Has anyone come across a circuit for a manually-adjustable, or even
automatic regulator circuit
to control a 24V car alternator?

The crude way is to use a voltage divider on the sense input. For the
GM 12 V 120 A alternators used on the 86-87 turbo V6 Regals, an example
diagram is at: http://www.gnttype.org/techarea/projects/gnalt.html The
idea is to use a throttle or boost actuated switch to kick up the output
voltage to give the ignition and fuel pump a boost at wide open
throttle. On these alternators the dash volts light is a 194 and it
goes from battery to the L pin on the 4 pin connector, to provide both
sensing input and idiot light. The internal resistance of this bulb is
crucial, you will blow things up if it is shorted and make things very
unhappy if it is open. The S terminal, on the other hand, is for
sensing only and can be connected directly to the battery. I don't know
the limits on output voltage but 16 V is what is commonly used in
racing - enough to make a real difference in ignition and fueling but
not quite start popping lightbulbs during a sub-12 second quarter mile
blast :-). This works on all the GM alternators I've seen from the 70's
and 80's. Somewhere in the 90's the engine computer started controlling
the alternator and I'm not familiar with the details on those. A
commercial external regulator (I think; it may just be a divider too) is
available from Precision Turbo & Engine in Indiana, USA.

--
Regards,
Carl Ijames carl dott ijames aat verizon dott net
(remove nospm or make the obvious changes before replying)