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Re: Choosing a replacement alternator - restrictions?
mb, generally a higher capacity alternator is longer, form front to back. And since the pulley is up front and has to line up the same way, they usually make the alternators extend further BACK as the capacity gets larger. So the position of the mounting leg(s) and pulley stays the same, and you need another inch or two aft of where the existing alternator is.
You'd want an alternator from the same series, so it bolts up the same way. With the same pulley diameter so it runs the same way. And generally if you stick to the same series, they are built to operate in the same rpm range so you don't have to worry about matching that, although you certainly CAN see if there's a better match, i.e. a different pulley size that will give you optimum charging at a lower engine speed.
You've got 180AH of batteries, let's say you take good care of them and don't cycle them below 50%. So all you need to replace is 90AH of power, and since the batteries will only accept 1/4C (1/4 of their 180AH capacity) during charging...you probably can't pump more than 45A of power into them, even with an external regular, unless they are deeply discharged.
So a common 50 or 55A alternator would be a sensible step up, maybe even a 60A alternator, since they all cut back their power as they operate and heat up, and a 60A alternator might actually cut back to 45-50A maximum power well before a half hour was up.
You might also want to consider an external regulator while you are doing this. Any alternator shop can also modify your alternator (bring out 1 wire and add one switch) so you have the option of external regulator, with the internal as a 'spare'. An external regulator is better for deep cycle batteries, it charges them faster and with voltage levels that are optimized for longer battery life. Even if you just modify the new alternator now, and add the external regulator later.