Trade: 3400 Alternator/Brackets for 3100

[neophile_17]

Hey guys,

I'm doing an engine swap and need the extra space of the smaller 3100 alternator and don't need the juice to power a concert stadium complete with light show. Sooo I'd like to trade a large 3400 alternator which looks to be a new rebuild for a smaller 3100 alternator. I have a picture of the brackets and the 2 alternators I have although I prefer to only ship one. Picture can be emailed on request since I can't seem to post it. I will have it tested at the auto parts store first.

We will exchange the alternator, alternator bracket, engine hook, and bolts. I think these are the only items needed to up/downgrade alternators.

Keep in mind that you will need longer serpentine belt and space for the bigger alternator. I don't know but the wiring harness may also need to be extended.

Thanks,

Sam

[robertisaar]

what did they come off of?

[neophile_17]

The newer rebuild is from an 2003 Impala and the stock unit I don't recall as I bought it over a year ago but I try to get 2000 or newer parts. I always try to pick these things from cars that have an obvious reason for being in the yard that doesn't affect the piece I'm trying to pull.

[robertisaar]

hmm..... GM rates the one i have at 105 amps, looking on rockauto, showing the same for a 3400 impala. they look nearly the same size though? unless you have one from a 3800?

[neophile_17]

That is does. Hmmm. After discovering that the two alternators are different sizes while hunting for a spare I read a couple posts of guys trying to upgrade their 3100 alternators to 3400 units. I guess they aren't all high output but I wonder why GM would use the larger size to get the same output. Maybe it offers more power at a lower RPM??? Oh well I guess I'm just going to spend another couple hours going to the yard to get a small case alternator and bracket.

Sam

[neophile_17]

Because I'm a little obsessive I looked into it some more and there appears to be a 100A and a 105A CS130D with the 100A version putting out about 10A less at most points and almost 25A less at idle. Below is the graph from AC Delco on the two.

http://www.acdelcotechconnect.com/images/tas_alt_cs130d_curve.jpg

I have no idea what rebuilders do when they remanufacture these things and how they label them afterwards...

Oh and they say the alternator turns about twice engine RPM for point of reference.

[neophile_17]

Hello,

So I figured I update this since I did manage to get myself a smaller case alternator at the JY and I spent some time trying to figure out the difference. Maybe it'll help some one else out. I won't pretend to know anything for certain but the internet wasn't very forthcoming with pertinent info and so I'll share my experience with a limited number of cars in one self service JY.

1) There are at least two different case sizes used on 3100/3400 cars but the size of the engine does not have a bearing on which case you will get.
2) Every small case alternator I saw had a smaller pulley as well which causes the shaft to spin faster and partially compensates for the diminutive size. (Random thought: If you were low on charge a large case with a small pulley might get you where you need to be. Similarly you could use a large case pulley as an 'under-drive' pulley on a small case alternator.)
3) To switch case sizes you need a bracket for the case size you're going to.
4) If you're mounting the alternator in the stock location and swapping the engine into a Fiero then you definitely want the small case version.
5) There are more mounting bracket styles than cases. Oldsmobiles tend to have a 3 point mount style and other cars have a 4 point mount style but this may be related to me having too small of a sample size. There are at least 3 distinct mounts.
6) If you're really obsessive about weight there are a couple different engine hooks that go behind the alternator bracket- one of which has several ounces less metal but is also a little weaker if you aren't using a balancing beam to pull the engine. Come to think of it the diet conscious should just replace the bracket with an appropriate size aluminum spacer and stick the steel hook on the shelf.

Hope this helps someone.

Sam