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Thread: 3G alternator swap

  1. #1
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    Default 3G alternator swap

    I'm lookin at doing one of these, but the 3g install thread is missing the core info at posts 49 and 50. Would anybody like to either link to some info or explain the process? Thanks.

    1988 Thunderbird Sports Coupe 5.0L (Stella)
    306ci, GT40 intake, 65mm TB and EGR, GT40Y aluminum heads, T5 swap, 8.8 disc rear w/3.55s, 11" front brakes, MM front & rear control arms, front coil overs, Koni yellows
    Stella 2.0 Overhaul
    Retired DD: 1988 Turbo Coupe, 5 speed, 99.9% stock (Michelle)
    Current DD: 2015 Volkswagen GTI SE, 6spd manual (Claire)

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    1979 Ford Fairmont

    5.0L/4R70W/8.8"/5-lug/3" Exhuast


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    Well worth it,and doesn't take much time or money.I got my 3G from a 3.0 Taurus for cheap,polished it,and after 3 years,still going strong.
    '88 Sport--T-5,HURST shifter,Trick Flow R intake,Ed Curtis cam,Trick Flow heads,Scorpion rockers,75mm Accufab t-body,3G,mini starter,Taurus fan,BBK long tube headers,O/R H-Pipe, Flowamaster Super 44's, deep and deeper Cobra R wheels, Mass Air and 24's,8.8 with 3.73's,140 mph speedo,Mach 1 chin spoiler,SN-95 springs,CHE control arms,aluminum drive shaft and a lot more..

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    Doesn't everyone go 6G now? Better idle power output with the same bolt pattern and wires.

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    1. Locate the point where the pair of MAIN wires (black/orange) join and become one.
    2. Cut at intersection., leaving one MAIN wire.
    3. Splice in new piece of 10 Gauge wire, solder, and use heat shrink tubing to insulate.
    10 gauge will suffice? I was under the impression the stock wire was larger. My car is 15 miles away and I can't really check right now...
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

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    Isn't that to wire it up alongside the stock wiring? Even then, it isn't the best (voltage drop!). Either way, I wouldn't go less than 6 gauge with 4 or larger preferable. 4 gauge is good for a maximum of ~150A. 10 gauge is good for a whopping 60. Two 10 gauge runs will effectively get you up close to what 130's can put out. The wires WILL heat up though.

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    Sounds good, thanks for the info guys. I have a lil' somethin cookin up while the 306 is gone .

    1988 Thunderbird Sports Coupe 5.0L (Stella)
    306ci, GT40 intake, 65mm TB and EGR, GT40Y aluminum heads, T5 swap, 8.8 disc rear w/3.55s, 11" front brakes, MM front & rear control arms, front coil overs, Koni yellows
    Stella 2.0 Overhaul
    Retired DD: 1988 Turbo Coupe, 5 speed, 99.9% stock (Michelle)
    Current DD: 2015 Volkswagen GTI SE, 6spd manual (Claire)

  8. #8
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    Quote Originally Posted by Seek View Post
    Isn't that to wire it up alongside the stock wiring? Even then, it isn't the best (voltage drop!). Either way, I wouldn't go less than 6 gauge with 4 or larger preferable. 4 gauge is good for a maximum of ~150A. 10 gauge is good for a whopping 60. Two 10 gauge runs will effectively get you up close to what 130's can put out. The wires WILL heat up though.
    nope.........to part of what you are saying although you are so damn close to being right, its arguable nonetheless.

    two considerations before selecting wire.

    1- Will the wire be in a "free air" rating
    or
    2- will the wire be in "raceway or cable tray" , ie- enclosed or surrounded by a covering

    The two situations above drasitcally change the ampacity of a conductor as well as its allowable voltage drop. The NEC is just now catching up to the world of DC power as in ref to the nec 2002 on back there was little data. The only place a man could find DC wire rating would be in teh Telcordia or GR1275 standards.

    Of your above two criteria,, the temperature rating must be selected.

