8457 Power Puller is a motorized TECHNIC set of a 'power' tractor and pulling sled. The tractor pulls a sled which becomes progressively more difficult to pull, mimicking the behavior of actual tractors in a tractor pulling event. As well as housing the elastic string that causes this effect, the trailer also carries a battery pack to power the motor. The set contains a total of 20 pistons and instructions for a number of different engine configurations. For example, the tractor can have five engines with four pistons each, or two engines with ten pistons each and so on. Although the set only comes with one electric motor, it comes with instructions for adding a second one. The model uses a flex system to steer the front wheels, which some find less effective than the traditional rod and g