I was just making a statement regarding running the 1515 2.5d in my truggy on 6S and it getting warm. In fact I think that I got it up to around 165*F once, but that was geared to bust butt and I was running in grass.

As for monster trucks, I run the 1515 2.5d in my Monster GT on 4S and 5S lipo geared conservatively for about 35 mph. I can run all day with no issues. My MGT goes about 13 lbs with the 40 series Moabs on it.

I just made the above recommendations because it's what I would do if I had it to do over again. I just have to comment again, that when the going got rough the 1515 2.5d did heat up a bit on 6S.

Contrary to what some are advocating with regard to the Neu motors, I still believe that keeping the rpms to about 30,000 is a good target for a truck that's going to get bashed hard. For a 1515 2.5d, that means running it on 5S lipo or something similar.

My only concern with going with the 1521 is the torque. I already break tranny parts with the 1515. With the 1521, it might be worse.