Yea the O.S. .21TM is an excellent engine. One of my RC buddies has one and it is quite impressive, miles ahead of the TRX engines and better overall quality than the Picco Maxx engine. I've never had good luck with the TRX engines after the old Pro .15, which was decent but slow. The new ones run like crap but are fast in my experience. I'm not sure of how good it is by Dynamite makes a .19 drop in replacement, too. If you get the O.S. buy the THS T Maxx pipe to go with it, the stocker seems to choke it off but it likes the THS pipe.