That max torque of a motor is related to kV is a common misunderstanding, for same torque we need to balance the motor current with the kV. 120kV 120A current, 100kV 100A current. The coils magnetic field is given by ampereturns so if removing 10% of winding turns to get a 10% higher kV then the amperes need to be 10% higher for same torque.
One might think this will be a less efficient system since current goes up for the same torque but the higher kV means that less voltage is needed for the motor speed and this balances out to roughly the same power and efficiency as long as the copper fill of the stator slots is the same.
What about increased losses from the higher current then?
Resistance losses in your motor coils are the same regardless of kV if you have the same winding copper fill in the stator slots.
For concentrated windings like on most outrunners the wire transits between coils don’t add a significant amount of resistance but the distributed windings of a 6516x motor will add more due to the longer endturns so there are slightly higher losses per torque there in a higher kV 6516x motor. The motor leads on a 6516x motor don’t add a lot to the resistance, they are sized properly, neither do the mosfet losses in the controller since the resistance there is also low.
(Now i am expecting people to chime in regarding switching losses in the controller. I have seen calculations that showed that even when running lower duty cycle with increased switching it will not create large losses. This obviously depends on the controller so i guess that an inefficient controller can make a difference. If someone has figures on a common VESC then i’d be truly happy to see them)
The battery then? Let’s say we build a 12s10p battery for the 100kV motor, to get same speed with 120kV we need less voltage, 12*100/120 series voltage for the 120kV motor so it’s required a 10s battery, we can then build a 10s12p battery with the same number of cells for same power output. Resistance and energy content with these two battery configurations are the same.
I found that (if the chinese spec figures found on Alibaba are correct) the 65161 100kV motor is the better winding for resistance losses since it seems to have slightly better copper fill in the stator slots, if so then it will be able to produce a small percent higher continuous torque and power but the difference is small. I don’t think there’s a big reason to choose one before the other but going for max efficiency and laid back riding then 100kV has a slight advantage.
On the system side of things i’m not sure what determines propeller efficiency but i don’t think it matters since the drive system losses and efficiency is roughly the same for different kV motors. That means a 120kV motor can run the same propeller rpm as the 100kV motor but also at 20% more speed. That sounds like the 120kV setup has an edge, just note that a higher power setup will require more capable controller which will be a bit more expensive.
On the even higher kV numbers: again going by the chinese specs it seems that the 195kV motor has substantially lower copper fill so it’s then not the best choice.
Boundaries:
The above logic is true for the common motors we use but it’s not for the extremes like single turn motors (not good winding utilisation) and truly low inductance motors (problems with increased losses and difficulties to control the motor.)