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4.88s are in, and boy is it sweet!

Discussion in '1982-Present GM Diesel' started by AgDieseler, Oct 3, 2004.

  1. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    I had been considering a gear change for a while now, and after a some debate, I settled on 4.88:1. The 5:13s didn't have the pinion engagement for towing, and that's part of the plan. 4.88s set the rpms right where they need to be for my turbodiesel, so I got on the phone to Reider Racing and ordered Precision 4.88s and an install kit.

    How did it do?

    Performance Report:
    In a word, wow! If you put this thing in drive, you had better want to go forward. It's like I have it in 4-lo all the time. The Detroit chirps more frequently in turns, but it's not violent at all. Interestingly, in town performance isn't that different. Cruising on the highway is where these gears make themselves known. /forums/images/graemlins/grin.gif

    75 mph @ 2400 rpm gives 5.5 - 6 psi of boost and 550 - 600 on the EGT. Locked up in 4th, lean into it a little bit and it's easy to gain speed on hills with boost topping out around 12 psi before the 700R4 tries to grab 3rd. Thank goodness I installed extra sound deadener some years ago; I think it would have been a little loud otherwise.

    The real fun in all this was watching my friend set up the gears. I had never seen the guts of my 14FF disassembled, and had never seen a rear end set up. The attention to detail was awesome. Backlash is set at .010, and we ended up shimming the pinion to .023 to get a perfect pattern. Drive side was perfectly centered, and the coast side had the slightest bit of toe in. /forums/images/graemlins/peace.gif

    My speedo is highly inaccurate now, and I need to rebalance my rear driveshaft. One mod begets another. /forums/images/graemlins/rolleyes.gif

    After the 500 mile break-in, I'll see how well these BFGs can hold on to the pavement, and give a towing report.

    ...now if I could just find some time to install these 6.5TD injectors I have... /forums/images/graemlins/thinking.gif

    /forums/images/graemlins/cool.gif
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Deep gears are perfect for the 6.2 IMO...I love my 4.56's for the power, just need an overdrive tranny now.

    Rene
     
  3. Pookster

    Pookster 1/2 ton status

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    such tall gears!!!!!! Isnt this gonna kill fuel economy?
     
  4. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    We'll see, but I don't think so. The 96 runs in this band, and peak mpg is found at about 60 mph. After break-in (and a new driveshaft), I'll get a highway trip in there and see how it fairs. I'm shooting for 16-17 mpg.
     
  5. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    IMO the 6.2 responds well to deep gears...making it as snappy as a small block in non turbo trim. I run 4.56's with 40's and an SM 465 and it pulls pretty good. Cruise rpm is about 2500 or more and I still manage about 16.5 mpg on a steady cruise.

    I'm hoping to run either a 5 or 6 speed sometime in the future which should gain me 3-5 mpg easily.

    Your truck must be pretty quick with 4.88's, smaller tires and a well built turbo'd 6.2.../forums/images/graemlins/waytogo.gif

    Rene
     
  6. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    I have some mileage to report:
    First tankfull - 520 miles - 1/2 highway & 1/2 in-town (the worst stop and go you've ever seen!)

    16.5 mpg /forums/images/graemlins/thumb.gif

    I expect that a full run on the highway at 65 (2000 rpm) will yield 17 or 18 mpg.
     
  7. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Not to crash your thread but I think thats pretty cool that you have a built and mildly bombed 6.2. You don't see that much and bombed cummins like mine are everywhere. Cool to see something different. /forums/images/graemlins/thumb.gif
     
  8. joez

    joez 1/2 ton status

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    [ QUOTE ]
    Not to crash your thread but I think thats pretty cool that you have a built and mildly bombed 6.2. You don't see that much and bombed cummins like mine are everywhere. Cool to see something different. /forums/images/graemlins/thumb.gif

    [/ QUOTE ]

    Correct me if im wrong, but i thought yours was stock /forums/images/graemlins/confused.gif
     
  9. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    [ QUOTE ]
    [ QUOTE ]
    Not to crash your thread but I think thats pretty cool that you have a built and mildly bombed 6.2. You don't see that much and bombed cummins like mine are everywhere. Cool to see something different. /forums/images/graemlins/thumb.gif

    [/ QUOTE ]

    Correct me if im wrong, but i thought yours was stock /forums/images/graemlins/confused.gif

    [/ QUOTE ]Cummins engines like mine, bombed ones are everywhere. /forums/images/graemlins/rolleyes.gif
     
  10. AgDieseler

    AgDieseler Certified Scrap Producer Premium Member GMOTM Winner

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    Thanks, I appreciate it. /forums/images/graemlins/waytogo.gif

    It's been a lot of fun along the way.
     

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