It appears that "400" is for the length of the motor= 400mm. Graupner also uses that for their motors i.e. speed 300 = 300mm in length - speed 400 = 400mm in length. I looked at specs for the Kontronik motors but couldn't get the light bulb to turn on in the brain. There is no standard for motors used in the hobby thus the numbers only mean something to the manufacturer.