Torque multiplier engines, and associated methods and systems, are disclosed herein. An internal combustion engine in accordance with a particular embodiment can include a connecting rod operably coupling a pair of opposing pistons. The engine can further include a first bearing coupled to the connecting rod and positioned to engage a first cam groove of an inner cam drum. A second bearing coupled to the connecting rod can be positioned to engage a second cam groove on an outer cam drum. The first and second bearings can translate linear motion of the opposing pistons to rotation of the cam drums.