Tech opinions on alternator reduction drive

[Adsport]

interested in people's opinions:

Will the loss of drag from fitting an alternator drive reduction pulley be negated by the loss (if any??) of voltage / amperage supply to the ignition setup ?

i've been thinking of running a larger pulley on my alternator in conjunction with some other things i have going on at the moment , and forsee this being an issue and hence making the process completely not worthwhile .

when would alternators be making their regulated power output ?

and if people think a reduction drive is worthwhile; what kind of percentage ?

[oldcorollas]

you can find output vs rpm graphs on bosch website.. can probably find similar for denso.
the only reason to run a reduction drive on the alternator is if your engine rpm will overspin the alternator to a speed that it does not like.

the drag/load from an alternator is more a function of the electrical load of the car, and from charging the battery after strating and if it is low on charge.

if you want to reduce alternator load.. then downsize the elec consumption of the car, or fit a smaller alternator (so that it can supply the required constant load, but will charge the battery slower)

that said, a 100amp alternator, with maximum load, is only 1.4kw... it should never need to output that unless it is trying to charge a fully flat battery?

edit: that said, i did underdrive my 4K alternator, as the bosch reg kept blowing from overspeeding the alt (redline raised 25%). as a result, voltage would drop at idle, but would still charge from about 1500rpm up.

the smaller "compact" alts have both higher max and min rpm, but have a slightly larger range as a result...

[Billzilla]

I ran a 1:1 pulley setup on my 4AGE a while back and it just made enough volts at a 1050rpm idle to not make that charge light come on. Knowing what I know now, I'd probably spin the alternator (assuming they're all similar) at around 1500rpm minimum.
You get a very slight benefit from the decrease in change in rotational inertia when changing revs, though it's not a lot. You'd gain far more by making the flywheel slightly lighter instead.

[mrshin]

Drive your alternator so that it can spend most of it's time in a speed range it is happy with - same with your water pump etc.
In other words, change the pully if you intend to keep your engine at high RPM constantly - e.g. circuit racing, but if your car is mostly for traffic light action, then leave alone.