I am replacing the SI alternator in my '87 GT with a CS130, but I'm not sure which pulley to use and hoping someone knows...

My '87 is a bit a Frankenstein, with a 3.4pr and an '88 cradle. I don't know how much of each car was actually used. I have the SI alternator off, and a brand new AC Delco CS130 sitting side by side. The CS pulley is tangibly larger than the SI pulley, but both share the same 4 (?) rib width, so either would work. I bought the specific part number for an '88 Fiero (#3351010), so I suspect I should leave the pulley, but I ultimately don't know what crank pulley I have, though I believe all the 60*V6 motors have the same pulley so I should be good.

Any comments?

You should be just fine.

I would suggest leaving the pulley alone.

Thanks - left it alone & bolted it up!

Advertise Amp are Peak Amp and alts needs to spin fast to generate it.
See example charts in my Cave, Watt Story

Bigger alt pulley is same results as "power" crank pulley.
either can slows alt and that can cause alt nearly never making full power.
IOW Depending on drive(crank) pulley maybe ok or you run out of power often even driving on highway w/ power hungry lights sound etc.
Many "Power Pulley" users have found out this the hard way... Worse case the battery can drain so low the car stalls on the highway and won't start.

you need more data to see final results.
diameter of crank and alt where belt are (and maybe others.) Think math is crank size ÷ alt size = y:1 ratio then Engine RPM X y = ALT RPM
or optical tach to measure RPM on each shaft.
Example:
Many engines cruise @ 2300-2500RPM needs an alt pulley set so alt spins 2 to 2.5 times engine RPM.
But
Many farm tractors and city vehicles often runs lower engine RPM and may need to spin the alt 3 times faster to make enough electric power.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 04-18-2018).]

Yeah, I understand how to compute pulley speeds, but unfortunately no matter how much math I do about pulleys all I could ultimately do is guess. I don't know what the car's consumption is, best or worst case, so ensuring the alternator's output is X amps doesn't really help me. Truly, I don't care if the alternator never makes full output, all I need it to do is make enough. All else being equal, underdriving it does make some sense - 80a was doing the job just fine, so I have no need of the full 105a output of the CS130.

My hope was only to replicate the factory setup, assuming that GM had done its homework. The choice is '87 SI pulley or '88 CS pulley, smaller vs. larger respectively. My assumption is that the crank pulley did not change between '87 and '88. If that's true, then I can also assume the pulley that came on the alternator is the correct one. Additionally, since we know the CS makes more amps at a lower rpm, it certainly stands to reason there is no need to drive it at the same speed as the SI.

I'm gonna run the larger pulley and watch the voltmeter. Worst case, I call AAA and swap a pulley.

[This message has been edited by thesameguy (edited 04-18-2018).]

22P lists same crankshaft pulley for all years.
P/N 10041616

TFS suggested the same, which just reinforced yours & Blacktree's suggestion and confirmed my initial guess.

I am waiting on some thermal wrap to replace the degraded stuff on the forward engine bay wiring harness, then the car can go back on the road... Unfortunately, it seems the carrier has misplaced my Summit order, so it may be some time before that happens. Of course.