I put in a Titan extruder with a Nema17 40N - DShaft .9 deg/step supplied by Maker Farm. I'm using a basic RAMPS 1.4 board with A4988 stepper drivers and running Marlin firmware. I'm using Repetier-Host and running into a problem (?) configuring the extruder speeds. Any time I set the speeds (extrude or retract) above 20 mm/s, the motor makes a horrible sounding screech, and the motor doesn't spin. Based on everything I read, I should be able to turn that up to something more like 40 mm/s. I suspected the Vref on the stepper driver (which is currently set to 5v), but upping or lowering the voltage didn't seem to improve things. I've also tried swapping the driver with another, but it made no difference. Also, things work perfectly for me at 20mm/s or lower. Also, as a side note - likely unrelated, but odd none the less, I had to change the DEFAULT_AXIS_STEPS_PER_UNIT from the recommended 837 down to 802 to get it to move the right amount of material. I can't imagine why that would be, but perhaps it's somehow related. Can anyone give me any suggestions?