120amp + is some pretty serious ICE!
If you're running non competition levels I'd advise against going beyond 70 amps, remember a bigger alternator will pull power from the engine. If you're not using it you'll be gimping your performance as a result.
But no, you won't need to rewire the whole car. the electrics will only pull the currant they need, the alternator doesn't pump it about the car like water or gas, for example :)
Did the same myself just a week ago.
No you won't...well not entirely true but close enough for the purposes of this. A 45A and a 75A alternator will both draw almost exactly the same amount of power from the engine to turn them (actually the 75A may take a tiny bit more, but you'd be lucky to be able to measure it) - any additional power would be mostly for the slightly bigger cooling fan on the large capacity alternator, together with someone called "iron losses", but so small as to be not worth worrying about.
Edited by Captain Mainwaring, 11 June 2013 - 12:40 AM.
