I spose a good thing about todays current batteries and brushless motors allow you to basically run a 40,000 rpm motor or around that size and simply limit the throttleon the transmitter much like a carb restrictor in a nitro engine, this allows you to have the biggest if need be and also something smaller if you choose to go smaller, todays ESC`s have no worries having limited throttle and would behave much like a smaller wound motor having the throttle limited. I have a GTB 6.5 in my XXX4 G+ but if the track wont allow that kind of power and speed i can simply limit the throttle on the transmitter. Then i have a 7.5 or 8.5 etc.