The TRX .15 is an older model engine from Traxxas. It is still in production and is still included in the Nitro Sport and Nitro Stampede. It replaced the TRX .12, and was replaced with the TRX 2.5. The TRX .15 is available in a sport and pro model. The pro model has a bigger cooling head and a two needle carburetor. The sport model has a single needle carburetor. The sport model was originally in the Nitro Sport.