I’m curious as to why you are concerned about the rpm’s of the Arri 16 Variable Speed Motor.
I've never seen rpm figures for the 'Wild' motor printed in Arri literature. There were rpm figures listed for the 24fps sync motor however, but that had to do with differences between 50Hz and 60Hz, and 24 or 25fps.
This 8 volt 'Wild' motor was the 1st motor Arri offered for the 16S when the camera was introduced in 1951. (The history lesson of this post)
This is a pic of the 1st generation 16S with the wild motor. Notice, there was no opening for an external magazine. Just 50 or 100ft rolls.
This is the finder side......
The wild motor design didn't change much until Arri changed the voltage requirements from 8 volts to 12volts in, or around, 1980.
The math to figure out the rpm's is fairly simple, in my opinion, but if I'm wrong, someone please correct me.
The drive shaft where the motor fits into .....
has to rotate 3 times to achieve 1 exposure. This includes the shutter rotating, the un-exposed film pulled into place, the registration pin to register the film, the shutter to open and expose and then close.
You can easily check this by lining up the mirror from the front to cover the aperture, then rotate the drive shaft 3 times. It should stop in the same 'position' as you had placed the shutter originally. You could rotate the shaft with your finger on the coupling, or you can have your wild motor in place, make some marks on the inching knob and motor housing to give you a reference point, then count the turns on the inching knob.
If it takes 3 turns of the camera's drive shaft to expose 1 frame of film, then 24 frames would take 72 turns or revolutions of the motor's drive shaft. You want to run your camera at 24 frames per second, then your wild motor would be running at 72 revolutions per second. 60 seconds in a minute would give you (60 x 72) 4320 revolutions per minute, or 4320 rpm.
From a 1970 instruction manual...
However, your wild motor will handle 12 volts, in fact, Arri said that in a 1972 product brochure....
The wild motors on the 16S are designed to be run at 8 volts. They'll work at 12 volts, but the higher voltage damages the brushes of the motor and it will shorten the motor life.If running with 12 volts, don't use the 400ft mags and 8 volt torque motors. Those small motors are designed for 6 -8 volts, given 12 volts they will run too fast and strip the film when 12 volts is applied.. So the 12 volts has to dropped down to 8 volts. Easily done.
Arri says the wild motor runs from 6 - 40 fps. Well, I've run mine from 4 - 50 fps off of a 8 volt battery, without a 400ft magazine and torque motor. It depends on the condition of the battery.
Finally, I can see how you came up with the 2100 rpm figure. The 16S is not the same as a camera with a 180 degree half moon shutter where half the 360 degree shutter disc is black and the other half open, but it has a 'Bow-tie' shutter. It consists of two open segments (90 degrees each) positioned opposite of each other.
A bad pic of a Bow-tie shutter...
Butterfly shutters rotate at half the speed of a half-moon shutter. As explained by fellow forum member, and knowledgable of all things 16S, Tim Carroll:
"Okay, I am not sure what folks are referring to as butterfly shutters, but if you are really meaning bow-tie shutters, then the Arriflex 16SR does not qualify. The Arriflex 16S, 16S/B, 16St, and 16M all have bow-tie shutters. On the Arriflex, a bow-tie shutter doesn't expose the negative twice, it rotates at half the speed of the movement. The mirror does a complete revolution every two frames."
and by forum member Jonathan Benny:
"The disk with two 90 degree openings is turning at half the speed of what a shutter with a 180 degree opening would, therefore the same shutterspeed. Each frame is only exposed once and the film is advanced between each 90 degree opening."
"Arri S and 16BL segment the mirror itself with a small stripe so you see your "48 flicks" so to speak in the viewfinder but this has no effect on how the film is exposed when the shutter is open."
The 90 degree opening is moving half the speed, therefore it's exposure is the same, or equivalent to a 180 degree shutter.
There you go, something to think about. lol.