    Lets use the 10awg as an example

    In raceway or cable tray rating of #10 at the three temp catagories as ref by the nec...
    at 60deg C its good to 30A
    at 75deg C its good to 35A
    at 90deg C its good to 40A

    Free air rating of #10 at the three temp catagories as ref by the nec...
    at 60deg C its good to 40A
    at 75deg C its good to 50A
    at 90deg C its good to 55A

    In conservative cercumstances, we use the middle temp range as our guide for both free air and raceway/cable tray options.
    If your wire is going to be enclosed, its heat buildup is greater so follows the voltage lost across the length of wire. Lenght of wire is critical in DC power.

    Now, in an automobile engine bay, you have a power plant producing watts of heat dispation directly effecting the surrounding wiring. IN this instance, it is far better for you to use the 60deg C "in raceway or cable tray" column in the nec. However, better isnt always practical or cost effective as you will see in the conclusion to this post.

    Why? Not only is the wire going to produce its own heat by way of current draw and the natural characteristics of copper (K factor) but the insulation around it also plays a heat holding roll. In addition, the engine running offers radiant heat to the wiring and thus forces you into a more conservative wire size scheme.

    Take a look at some of your wiring inside the car and read off the fine print stamped along the conductor insulation.

    You cant just size wire by its ampacity predictability in DC power, thats just totally retarded.

    So, your ampacity goal is 130A lets say.

    6awg at 90deg C free air rating is 105A
    4awg at 90deg C free air rating is 140A
    2awg at 90deg C free air rating is 190A

    If your wire will dangle in the wind so to speak with no covering on it,then the free air rating applies.

    To correctly calculate the actual KCMIL (circulair milimeters of copper) required to carry the hypothetical 130A,

    Kcmil= Kfactor x wire loop lenth x ampacity / voltage drop
    Kcmil= 11.1x 2 x 130 A / .10 VD

    11.1 is my k factor constant for copper (in dc power only)
    2 is my wire distance (there is only one hot feed, no return on the alt as its bolted to its own ground source assuming your plant is grounded.
    130a is my full duty on the alternator
    .10 is the voltage drop loss across that 2' distance of wire that i am going to engineer and allow for.

    now i can calculate the above and i come up with ........
    28860 circulair mills of copper required to carry a 130A load across 2' of distance.
    28860 cmil converts to NEC chapter 9 table 8 wire size as closer to 6awg than 4awg. Since the Kcmil wire size is greater than 6awg, we default to the next higher size which is 4awg.

    so,,,,,,,
    4awg not only meets the DC power current carrying conductor properties of the application, it also fits the ampacity requirement of a "free air rated" wire at 90deg C.

    In reality, all of these 3g's running parallel with a smaller guage wire seems odd to me. I would want to replace those black/brown wires with a single conducter fitted with a fuse rated at 80 or 90% the value of max ampacity of the alternator.

    In short, a DC power conductor must meet two criteria to qualify for the job.
    IT must be the correct circulair mils of copper and be the correct size for the amperage it will carry.

    Seek's example of laying another run of #10 in parallel with the existing harness does /does not add to the total copper required, you just have to understand that the two blk/brn wires splice forward to one black brown and then into one single yellow wire. Those splices get corrosion over time so its best to eliminate them all together. The 5.0 final alt landing on the starter relay is configured a little different than the 3.8 but they are close enough.

    Just my 2cents worth.
    Last edited by jcassity; 06-09-2009 at 01:31 AM.

  9. #9
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    Holy crap, now that is a response!

    4 gauge it is then.

  10. #10
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    when i did mine, it was a lot of trial and error, i guess i'm lucky i didn't fry my wiring or the alt.

    my alt puts out 191 amps at 2k rpms though (according to autozones tester)......i have no clue what it came off of because i found it under about an inch of dust in the shop i work in. Reg was bad, so i went to the junkyard and pulled one off a junk alt that had been laying in the mud for years.....came off of something with a 10 rib belt.
    Currently Birdless but never Foxless

    86 Mustang GT

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