Alternator Sizing - Amp requirements

67Dart273 said
Is this a monster with a really big fuel pump? If not, sounds like your fans are the greatest draw, and in the Sac area, you likely need them a lot at low speed

The fuel pump is just a Mallory 140. I just called Holley, and they said it draws 5 amp max, while the ignition I'll be running draws .7 per 1,000 rpm - so lets say 7 amps there. Looks like a line lock draws about 2 amps. After some searching, looks like the fan I've got pulls about 35 amps on high speed, which is about 50 amps total so far.

I'm just unsure what the load is of high beams/brake lights.

BillGrissom said
When adding up, try to find "running current". I suspect many spec's such as radiator fans are for starting current, which drops greatly after the motor is spinning. At a high level, if your battery stays charged over a week of driving, your alternator is doing its job. One way to know is to put it on a 10A smart charger every Saturday morning and see how long before the charger shows "full". Another approach is to copy a modern car with similar loads, since the designers surely put in the smallest alternator which suffices, as every penny counts.

With a bigger alternator, you likely need bigger than factory wires, and probably bypass the alternator route into the cabin (to dash ammeter) to return to the battery. Search "MAD Bypass". I put large diodes in the underhood shunt route so current still routes thru my ammeter until the diodes start conducting it direct to BATT+.

Thank you! The car will have all new wires, as I'll be wiring it from scratch eventually. There will be no bulkhead connector, or ammeter.

Demonx2 said
I recommend a call to Powermaster. They can share alternator output curves and you can pick from there. As @67Dart273 notes above, 1-wrie alternators need a big charge wire. I designed up my own wiring like you are planning to and since I have a 1-wire alternator, I ran a 4-ga wire from the alternator all the way back to battery in the rear. (I have it fused on both ends in case of a short anywhere!) With a 95A alternator, I can tell you that at 1000 engine rpm, and with 2 electric radiator fans (approx 35A total), 1 trans cooler fan, and all headlights/brake lights on, the voltage measured at the battery matches the voltage up at the back of the alternator so the 4-ga wire is apparently doing the trick in terms of minimal losses. I share this just to say the 95A rated output is sufficient for the loads I note.

This is great information. Thank you! Do you run an electric fuel pump as well? Which alternator did you go with?