A poster on another forum asked about the performance differences between the 2 size motors:
Since the HV 550 motor has a stator and rotor magnet that are twice the length of a 540 motor, it will have twice the torque of an equivalent Kv 540 motor.
HV 4.5 550 (5000Kv)
Bal. 8.5 540 (5000Kv)
Therefore, if you are running two motors (one 540, one 550) with equivalent Kv in the same vehicle under equal driving conditions, the 550 motor will only have to work about half as hard and will heat up about half as much as the smaller motor.
I am responding to a racer (on RCTech) who burned up our Ballistic 8.5 in a 4x4 SC installation. Our warranty does not cover burnt motors or stators; but, both ROAR and 540 motor sellers (including Traxxas!!) promote the smaller 540 motor for use in the heavier 4x4 trucks.
Customers are really not happy with motor companies when their motors burn up.....
Companies are also not happy informing customers they're not getting a free motor/stator replacement....