First of all the Traxxas trx 2.5 only puts out 1.33 horse power. Don't go with a .21 because then you will have to spend alot more money just to get the big block conversion and everything else that is needed. Get the Fantom .18 I have heard alot of good things about this engine...It is easy to tune and has Alot more power than the trx.. Hope that helped