Traxxas recommends the High-Output BEC, or battery eliminator circuit, as a required accessory when installing the Traxxas #2255 High-Torque 400 servo in a TRX-4 or TRX-6 model. The Traxxas High-Output BEC ensures maximum performance with a perfect factory fit. The kit includes everything needed to install the BEC into TRX-4 and TRX-6 models including: BEC, wire harness, and extended receiver box cover with seals.