So I came across a new high output 3g after market alternator at a good price but I'm not sure the stock pulley will be adequate. Its rated at 240 amps and it came with a v pulley that's much smaller in diameter than the stock pulley. I can't use the one that came with it since its not a dual pulley. Does anyone know where I can get an adequate pulley for this thing. I've looked around the internet and can't find anything about this and i've never really dealt with pulleys before so any advice is appreciated lol.

I have the stock pulley on the new alternator now but haven't tried it since I don't have the cables I need yet. I'm going to try it and check the voltage ,but if the alternator was made for that small pulley I don't think the stock pulley will be fast enough.

 

two things you need to know about pulleys, make sure it is the right width and make sure it's inline with the drive pulley.

does the aftermarket have an operating rpm range listed?

 

i have twin 130a 3g's on my van. the truck came with a monster of a pulley, 3 3/8" IIRC. when i installed the alt, i found a pulley from an 80-ish dodge truck that bolted right up and was smaller, 2 3/4 or 3, somewhere in there. but its for the narrower belt, so its not ideal, but works well enough.
when i added the second alternator (for my inverter system with its own batteries, divorced from all else in the truck), i took a pulley from an 80-ish ford with the 300-6 because of its smaller diameter, which is nearly as small as the serpentine belt pulley the alternator came with. it too is the narrower belt. and its too thick in its mounting, so i had to mill it down inside to be able to get the nut all the way on, and shim it out from the alternator with a washer. it was a bit of work, especially with my crude attempt at a mill.
i think it might be possible to use a pulley off an old volvo 240, as those are the same outer dimensions as the last ford pulley i mentioned, but i think and better able to mount up to these alternators. but I HAVEN'T TRIED THIS ONE.

all this said, pulley size makes a HUGE difference in alternator performance. with my first alternator, running my inverter with my compressor (10 amps @ 120v), engine idling, the inverter hits the low voltage alarm after a couple of minutes. keeping the engine at a much higher RPM lets it keep up. to contrast, the one i just installed with the pulley i milled out, remains over 12v under the same load with the engine at idle, and though i haven't tried it for a prolonged run, it seems it would keep up.
all that said, i plan to pull a pulley off one of those old volvos and put it on my first alternator sometime soon.

and BTW, all the pulleys i mentioned above are for 2 V belts.

 

I replaced my original 70 amp Alt with a 130 amp 3g alternator from a 1997 Taurus. I replaced the serpintine pulley with the original 2 vbelt pulley from the old 2g alternator. Haven't had any problems since.