I remember reading that school bus alternator article too! I think it was in an early 2000s issue of Good Old Boat, is that possible?
This general topic has been discussed at length at moyermarine.com. Here's what I've taken away from it:
- 1HP is theoretically about 750 watts, which at 12V is 62.5 amps. At 14V it's 53.5 amps. Considering the effects of energy loss, figure 3.2HP to create 100 amps or 1200-1400 watts of actual output. The A4 has enough juice to do this, even underway, but since the engine doesn't run at close to 30HP output you're going to feel it if you're underway.
- Single belts start slipping around 100 amps; you could outfit the aux drive and the alternator with double pulleys to transfer more power, and it is not too difficult to get a custom pulley made for the aux drive, but...
- The alternator has to be spun at an insane speed (by A4 standards) to put out anything close to max output. The pulley on the alternator would have to be so small to achieve that speed that there would be very little surface area and belts would start to slip, so...
- Alternators are rarely able to put out much more than half their rated output
I installed a pretty good-sized inverter to provide power at my mooring for tools including a heat stripper and a Shop Vac. My takeaway from this discussion was that to get a meaningful amount of juice out of my A4 for this project, a pretty big alternator would be necessary, and that there was no easy way to get 1500 watts out of the inverter without tapping some of the battery reserve.
I ended up getting a good deal on a used-and-turned-out-to-be-broken 120-amp Moyer alternator, had it rebuilt, and it's worked quite well. Single belt, I think I had the pulley replaced to fit the stock A4 aux drive, it doesn't slip, but probably never puts out more than 60 amps even when the heat gun and Shop Vac on the inverter are dragging down the battery voltage and the engine is spinning at 2300 RPM.
So Randy, if you're thinking what I was thinking, I would encourage you to go big with your next alternator or stick with the status quo. I don't think a "slightly" larger alternator is going to do anything for you.