Alternator sizing

[Tom Vogel]

I have a 140Amp powermaster to go on my F1R procharged BBC. It has a huge fuel pump, big duel elect. fans; 7AL ignition and AC. My mechanic says he thinks the 140 amp alt will be inadequate when cruising,... that it wont keep the battery charged at lower RPMs. He suggests dual batteries.
Any opinions? TIA, Tom

[GetMore]

If the alternator won't produce the power needed extra betteries won't help you all that much.
What rpm will the motor idle at? What is the redline? What is the ratio of the crank pulley to the alternator pulley?
You should be able to check the output curve of the alternator and see how many amps it can put out at idle in your car. Then figure out how much power you could possibly draw and see if you've got enough or not.

[gmachinz]

Most 140-amp alternators do provide about 60 amps @ idle however, at least 20 of that is used up with just basic lights, radio, etc. so of the 40 remaining, add up your total amp draw from all of your accessories and see where that leaves you. Most 1-wires are RPM based in terms of total amperage output so the alternator RPM has to be moving at a pretty good clip to put out more power. You can cheat it by swapping to a small diameter alt. pulley but even that's just a bandaid. My advice is to go with a 140-amp, three wire GM style 6" cased alternator and wire your charging system accordingly. It'll be much more reliable. Email me if you need details-there are a lot of ways to go with this.
You need to add up the total vehicle draw @ max. amperage ratings, then pick the alternator according to power demand. Ideally, you want an alternator that can supply 60% of the total amperage that can be drawn from your vehicle-this will ensure your car stays charged, alternator won't be over-worked, etc. When the numbers are close, the last thing you wan tis an alternator that is RPM based in giving you enough power-if you need it at idle-2500 RPM primarily, you should most likely go with a stock style 140-amp three-wire design.