it would be less of a strain though wouldnt it? ur alt only spins as much as it is going to it doesnt get harder to spin when u draw from it does it? so in a sense u take the fan off of the engine and run it using the electricity u are already priducing anyway...right??
Bobzooki's first law of Automotive Physics: Nothing is free.
If you have a MORE EFFICIENT alternator, it will produce more power for a given load on your engine. Guess what? Nobody is trying hard to make a more efficient alternator.
So, if you take out an 80 amp alternator, and put in a 100 amp alternator, it's gonna be harder to turn when you're drawing full power. That's a bigger load on your engine.
So, you say, when the electric fan kicks on, the RPM's don't drop. That's because of two things. (This is hypothetical.)
1. A 5 amp draw on an 80 amp alternator isn't exactly going to break the bank - it'll take a little more force to turn it, but not THAT much.
2. When the fan turns on, it draws its initial current surge out of the battery, and the alternator load gradually increases as the battery voltage is drawn down (and the voltage regulator demands more current to charge it back UP). So, you would notice it a lot more, if the battery wasn't there.
You could try this experiment:
Start the engine, and disconnect the battery, so you're running entirely on the alternator. Turn on the fans (rig it). Now you will se the ACTUAL draw that fan puts on the electrical system. The battery just softens the blow.