The Novak HV 4.5T is so popular in USA with SCT that they started manufacturing it again after a slow run with 1/8th electric cars...
Tried a HoBao 4WD SCT running one on tuesday night and was impressed enough with its smoothness and torque that i've ordered one for my Losi 4WD SCT... will be running it with the same Fusion Comp speedo i ran in Hong Nor SCRT-10, which is compatible with 550's and rated at 150A.
for 4WD SCt the 550 motors are really recommended... my 540 size 6.5T was getting hot indoors, even with 2 cooling fans on the motor it was just acceptable on short tight tracks... but was stupid fast
The 550's seem to use less battery power with same gearing and have lots more torque... but more importantly don't get hot and will handle the heavy drivetrain better.