Changed my 640 over to 12 volt last fall. Read alot about the 2 different types of alternators and decided to go with the 3 wire over the single wire. My understanding is the 3 wire will charge at pretty much any RPM. Whereas the single wire needs to be revved past 1700 RPM (or so) to initiate its charging circuit every time the motor is started.
Please chime in if my info is incorrect.