4400 - Traxxas® makes it easy to upgrade TRX® Pro .15 nitro-powered models to the awesome power of the new TRX® 2.5 Racing Engine. All the necessary hardware to bring older Traxxas models up to full TRX 2.5 spec is included such as: header, pipe, pipe hanger, coupler, linkage, and additional components specific to each model. Major components come pre-assembled and pre-adjusted from the factory. Just bolt them on! Detailed instructions are included. TRX 2.5 Racing Engine (model 5207) sold separately.