What do 1G, 3G, etc mean on alternators?

The "G" is for generation. This may be a good read for you..

Ford Mustang: Mustang Tech: Electronics and Ignition: 130 Amp 3G Alternator Install

the problem with many one wire conversions is that the alternator is not self energizing, and as such requires a slight load to be put on them at around 1000-1500 rpm before they actually start charging.

I don't know if that's much of a problem tho, whenever I start mine it usually buzzes up enough to energize the alternator.

I changed out my small case 1G 65A for a large case 100A (mid 70's). It uses the same number of wires and the same voltage reg. as my stock one did so there was only some wire splicing at the connections for the rear of the alt., pretty simple. I only needed to loosen/lower the lower mount for the alt. and use a ~1" longer belt. I get full charge (headlights are at full brightness at start/idle with no revving) at idle. I have been happy with the upgrade and it still looks fairly stock unless someone notices that the alt. is a little larger.
Here's some pics of the initial install before I tore the car down and repainted everything, the wiring will be bundled/cleaned up at install. I picked the alt. up for ~$40 new uninstalled on fleabay and had it tested = perfect. You can get them at any parts store with a warranty.




I have added a rubber coil cap over the red '+' terminal on the alt. to protect from accidental grounding with a wrench.
Just another option,
Jon