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doubler question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Taz, Apr 20, 2001.

  1. Taz

    Taz Registered Member

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    has anybody looked into maybe changing the high gear on the NP203 case? I was thinking of getting the doubler, and if you would change the higher gear you could have an effective 'overdrive' and with an overdrive tranny you would have double overdrive increasing gas milage. that way I could use the lower gear when 4 wheeling and the high gear on the road.

    thanks

    (I posted this on the ORD file but didn't get a response, so I thought I would try it here)
     
  2. Taz

    Taz Registered Member

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    the reason I am asking this question, is that if this can be done it would be an easy sell with my wife. She agreed that I should get the 700r4 tranny (by jet) and I thought this would be a nice addition

    thanks again
     
  3. BUDDY

    BUDDY 1/2 ton status

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    I think that's a GREAT idea. The only thing I can see wrong with it is that you might have to run a lower gear in your axles, which would make that change totally useless. It probably depends on your own personal preference though. A .75:1 OD high gear in the range box of the 203 would be a big sell for me though. I'd probably shell out an extra 500 bucks for it, because of the gain in gas mileage.

    My .02,



    72 K-5, 33x12.5's, 350/350/205,12-bR,d44F soon to be 39.5'x15.5s, EFI502/nv4500/doubler205, d60F, d70R. Stay tuned.
     
  4. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    I see what your asking but the problem can be solve else where without a seto of custom made gears for the 203. The OD will get you some better MPG on the 700 but it has to be a ballance. If you read here long enough you will find all sorts of complaints about the 700 not holding OD and hunting back anf forth. This is partly caused by wrong gearing to adjust for the tire size. The other part is a 3 bedroom ranch has a better drag coefficent then these trucks. Weighing 5,000 lb isn't helping either. Idealy running down the hwy you want to be turning about 2200-2500 RPM for best MPG. Lower than that and the 700 is going to start hunting gears. You would have to be someplace where you never have hills to deal with and alwasy have the wind at your trail. You would have to deal with any loss of speed and make gradual increases in speed. Above that and your just turning more RPM then needed if the motor is heathly. However you want to keep the RPM low enough that if you do drop out of OD that your in the power band for the motor at about 3000 RPM for Passing or pulling a steep grade.
    SO bottom line is with the 700r4 you are not going to see a improvment in MPG if you get it where it's turning 1500 rpm. The engine has to be up about 1/3 the way on the power band to over come the mass and wind drag or The tranny won't like it and it will be back and forth with OD. That alone is not all that great for the Lock up converter being cycled non stop.
    With a 700r4 and 33's you want to be atleast a 3.73 gear asuming the stock 1:1 high range in the case. You would probably be better off with a 4:11 and 33's. 4:56 with 35's would not be out of the question.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  5. Butch

    Butch 1/2 ton status

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    Also if you have an overdrive in the doubler all of your first three gears would be higher. You would in effect be in 1st gear overdrive starting off from a stop and it would kill your performance there as well. What we really need is two speed axles like the 1 1/2 ton trucks have then you truly could have the best of both worlds. I run 410 gears and 35's with a manual and a doubler. At 70 mph I turn 3000 rpm's and for that reason only get 6 mpg. I would say if you want a K-5 forget about gas mileage and focus on function, otherwise you will just get depressed by the whole idea.

    I thought I was wrong once,
    but I was mistaken
     
  6. Ryeguy

    Ryeguy 1/2 ton status

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    You can't change the high range in the NP203 without
    significantly changing its design. Changing the low range
    would mean a set of custom gears.

    The TH700 is a good choice if you build it to handle the
    torque load, and if you can handle it's overdrive ratio. Nice,
    low 1st gear there, too.

    FYI, contrary to A/A's book, you do _not_ need a custom
    TH700 output shaft to mate a NP203 behind it. All you need
    is a spacer to go between the NP203 mount and the TH700
    flange. We've done this 'round here with no problems, and
    it saves quite a bit of $ when you're looking at custom drive-
    shafts anyway.

    --Rob
     
  7. pcorssmit

    pcorssmit 1/2 ton status

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    Would it be possible to simply shorten (cut off) the output shaft on the 700, or are the splines not long enough? This would save having to dissasemble the trans as well. I've been needing a new trans for some time now, and couldn't make up my mind on the t-case. I think I'm just gonna keep the 208 and convert it to a short shaft for now, mostly since it will save me over a grand. It would be nice, but I really haven't found the situation where I need the 4:1 low.

    Pete

    '83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  8. Ryeguy

    Ryeguy 1/2 ton status

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    &gt; Would it be possible to simply shorten (cut off) the output
    &gt; shaft on the 700, or are the splines not long enough?

    The splines aren't long enough. The spacer is around 1-1.5
    inches long (forget the exact measurement but could find out
    easily enough, but it's easy enough to figure out anyway).
    No trans. disassembly is required to put in the spacer.

    --Rob
     
  9. Bigd

    Bigd 1/2 ton status

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    Would have too agree with Ryeguy on the high gear change for the 203 range box.When it's in high the input and the output are directly connected.No gears in involved even though they are spinning all the time.I think the only thing that would be possible is changing the low range but it would be expensive if you can even get different size gears in the case.Just my 2 cents.
     

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