Finally i figure out the reason why the faraday disc generator experience less back torque. I got the idea based on my experiment here http://www.overunity.com/14323/i-think-i-discover-something/ (http://www.overunity.com/../../14323/i-think-i-discover-something/)
Assuming that the magnetic field rotate as the magnet rotate, the faraday generator have less back torque because the external circuit are exposed to less magnetic field thus producing less Lorentz force. On the other hand, even though the disc are exposed to high magnetic field its mechanical attachment to the magnet prevent it from producing forward torque (aidding the rotation).