The 1521 works better for bashers than the 1515. I like large tires, Moabs, Big Joes ect. on my trucks and that puts a strain on any motor. The 1521 gives wheelie popping power while keeping things very cool. LOL-I had a set of 8 1/2 inch Jumbo Kongs on and was doing 40 foot long wheelies. I also like to throw on a set of sand paddle tires and go hill climbing. The 1515 I had was at its torque limit and my 1950 Lehner was beyond its limit. Feigao? HaHaHa poor Feigao de-magnetized.