How to: GM12SI alternator on a small block

Electrical and Ignition

  1. goldduster318

    goldduster318 Overzealous Car Modifier

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    I've had problems off and on with the original Chrysler dual field alternator and regulator. I've been through a few regulators, alternators, etc...I was also never too happy with the performance at or near idle. I was driving the car recently, the voltage regulator malfunctioned, and the voltage started fluctuating wildly (this was at night too), so I decided I wanted to go to a simpler, internally regulated alternator...and one that is easy to get if it broke later, not expensive, and also could fit on the car.

    My car has Magnum heads so alternators are a tight fit to the head, as there is a boss that is very close to the back of the alternator.

    I went down to advance auto parts and bought an alternator for a 1985 Pontiac Firebird 305. This will give you a 10si or 12si alternator with the "3 o'clock" clocking, which will put the BAT stud and cooling intake in the correct location outside the confines of the engine block. The regulator connector will face down. Interestingly enough, if you order a P7127-M3, listed for a 10SI, 63 amp application, they gave me a 78 amp 12si alternator, when they would normally charge $80+$16 core for this alternator, I could get it for $45 + $10 core. 10SI and 12SI delco alternators are the same physical size, so there is no difference. You can get a 12SI in up to 94 amps in a stock application, and it will look just like what you'll see here.

    Also, you'll need to buy a few additional parts. You'll need to buy a universal GM alternator bracket (Mr Gasket makes the one I used for $10.99), a 50 ohm, 10w resistor, and a rectifier diode. The resistor and diode are available at Radio Shack under part numbers 271-133 and 276-1661 respectively. This will cost about $6, and you'll have extras.

    So, for the brackets, you'll need to modify as follows:

    [​IMG]
    Slice the stock brace as shown. You'll need to reverse the direction of the bend, as the mounting lugs on the delco alternator will stick out instead of be closer to the head like the stock setup. Its amazing that the offset from zero is the same, just in opposite directions. I used a cutoff wheel and flap disk to do this.

    [​IMG]
    You'll need to slice a stock 70+ non-a/c bracket up (basically cut the slotted section off, and mock up the Mr Gasket chevrolet bracket, so you clear the alternator fan and have adjustability. Mine ended up looking like this (my welds aren't that great, I know). If you do an overlap style weld like this, it will line up right and be very strong. If it doesn't look perfect, don't worry, its hard to see once installed.

    [​IMG]

    Next, you'll need to get the soldering Iron out. Hook the diode and resistor up as shown. The grey stripe on the diode will be on the end that will be hooked to the "1" terminal on the alternator. Hook the other end to a fused, keyed 12V source. I had a wire feeding my electric choke, so I tapped into that. This wire will allow the alternator to be excited so it will charge. I covered all of this with electrical tape...use that or heat shrink tubing. The diode keeps your engine from having "run on" which is caused by electricity backfeeding into your ignition with the key off, and the resistor simulates a "194" light bulb, which would normally be in the idiot light circuit on a Chevrolet of this era.

    For the "2" terminal, you'll need to extend your blue wire from your old alternator field wires. This provides the sensing point on your dash harness. It will maintain this point (the splice for the ignition system feed) at 14.3-14.7V. Nothing special needs to be done here. Test this for high resistance and damage just to be thorough.

    Connect your old "BAT" terminal to the "BAT" terminal on the new alternator. My car has 10 GA, and feeds power directly to the starter relay...you'll need to upgrade this if its stock, the stock wiring can't handle 78 amps. I am planning on putting bigger wire than 10ga in soon. I'd suggest removing the ammeter and using a volt meter. My car feeds the dash harness off of the starter relay. MAD Electrical has some ideas on this.

    The finished product will look like this:
    [​IMG]
    [​IMG]

    Now, should there be alternator failure, not only did I buy an alternator for $55 with an internal regulator and lifetime warranty, I can get one of these nearly anywhere. Most auto parts stores carry a 10SI or 12SI right on the shelf. The performance is excellent. Even at a 850rpm idle, there is almost no voltage change with the lights, radio, and wipers on.

    I know not everyone will like this...some are purists, but this works great, and costs under $100.

    I'd be happy to answer questions about this...and hear your thoughts.
     
  2. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    I used to run a Delco back in the mid '70's when I had the 340 in the old '70 Sixpack RR body. Don't worry about application and clock position, YOU CAN CHANGE THAT

    I don't recall, anymore, it seems to me I hack-sawed "something" off the rear of the alternator to clear the head. (It's only been 40 years!!!)

    Also, you really don't need that resistor BUT YOU DO NEED the diode. This is to prevent the diode trio from becoming the charging source under certain conditions.

    We used to sell a lot of these for tractors, etc, and I used to make a little pigtail. Just jumper terminial no2 to the battery stud, and put the diode in series with the no1 terminal.

    Also, I don't have the number handy, but the "right" regulator will turn any of these into a "one wire" if that suits ya.

    [​IMG]
     
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    • goldduster318

      goldduster318 Overzealous Car Modifier

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      Yeah, I'm aware that its easy to remove the bolts and change the clocking...but since I was buying one, I figured i'd get one that I didn't have to mess with.

      I used the blue wire to cover the voltage drop in the ancient wiring harness. If I hooked it up one wire style I'd loose about a volt, and the lights would be dimmer...the wire was there, so I used it. What you're saying is always an option though, though for me i'd keep that to the tractors.

      While I also figured I could get by without the resistor, if the regulator failed, it would have less current passing through that circuit - and maybe extend the driving distance...using ohms law: V=I*R. The original GM cars had a 194 light bulb there, so simulating that couldn't hurt.
       
