During my travels to and fro various electrical jobbers getting Alternators
and Starters "rebuilt" I came to make the acquaintance of several techs doing
the work. During this period I came to understand the modular nature of the
Bosch alternator. For a given central segment, there were many front
assemblies, still more pulleys, multiple back ends, several different
regulators and basically one diode board. The difference between a 65 amp
alternator and a 130 amp one was that the diode board was populated with
twice as many diodes as the 65. For any given center section (defines a class
of alternators) there is a maximum number of diodes that can be accommodated
on the board, therefore a maximum rating. As you know, the current carrying
capacity of a diode is a strong function of the temperature. In order to meet
the 65 amp continuous rating, the alternator probably has to be immersed in
solid CO2.

In any case, the only way to look at these ratings is that a 130 amp unit
will put out twice as much current as a 65. Of course, for any given current
draw (say 47 amps  as in when I'm heading across Texas with the stereo
cranked and Wagner on the deck) the more the diodes, the less the diode board
temperature and that translates into longer times between stops at the
Alternator jobber.

Alistair's tip about the Audi air baffle on the back of the alternator
translated into big lifetime savings and increased output. The cost of having
the tech mix and match the Porsche 944 tail with the Vanagon front and the
928 diode board was minor. The confusion at a VW show when a purist tries to
figure out when VW mounted this alternator is truly priceless.

Frank Grunthaner