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'79 350 CJ -Better highway driving - lower RPMs - advice

Discussion in 'Jeep CJ' started by loudnproud, Jul 26, 2019.

  1. Jul 26, 2019 at 3:37 PM
    #1
    loudnproud

    loudnproud [OP] Member

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    1979 CJ7
    350 ci 400 tranny. 35s/3.5 lift. fiberglass tub / air lockers
    Hello all, I need some advice and insight. My CJ is pretty heavily modified. I would like to lower my RPMs when running highway speeds of 65-70 mph. Currently I am about 2800-2900 RPMs at these speeds. And yes my Jeep handles fine at these speeds. Although built for climbing, I will use it as a summer driver and just hits some trails.

    I'm looking for insight on how to accomplish this. My first thought was to change rear end gears. If this is your suggestion, what gears would you change to? 383? 350? I'm afraid to change too much with my large tires.

    A mechanic thought that we may be able to change gears in my Quadra-Tech transfer case.

    Thoughts? Do you have experience in this or is it an educated guess. I look forward to the input.

    Here are my specifics:
    350 ci
    400 turbo trans
    Dana 44s
    409 gears
    35s
    American Motors Quadra-Trac part time conversion (Borg Warner 1339) without overdrive
     
  2. Jul 26, 2019 at 4:35 PM
    #2
    aggrex

    aggrex Well-Known Member

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    Welcome @loudnproud ...not too sure about the mechanic who suggested changing gears in the TC as they are not involved during hwy speeds . Changing axle gears is $$$ so to decrease hwy rpms my suggestion is run larger tires which can be less $$.
     
    JKBob 25 likes this.
  3. Jul 26, 2019 at 5:05 PM
    #3
    Jim Beam

    Jim Beam Well-Known Member

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    Changing axle gears would be your best bet since you have an automatic, V8, big tires. the low axle gears you don't really need and you can always get a 4:1 kit for the t-case later on. Although in my opinion driving a CJ at those highway speeds is kinda dangerous anyway.
     
    DustStorm4x4 likes this.
  4. Jul 26, 2019 at 7:45 PM
    #4
    JKBob 25

    JKBob 25 Well-Known Member

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    Hey @loudnproud . I am no expert in this topic by any stretch. But just my 3 cents..
    I'm running 3.73 gears sitting on 35s. Stock t-case. And im running about 1900 rpm at 70 mph. And getting 19mpg. Now I know CJs and JKs are nothing alike. But the gearing is the same. Correct??
    I know @OFFGRID has family things to deal with. But he is our for most expert on the shit.
     
  5. Jul 28, 2019 at 10:03 AM
    #5
    loudnproud

    loudnproud [OP] Member

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    350 ci 400 tranny. 35s/3.5 lift. fiberglass tub / air lockers
    Thanks fellas for the advice.
     
  6. Jul 28, 2019 at 10:06 AM
    #6
    loudnproud

    loudnproud [OP] Member

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    350 ci 400 tranny. 35s/3.5 lift. fiberglass tub / air lockers
    Hopefully someone with the same set up can steer me to the correct gear ratio.
     
  7. Jan 9, 2020 at 4:37 AM
    #7
    CZJ7

    CZJ7 Member

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    Any changes since July?

    aggrex is correct as TC ratio changes affect Low Range, but High Range is always 1:1. That leaves the axle ratio and Jim Beam is on the closest track. Without an overdrive you’re stuck with a wailing 350 on the highway. Axle ratio change is the only cure either by higher gears (lower numerically), or bigger tires (same effect).

    Without knowing exact ratios available for the D44 offhand, for that 65-70 mph speed, some ballpark numbers are:

    3:83 gears will yield about 2650 rpm
    3:53 will give 2450
    3:23 will give 2250

    The downsides are that both axles have to be done, and off-road if in low range will suffer—that’s where the TC ratio change will come into play. Of course, depending on what kind of off-roading you do (trail runs and fishing trips vs. harder-core rock climbing and “hey y’all, watch this” kind of stuff), the 350 most likely has more than enough muscle to not bother with the TC ratio change.

    Speedometer drive gearing will have to be addressed too, as any final-drive ratio changes will mess with it.

    Hope this helps.
     
    aggrex likes this.
  8. Jan 10, 2020 at 8:00 AM
    #8
    loudnproud

    loudnproud [OP] Member

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    Thanks CZJ7.
    I haven’t taken any steps yet. I should have enough $ saved up to address it this spring. I’m think a 3:50 ratio will be best for me. I still want a decent hole shot that doesn’t lag too much. I appreciate your insight. Very helpful!
     
    CZJ7 likes this.
  9. Jan 10, 2020 at 11:32 AM
    #9
    CZJ7

    CZJ7 Member

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    Most welcome sir!!

    I’ll bet your holeshot now is amazing....quite the sleeper!!!

    There is another alternative....swap in an overdrive transmission. Hole shot preserved and highway rpm solved. A 700R4 or 200-4R would do it. Additional mods like cross member mounts, TC compatibility, and driveline length come into play.

    Axle gears are certainly cheaper.
    $$$ fix anything!
     
  10. Jan 13, 2020 at 7:32 AM
    #10
    loudnproud

    loudnproud [OP] Member

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    1979 CJ7
    350 ci 400 tranny. 35s/3.5 lift. fiberglass tub / air lockers
    Hole shot is okay. My CJ is fairly heavy and those 35s don’t help! I will look into these options. Thanks
     
    CZJ7 likes this.

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