Which alternator? amps vs volts ?

[Egor Rapoport 10+ year member]

Hey guys,

I got 1995 mark VIII which has a stock 120amp alternator.

I am running Hifonics 100Wx4 amp for speakers and Memphis Mojo 4000W @ 1ohm for subwoofer. I also have an Optima Yellow Top battery.

Currently with stock setup my voltage would drop to 11-12V and my Hifonics amp would shut down.

I bought a high output alternator (mitsubishi). It is 220amps @ 1200rpm. problem is that when this alternator idles it gives 13volts @ 600rpm. Once you rev it up to 1200rpm it provides 14.4 volts as needed. Having this alternator improved voltage drops by it is still not very good.

I am also getting 2nd battery Optima red top to help voltage drops.

The guy who I bought high output alternator says that this is how ALL high output alternators work and i either need smaller pulley (which he cant find) or he is offering to take the alternator back and give me a refund.

Here is my question.

Do I:

A. Stick my stock 120amps under the hood which does do 14.4 volts at idle but less amps then stock

B. Keep 220amps alternator which does 13volts at idle but more amps then stock

Also, explanation would be much appreciated!

Thanks in advance!

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[anwaypasible 10+ year member]

that sounds like a voltage regulator problem.

see, voltage comes first and then amperage.

this isnt an exact example.. but it will give you an idea of what is going on.

say you have 1volt with 0amps

and then you have 2 volts 0amps

but then you use a voltage regulator to push the 2volts back to 1volt and that gives you 1volt and 1 amperage.

there is an equation that exists.. and i dont know it, nor do i know how complicated it is.

but i can guarantee you that if you have 1volt at 10 amps.. you can also have MORE volts with less amperage.

its a vice-versa situation.. and sometimes these things can be adjusted.

that is why there are adjustable voltage regulators.

if you adjust the voltage regulator to give higher voltage.. then the amperage at the new voltage is going to be less.

i'd say keep the big alternator and instead of purchasing a second battery that runs at 10 - 12 volts.. purchase a capacitor that works well enough to keep the amp from shutting off at engine idle.

also make sure your capacitor is big enough to last while you are at a stoplight.

the cap isnt going to do you any good if it goes dead before the light turns green.

but once the light does turn green.. the big alternator should be able to power your system and recharge the cap with no problems.