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Questons for diesel owners

Discussion in 'The Garage' started by JHG, Jul 30, 2004.

  1. JHG

    JHG 1/2 ton status

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    I am going to swap my 350/TH350 combo out for a diesel and TH400 or TH700R4.

    The main reason is that gas price is horrible ower here (5.66 USD/GALLON), but also my TH350 passed away the other day (RIP) so now I have to start doing something.

    Now I have few options.

    My truck has D44/12 bolt with 4.88 and 38" tires. I did change the truck for 44" but havent bought them yet.

    I can purchase a 6.2 with TH400 and NP205 or with TH700R4 and NP208 (same as is in my truck right now).

    I can also get D60 and 14 bolt FF with no spin and 4.56 or 14 bolt SF with Air lockers (ARB?).

    I know that TH400 is much stronger in OEM form than TH700, but if I chose TH400 and 4.88 then my engine will be at ~2200 rpm at 60 mph.

    If I have TH700R4 my engine should be turning ~1600 rpm at 60 mph.

    If I should choose TH400 and 4.56 then my engine should be at 2080 rpm at 60 mph.

    If I will stick with 4.88 then I will probably use my current D44/12 bolt combo untill I break it but if I will got to 4.56 then I will upgrade to D60/14 bolt.

    ***
    So my questions are:

    How much affects it will have on my mpg if 6.2 is running at 2200 or 1600 rpm.

    Would the 6.2 with TH400 and 4.56 be horrible to drive?

    I know that 14 bolt FF is a lot stronger than 14 bolt SF but I like the idea of having a selectably lockers. Would is it that much weaker that I would regret heading the SF way?

    Thanks,

    Jon H.
     
  2. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    What year is the th700 out of? What kind of condition? I would prefer the th700 myself. I doubt you would break the SF14B, it can handle up to 38" tires, it's almost identical in strength as a D60SF.
     
  3. u2slow

    u2slow 1/2 ton status

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    I have a 6.2/TH400 in my Suburban. I don't think I'll ever blow the transmission, but its the last transmission I'd pick for that engine.

    The wider ratios, lockup converter, and OD on the TH700 make for excellent fuel economy. I've been told ~1800rpm yields the best economy on the 6.2L.

    IMO, run your locked 4.88 1/2-ton axles with the diesel until you break something.... then go for the D60/14FF /forums/images/graemlins/thumb.gif
     
  4. JHG

    JHG 1/2 ton status

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    I did not make it clear in my first post, but my truck is going to be a 44" (they do fit), and my 38´s will only be summer tires.

    I have a two possible combos:

    1) Current axles (D44/12b) with 4.88, 6.2 and TH700
    2) Swap in D60/14 bolt (SF or FF) with 4.56, 6.2 and TH400

    I will buy 1987 or newer TH700 (if I head that way).

    Right now I am thinking about 1). Thats a lot cheaper and less work.

    I know that number 2 is a much stronger combo, but I am afraid that 44´s and 4.56 will be horrible to drive (with 6.2), and I am also a lazy bastard /forums/images/graemlins/smirk.gif

    The downside to choosing combo nr. 1 is that if the TH700 cant take the stress of my wheeling, then I have to swap a stronger transmission in the truck.

    Do you think that TH700 could live with this combo (I have a HUGE cooler and a trans temp gauge)?
     
  5. GMCLegacy

    GMCLegacy 1/2 ton status

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    44" rubber and 1/2 ton running gear is nowhere near ideal IMO, also a 700r4 isnt going to like those large tires with 4.56 gears. thirdly, i how efficient do you expect a rig on 44s to be? a diesel is going to help, but probably isnt the answer.
     
  6. JHG

    JHG 1/2 ton status

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    I know that 44´s are not going to give me great mpg, and my K5 will sertanly not be some kind of a economy box, but I should be getting better mpg with 6.2 than with my 350 (I have another vehicles to use to go to the mall).

    In Iceland we drive mostly on snow (no rock cravling or mud) and 38" tires are on the limit for such a heavy truck (we cant dig through the snow and have to float on top of it).

    The mpg factor is important because if I have to bring to much oil when I travel, then my truck will be heavier, and of course the trip will be more exspensive. Its getting to exspensive to wheel with my current combo (and my 350 is getting tired) so I have to change my engine sooner or later. Also my transmission (TH350) blew up few days ago (have used it without problems for many years). So now is the time to check every option /forums/images/graemlins/hack.gif /forums/images/graemlins/grind.gif /forums/images/graemlins/weld.gif.

    I am trying to change my truck that way I can wheel with it (dont want to buy japanese diesel truck) without having to spend to much money. So it has to be a compromise between fun and economy.

    If I will choose TH700 then I will use the current driveline (untill I break it) with 4.88.

    I have been thinking of option no. 2, that is D60/14b/4.56/TH400 but I am afraid that it will be to slow.

    I have also been affraid of that 1/2 ton axles will not live a long live with 44" but I have talked to many using it without a problem (I think there is not as much stress on the axles in snow driving as in rock crawling). Everybody use the motto, use it untill you break it, then upgrade.

    Because I want to know more of combo no 2, I ask diesel owners, is 6.2, 44", TH400 and 4.56 out of the question or is it possible to live with that?
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I run 40's, 4.56's and a SM465 4 speed with my 6.2 and it isn't any slower than when I had my old 350.

    I'd run the one ton stuff with the TH400 and the 44's. See how it is...at the very least it won't break and it'll likely get you 15 or more mpg (to the US gallon) I average close to 16.5 on the highway...

    If you find you need more gear then swap the gears for 5.13's later.

    Rene
     
  8. u2slow

    u2slow 1/2 ton status

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    trustyk5 is running the 6.2L, NP205, D60/14FF w/4.56, and 40's but its an SM465 (4-spd manual).

    Also know a guy with a Suburban running 6.2L, NP205, D60/D70 w/4.88, and 44's but was using a Clark 5-spd (direct 5th).

    Both get 15+ mpg IIRC.

    The TH700 should live well behind the diesel. Stock 6.2L trucks ran 3.08, 3.42 or 3.73 with TH700. 4.56-4.88 gears *should* negate the tire size quite effectively.

    You have more overall gearing with the 208 t-case, (2.63 vs. 1.96) and the 1st gear on the TH700 is deeper (3.06 vs. 2.48)
     
  9. JHG

    JHG 1/2 ton status

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    Thanks for your answears, now I have the answears I needed, that is, combo no. 2 is possible to live with and TH700 should be able to survive /forums/images/graemlins/smile.gif

    Now I will have to start buying stuff /forums/images/graemlins/deal.gif, and then /forums/images/graemlins/hack.gif /forums/images/graemlins/grind.gif /forums/images/graemlins/weld.gif and finally /forums/images/graemlins/woot.gif

    Jon H.
     

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