A harmonic balancer (also called crank pulley damper, crankshaft damper, torsional damper, or vibration damper) is a device connected to the crankshaft of an engine to reduce torsional vibration.

Every time the cylinders fire, torque is imparted to the crankshaft. The crankshaft deflects under this torque, which sets up vibrations when the torque is released. At certain engine speeds the torques imparted by the cylinders are in synch with the vibrations in the crankshaft, which results in a phenomenon called resonance. This resonance causes stress beyond what the crankshaft can withstand, resulting in crankshaft failure.

To prevent this vibration, a harmonic balancer is attached to the front part of the crankshaft. The damper is composed of two elements: a mass and an energy dissipating element. The mass resists the acceleration of the vibration and the energy dissipating (rubber/clutch/fluid) element absorbs the vibrations.

Over time, the energy dissipating (rubber/clutch/fluid) element can deteriorate from age, heat, cold, or exposure to oil or chemicals. Unless rebuilt or replaced, this can cause the crankshaft to develop cracks, resulting in crankshaft failure.

There has been a trend at times by some "performance enthusiasts" to remove the harmonic balancers on their cars, usually when the balancer is attached to the crank pulley. The argument is that they aren't necessary and their mass reduces the performance of the engine. Others argue that this is not worth it, because the danger of damage to the engine from the vibrations the damper is intended to prevent is too high. Certain cars, however, do not come equipped with an external balancer on the crank pulley, and as such, can have the pulley replaced with a performance oriented product.

While net engine output can be increased without harmonic balancers, in professional race cars harmonic balancers are still commonly equipped, for reasons ranging from safety concerns to regulations. Almost all modern car manufacturers, even "performance" car makers and specialty tuners, include a harmonic balancer on their vehicles, and removal voids vehicle warranty.