I would be grateful for any info on the subject, to satisfy my curiosity.
PS: Current best theory I can find is:
Find out the pitch of the engine in hertz, and then that is the number of cylinder firings per second, divide by 8 and you have revs per second. Multiply by sixty and you have rpm. I've never done this, and I may be completely wrong, but intuitively, that's how I think it could be done.