Hi 971,

The difference between the speed 400 6v and 7.2v is that the 6v is a hotter motor and will draw more current, less torque but higher rpm's. The 7.2v is softer in that it will withstand higher voltage and current draw and thus more torque output, albeit at slower rpm's.....used to use these motors a lot in electric flight before it all went brushless......

Cheers

CaptB