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The factory alternator on early cars was some where in the range of 40 - 60 amps.

There is an article in the tech section that covers this subject.

http://www.fordmuscle.com/archives/2004/07/6GAlternator/

If you are running a decent modern stereo, halogen lamps, electric fan, electric fuel pump, hi power ignition...maybe an electric water pump. You need to add the current draw from each item to determine your amp deficit. Any thing over 40 amps is being pulled from your battery.

General ROT, rule of thumbis 80 to 100 amps at a minimum would be worth the swap. More woudl be better.

To complicate the mattter, you need to know how much the alternator is putting out at idle, that is as if not more important that what it puts out at speed. If most of your driving is in town, lots of stop 'n Go or cruising time...even a high total amp alternator may not create enough juice at or below 1000 - 2000 rpm to keep your battery charged.
 

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Beoweolf is right, decide what electrical accessories you want then add up the current draw, you might need a 130A 3G, I am getting by with a 95A on the Cougar, it has an elec. fuel pump, and elec. rad. fan and electronic ignition.

I put a 130A 3G alt on the '88 GT then added an elec. fan.

_________________
'88 Mustang GT, T-5
'67 Cougar, Modified -- Loudcat

<font size=-1>[ This Message was edited by: wdflarity on 4/8/06 10:28am ]</font>
 

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Bigger is better in this situation
 

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bigger may be better,but keep in consideration of your lead wires for the altinator. I upgraded the alternator on my 1984 F150 with a 302...went from a 55 amp to the 100 amp old style, and pulled the entire harness off another truck; I am going to be using that same piece in my Torino once I swap over my correct pulleys for my 351C CJ.
I'd like to do the G3 conversion, but I need to get other things done first before I worry about the electrical, like getting my '99 C.Vic 12.5" brake rotor conversion done first.
 
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