I've been using this direct drive (gearless) extruder with 3 mm ABS for about 6 months now and lately with PLA . I took the original Sells Mendel extruder and split it into two in order to use a spring to apply tension on the idler. The advantage of this extruder over the newer MK7 designs that use less plastic is that you can print the whole extruder except for the hobbed bolt or MK7 gear.
I made two versions: One uses the MK7 pinch wheel and the other uses an M8 hobbled bolt that's been cut and drilled to mount on the motor shaft. The advantage of the smaller diameter 8mm bolt/rod is that it can apply more force to the filament.
It can surely go faster if 1.75 mm filament is used. The design for the hobbed bolt should work for 1.75 mm filament if a MK7 gear drive is used instead.
I've also included a 40 mm fan bracket.
There are built-in support that must be drilled with a 4 mm drill idler mounting bolts.
You'll need to either buy a MK7 pinch wheel. The better choice is to take an 8mm hobbed bolt, cut it, drill a 5 mm hole down the center and then tap across for a grub screw to fix it on the stepper motor shaft.