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      • duster510

        duster510 Member

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        hey, so that top bracket (with 3 holes) needs to be flat to get the pully to line up??
        how long is that spacer (that alt piviots on)
        im doin this to my duster, im tired of nothin at idle!!
        thanx
         
      • Mopar to ya

        Mopar to ya Well-Known Member

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        Blasphemer! Chevy parts in a Mopar?!? There is a special place in Hell for people like you! Now that I've said what everyone is thinking, that's good work. And you no longer get the lights that get brighter on revving and dimmer at idle. I like it!
         
      • Southernman

        Southernman Well-Known Member

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        Hey Gold Duster, nice job on that conversion. I've done many myself similar swaps over the years, and they worked great, as long as I didn't expose them to high rpms too much.

        I now convert my engines to run the Toyota (sorry guys) alternator and Mancini Racing sells the brackets to install them. The Toyota alternators can run at 7000 rpm+ 24/7 and they have a very low HP draw. I've never been able to wear out the Toyota unit, but the GM units, while a nice option, tended to have a shorter life cycle in higher rpm applications. Just my experience with them.

        Anyway, that's a nice ride you have there and good luck with your upgrades.

        Southernman
         
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        • goldduster318

          goldduster318 Overzealous Car Modifier

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          You take the top bracket and turn it around...it usually bends closer to the block where the alternator bolt is, When you turn it around, it will no longer bolt to the lower of the 2 water pump bolts, so you have to slice it for it to look right like I show in the picture.

          if you take the stock long alternator spacer and add ~3/8" to it (I used a nut that is one size larger than the bolt), It will fit perfect. Remember you need to take a 6" bolt and cut it down to around 5.5" of length.
           
        • 68-GTS

          68-GTS Super " Senior " Member

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          Interesting.... Is it possible for you to post the parts needed for the Toyota conversion as well ?
           
        • Southernman

          Southernman Well-Known Member

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          Here you go. This link is to Mancini's site where you can buy the entire kit (with the Toyota alternator - they call it a Denso, but it's the Denso used on most Toyotas today) or just the brackets.

          Once installed, your voltmeter will be extremely steady - so much so, I've had buddies say they wondered if the gauge was working. It'll pull 13.8 Volts regardless of engine rpm, your lights will not fluctuate - even at very low idle, and it will proabably outlast most of our hotrods.

          As you can tell I'm a big fan, and sorry to all you purists out there. Here's the link:

          http://chucker54.stores.yahoo.net/delialbrpk.html

          Southernman
           
        • 68-GTS

          68-GTS Super " Senior " Member

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          Thanks a bunch
           
        • Abodybomber

          Abodybomber Breaking street machines , since 1983.....:) Legendary Member

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          Sweet tech,guys.
           
        • famous bob

          famous bob mopar misfit

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          I`m thinking about a 100amp-1 wire marine alternator, powder coated black, ( mustang style) as i`ve rewired my whole car and eliminated the voltage reg. any thots on this??-------thanks, bob:happy1::coffee2:
           
        • goldduster318

          goldduster318 Overzealous Car Modifier

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          Please don't use a one wire!!! its just a fancy battery charger that cannot easily account for additional load in your car. Please read all the pages of this: http://www.madelectrical.com/electricaltech/remotevoltagesensing.shtml

          My 78a Alternator shown in this topic works phenomenal. With any electrical load (and changing ones), the volt gauge is rock steady.
           
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          • hefty lefty

            hefty lefty Member

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            We have to ask what pulley sizes you are using on the ND vs. the GM alternator. If the ND holds up better than the Delco but has a bigger pulley and turns slower it is an unfair comparison. You can rebuild a Delco 12SI for a lot less than a ND as well, in fact some of the NDs are throwaway and are not rebuilt.

            I always get frustrated when people say they have a Toyota, Honda, BMW, Jaguar, etc, etc alternator. None of the European or Japanese car makers make their own electricals. They are a Lucas, Bosch, Marelli, Paris Rhone, or a Nippon Denso or Hitachi, or what have you. They may be used on a Toyota, Nissan or Honda or a BMW, Peugeot, Rover, etc. but often these makes use different vendors. It helps to know who made the unit itself and what an OEM application was, but even one model of car had several vendors and models of part.

            GM always used Delco (almost always), Ford had Autolite/Prestolite and Chrysler built their own usually. This is unusual in terms of the worldwide market. I think Chrysler and Ford both had some Leece Nevilles for heavy duty applications.
             
          • daredevil

            daredevil Well-Known Member

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            I did it and then in two months changed to electronic voltage regulator and two field alternator.Not enough charge at idle.
             
          • goldduster318

            goldduster318 Overzealous Car Modifier

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            The GM SI alternators usually have 2.6" pulleys.

            I'll agree on the Densos being throw aways...In several cases I have seen both on Mopars and Toyotas, they have a tendency to seize quickly if they've been parked for any length of time in a damp environment. I'm not sure why that is. I actually got one for free because of this and I originally intended to use it. I tried to use the impact to remove the pulley and the case cracked when I used the impact.

            I really find the purist thing annoying...can't use a delco alternator, but many mopars came with Saginaw steering pumps and columns...which were clearly also made by a division of GM.

            The SI alternators are low cost, easily adaptable, easily rebuildable pieces that work well. You also don't have to take it apart to press the pulley on and off.
             
          • HotLines

            HotLines Realist - Free Thinker

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            If I am not mistaken, there is a member of FABO who makes a setup for the Toyota Alternator on a small block and I know I am not mistaken.

            My Signet runs a Mopar unit 2 wires and reworked at 100 amps, I mever have problems with it
             
          • john27pa

            john27pa 74 360/904 Duster

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            Is there a smaller pulley we can buy for the mopar alts? Im running a 100 amp alt from a 78 cordoba in mine and idle sucks. Maybe I will just bump the idle up to 1000rpm?
             
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