Also - because this is fun - I decided to build a hybrid bowden/direct multi-material extruder. This attached the splitter to the top of the direct extruder:
The goal here is to allow me to print with multi-materials, including ninjaflex. The direct drive stepper takes care of making sure the softer stuff extrudes correctly, while the splitter and bowden steppers take care of moving material into the splitter. In theory at least, I can switch materials just like the original MM design, but have the advantages of a direct extruder.
A couple of interesting things to note while testing this. First, I realize I'm going to have to mess with the Slic3r code because the distance to unload/load when switching materials has increased (maybe its easily configurable somewhere?). Second, The original MM electronics came with a spare 4-pin socket which lets you plug a stepper directly into the stepper signal coming in from the Rambo board. That means, in theory, its easy to run the direct stepper whenever any other extruder stepper is running (since that stepper signal is common to all); and that's what we want. I notice the new MM electronics don't have spare socket anymore. Third, I realize I'm running two steppers in parallel off the stepper drive. Since this is what the Rambo does for the y-axis I figure this isn't a problem. However, I can only get the new Prusa steppers (the ones that came with the MM upgrade) to play well together. If I try to use the original extruder stepper, or another random stepper I have, then they start skipping. Not sure why yet.
Here's the ninjaflex test, again at full speed: