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350 TO 454 SWITCH

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by iLUZON, Nov 7, 2002.

  1. iLUZON

    iLUZON Newbie

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    Switching from 350 to 454 in a 1984 1 ton crew 4x4 would like to also switch th350 to th400 .Have np205 on th350 would like to use on th400. Input shafts different sizes.How to fix? Also what about tranny mounts,drive shafts and any other surprises.The th350 has a shift kit and would be simpler to keep, but people tell me the switch is worth the hassles and $.Towing 7-10 tons, 200 to 400 miles a month.
     
  2. Sandman

    Sandman 3/4 ton status Author

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    If you just do the 454, Its pretty much a drop in deal. Reuse the same motor mounts and all. You will have to do something about the exaust. The TH400 is longer by a few inches so the transfercase will go back. This might mean driveshaft mods and maybe drill new holes for the crossmember if they are not already there. As for the transfer case, To make it work on the TH400, you have to change the input shaft setup with a new bearing for it. The housing will need to be machined to take the bigger bearing. It might be easier to trade for a 205 that was behind a TH400 to begin with. If you have the money and time, I would go with the TH400 also. It will just take for work. Try this link for a really good 205 tech page:

    Offroad Design 205 Tech Page

    Hope this helps.
     
  3. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Keep the TH350 tranny as they can be built super strong (and many tranny guys say stronger than a 400), but that's neither here nor there. The TH350 has less HP soak and better first gear and second gear ratios than the TH400. By keeping the 350 you will have no driveshaft or t-case mods so you can use your existing setup, and you won't have to hook up the electronic kickdown switch under the gas pedal that a TH400 uses (350 does it via the kickdown cable which you already have). Not saying the TH350 won't fail, but when it does you can have it Beefed for less than it will cost you for new driveshafts and a 400 version of the NP205.

    As for swapping in the BBC in place of a SBC, it is basically a "bolt in deal" but you should definitely replace your motor mounts if they are old as the BBC is heavier and tends to rest on the crossmember if the mounts "sag", or even go with poly ones from www.offroaddesign.com./catalog/motormounts.htm. You should upgrade to a four core radiator so you don't overheat the BBC from here www.radiator.com/index.cfm?cmp=coloradok5&cmptype=bannerad (both places listed will give you discounts if you are a paid member here), you will need exhaust manifolds/headers for a BBC and might have to change your exhaust to let it breath properly (depending on what you are running now). Also the accessory brackets (A/C, smog pump, power steering, alternator/ etc.) are different on a BBC than a SBC so you will have to round those up as well. There might be more but this is what came to mind.
     
  4. 77fixer

    77fixer 1/2 ton status

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    First off, fill out your profile.
    I have not switched to a BBC yet but, I have switched my TH350/203 for a TH400/205.
    What is the 400 out of? Was it a 2wd or 4wd?
    I used a 2wd tranny with a 205 from a 465. I had to have the TC input hole bored to take the larger bearing and female input shaft.
    My TH400 came out of a 2wd truck. The output shaft was a match to the new input.
    The only things I had to buy were the TC input shaft, bearing, adapter and the boring of the TC. Of course, I rebuilt both while I was at it.
    With the amount of towing you do I would not re-use the the TH350.

    Brett /forums/images/graemlins/burb.gif
     
  5. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    </font><blockquote><font class="small">In reply to:</font><hr />
    With the amount of towing you do I would not re-use the the TH350.

    [/ QUOTE ]

    Why is that. The input sprag is notoriously weak (reason for stress failures of a TH350), but any tranny man/women worth their weight in tranny's knows to change it out for a HD unit during a rebuild. It will still cost less than drivetrain mods and swapping t-cases for this person. I'm not saying he has to use the 350, but their proven dependability has me wondering why you suggest he shouldn't? Plus the lower gears are better for towing.
     
  6. 77fixer

    77fixer 1/2 ton status

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    Maybe I overstepped my knowledge of 350's, I'm not a tranny expert by any means, but why do the heavier trucks use the 400.
    BTW, I did upgrade to the 34 element sprag. I have heard people talk of HP loss with the 400, maybe in a race car it makes the difference but in a truck... My truck runs much better, maybe I was used to the drag of the 203. Also picked up 2-3 mpg.

    Brett /forums/images/graemlins/burb.gif
     
  7. imiceman44

    imiceman44 1 ton status

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    I agree with leadfoot, it's much better to keep his setup and when the tranny fails, rebuild it HD.
    Still should be cheaper than changing the driveshafts and repositionning the cross member, and getting the 205 for a th400 and the th400.
     
  8. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    You are correct. The 1 ton trucks used the 400 as it was a "beefier" stock tranny, and beefy no matter how you looked at it, and will take a ton of abuse. If he had one, I would say keep it. Since he already has the 350, by keeping it, it would keep him from having to modify the drivelines and t-case. The 350 will fail at some point (all autos do), but it should be quick job for any "good" tranny shop, and with some HD parts, shift kit, and converter with torrington bearings, it should last a long time even behind a BBC in a fullsize. If he was going to put a lift (or bigger lift) in and had to modify driveshafts anyways, it wouldn't be as big of a deal to swap in the 400, but he would still have to worry about the kickdown and getting the necessary t-case/adapter.
     

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