associated to a motor shaft rotating around a motor axis disposed in a transverse direction, perpendicular to a longitudinal running direction of an associable vehicle, —a suction system comprising a filter box which delimits a suction volume which houses at least a first front suction duct and at least a first rear suction duct, respectively disposed in an advanced and retracted position in relation to an air/suction mixture inlet direction, each suction duct channelling the air/suction mixture before entering in the respective cylinders, —wherein said first front and rear suction ducts are fixed and have respective mutually different lengths.