Differential swap parts

Discussion in 'General Maverick/Comet' started by scs, May 24, 2016.

  1. scs

    scs Member

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    I just ordered a rebuilt 3.25:1 differential pumpkin to replace my 3.80:1 and bring my rpms down closer to stock and improve mpg (>8 mpg I hope). I figure I will drop from 3050 rpm at 65 mph to 2550 with 28" tires (roughly 800 rpm over stock). Everything I have read implies this is something anyone (even me) can do. So, my question is: Is it really this easy and what other parts do I need? Just a cover gasket?
     
  2. Crazy Larry

    Crazy Larry Member

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    You'll also need some gear oil.

    Don't forget to change the speedo driven gear. Otherwise, your speedo will be way off.
     
  3. 71gold

    71gold Frank Cooper Supporting Member

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    now would be a good time to replace axel seals if you haven't already recently.
     
  4. Craig Selvey

    Craig Selvey Indiana State Rep - MCCI

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    I also put copper washers on all the bolts.
     
  5. groberts101

    groberts101 Member

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    Far be it for me to rain on your parts parade here.. but if you are only getting 8 mpg from that motor?.. you have much bigger tuning issues going on here. And rpm drops alone won't completely eliminate the issue.

    I used to get mid to high teens mpg with similar cruising rpm and a much heavier car. Idle and light throttle cruise manifold vacuum should point you in the right firection here.. but my quick guess is that you need tons more ignition advance to up the torque output.
     
  6. Crazy Larry

    Crazy Larry Member

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    Totally unnecessary if you use a sealer.
     
  7. 71gold

    71gold Frank Cooper Supporting Member

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    only way I would is if I wasn't ever going back in again. ( I always plan on doing that).
     
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  8. Craig Selvey

    Craig Selvey Indiana State Rep - MCCI

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    The were there from the factory. I did not use any sealer.

    I don't plan on going back in.
     
  9. BigDog41364

    BigDog41364 Member

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    If there is no rouge of the gears i would put some on and double check the gear pattern just to double check there set up right. I would hate to put it in and it and have to pull it back out because its loud.
     
  10. NCFatBoy

    NCFatBoy Member

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    Where did you order it and how much was it?
     
  11. scs

    scs Member

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    Quick Performance out of Iowa. $599 + $40 shipping for a rebuilt open differential (which I want for this once as it will be my highway cruising diff). By the way, my original post said I would end up 800 rpm over stock with this setup. I meant 100 rpm over stock.
     
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  12. scs

    scs Member

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    My carb is a 600 cfm QuickFuel and it was tuned originally by a racer seeking high HP on his dyno, so I'm sure it's dumping fuel in to up the HP. But I think I'm gonna pursue some carb and timing adjustments to make it more efficient vs performance. If I can't find anyone local to do this, I'd consider pay you to come on down and work with it someday if your willing. I'll keep in touch via conversation, but right now I might be unavailable through late June.
     
  13. groberts101

    groberts101 Member

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    This is a common problem with inertia dyno operators as they only load the engine up at WOT from about 2,500-3,000 rpm on up. As you've obviously realized by now, that does absolutely nothing for simulating and tuning part throttle drivability. If you can find a Mustang or Superflow dyno around you they can run 1/4 mile and part throttle drivability simulation tests to tune an engine at lessor throttle angles. I haven't been there myself but I hear a lot of the AWD tuner guys talking about RS Motors down in Burnsville and they can tune from idle to redline.

    Otherwise I can talk you though most of the steps over the phone. If you intend to be driving the car much at all?.. I would most definitely tend to the issue sooner than later otherwise you will be muddying up the oil and potentially causing combustion chamber/valve deposits.. even undue piston/ring/cylinder wear as the fuel builds up on the cylinder walls and ringlands.

    Need to keep in mind that less advance = more unburned/wasted fuel.. more advance =.. well you get the point. People get far too caught up on tuning carbs for WOT but fail to get the ignition advance curves closer to optimized first. People are still caught up in the emmisions era when they ran 6-12° of base timing and ported vacuum. Without these stepping stones put in the right order you will never get the idle/transitional and low speed air bleed sizing where they need to be. Everything will be a compromised bandaid and the combo will never see its full potential. It's a back and forth process because one affects the other in the combustion chamber and heading out the exhaust pipe.

    What is the base timing(initial #) set at?

    It's extremely easy to lock out a distributor to get an idea of what this motor is missing. What distributor are you running? Please say it's an aftermarket unit. lol

    Another quick and easy test is to run the vacuum advance to a FULL manifold vacuum source. My guess is you have it hooked to a ported source which does nothing for idle and light cruise.
     
  14. Rasit

    Rasit Member

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    Let me know if you are looking to get rid of the 3.80's.
     
  15. 71gold

    71gold Frank Cooper Supporting Member

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    most people don't...if they don't drive it...:bouncy:
     

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