I have the TRX 3.3,Picco Max .26,OS .21TM and RB TM523. I have four Revo's.All have plenty of power but I think the OS .21TM has the smoothest power band,great low to mid rpm and holds a tune The RB is almost to violent for the track.Its cool that it comes with a header,mount and pipe.The Picco lacked top end put the new JL Red Dot version may solve this.I also ran the Traxxas resonator pipe.The 3.3 has great mid to top end but a lower quality engine.Stock up on engine bearings or get thr TKO ceramics.The OS .21TM is over priced but a great engine for the Revo for racing.It provides smooth power,decent fuel economy,light weight,bolt in fit with no issues.