Can I make this extruder work by changing motor? I have a Flex3Drive remote-direct-drive extruder I bought for my Ender 3 (with the original extruder motor), and I'm pretty happy with it except for abysmal retraction performance due to the 40:1 reduction, which requires the motor to turn about 480° to retract just 1 mm. This is minimally workable for PLA and rigid materials where I can get by with just 1 mm of retraction, but it takes just as long to retract 1 mm as the original bowden extruder did to retract 6 mm, defeating a lot of the appeal of a direct drive. For TPU I haven't been able to make it workable at all. By the time I retract 3 mm or so, which seems necessary to avoid stringing, the hotend has already melted and/or deposited a blob on the print. Failure to print TPU also defeats much of the point of a direct drive.
I'm running retraction at 8 mm/s, 500 mm/s² acceleration, which is already higher than the manufacturer of the extruder recommends, and about the fastest I've been able to get it to work. Based on this calculator and 8.9 mH motor inductance, that seems roughly expected. I have TMC 2209 steppers and tried playing with current but it doesn't seem to make much difference.
Otherwise, I love this extruder, and want to make it work. Is there a way I can salvage it by changing out the motor for something that can do higher RPMs and accelerate just as fast or faster?
In general I would use https://github.com/rcarlyle/StepperSim which takes into account more parameters.
You can play with voltage and current to see which combination gives you the best results for your motor.
Or you can change to a TMC driver with higher voltage (35-50 V) to keep torque at much higher speeds and push the current motor more.
Since the torque you require is likely not so high, you can increase the speed of your stepper motor with 3D printed herringbone gears, for example 4:1. They don't need to be super accurate, backlash is totally fine considering the ridiculous 40:1 reduction.