Everyone brings up the issue of the .12 being legal for racing and that's true if your running R.O.A.R. events, or your track follows the same rules. The truth is most tracks will let you race with a .15, or .18 engines, especially in the novice and intermediate class. Very few tracks that I've run across even checks your engine. The strictest track I've been to, only checks the "A" class trucks for engine size. In your case, I'd go with the .15.