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Need some good advice: Should I keep the TH350 with a 454?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blazer79, May 7, 2002.

  1. Blazer79

    Blazer79 1/2 ton status

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    Ok, for some of you, it may sound simple. Just toss the TH350 and get a TH400. The problems are:
    <font color=blue>
    1) I want to keep my NP203, so I'd need a $400 adapter for the TH400.
    2) The only 4wd TH400 I've found in a junkyard is mated to a NP208 and the guy wants $500 for the combo with no warranty on the tranny's condition.
    3) TH350 and TH400 are different lenghts, which would mean different driveshafts, more $$$
    4) I'm in Guatemala, Central America, so shipping something as heavy as a tranny is not an option.
    </font color=blue>
    Now someone mentioned that a STOCK TH400 is stronger than a STOCK TH350, but you can make the TH350 handle more power than a STOCK TH400 if you build it right. Is this true? What is the most torque and hp a TH350 can handle with all necessary mods?

    My 454 was mildly rebuild a few years ago, so maybe 350 ft/lbs of torque and 200hp is a down to earth figure, and I'm not planning on rebuilding this engine for about 2-3 years, but I need to think about the future and have a tranny that's ready for something like 500 ft/lbs of torque and 400 hp. Will my TH350 hold to this kind of power? Aren't drag racers using TH350s? I promise to change my ATF once every two oil changes.

    The guys at transmissioncenter.net rate their TH350 at 400hp max. They install a Transgo kit (about $70 form summit), replace a race which they sell for $29, and use a 700R4 low clutch instead of the TH350 one. A local tranny shop charges about $120 for labor.

    That's $219 vrs $650 of the TH400+NP208+New Driveshafts combo, or about $700 for a TH400 and an adapter for my NP203. I can blow my tranny 2-3 times and still have spare for towing. And still, there's a chance that the TH400 I find might still need $200 to rebuild.

    Am I crazy or does it make more sense to keep the TH350?

    Please feel free to post all of your opinions, good or bad, they are welcome.

    Thanks.

    PD Sorry for the long post.
     
  2. 84_Chevy_K10

    84_Chevy_K10 Banned

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    I can get you the necessary adapter to hook your TH 400 to an NP 203. In fact, how's half what you thought it'd cost at $200? It's not that big and heavy so shipping wouldn't be so bad.

    NP 208s are junk, slip yokes suck, you'll never forgive yourself if you go to a 208. I'm taking drastic steps to get rid of mine. It hasn't broken, but all the leaking and clunking from the slip yoke just sucked big time. I'm glad it's history.

    I would not even consider a TH 350 for your application. Yes, the drag racers do run TH 350s, but they're doing that to save power because the TH 400 uses up more power.

    Me personally, I refuse to have the transmission be the weak link in my truck. My 1 ton has a 350 TBI (215 HP I think) and it has a TH 400. When I got it, the transmission was burned up--but I had a TH 350 and driveshaft for it that would have worked fine and I could have installed it. Instead, I spent the $300 and got my TH 400 rebuilt. I think it was worth every penny. I refuse to have my transmission be a time bomb.

    In short, yes, you can make the TH 350 survive, but why? Why not just do the work now so you have a stronger transmission to begin with?

    That is, if you're bent on having an automatic anyway....my 4x4 is getting a 4 speed/205 combo.
     
  3. BurbinOR

    BurbinOR 3/4 ton status

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    Why not keep the T350 until, or should I say, IF it fails? I ran a T350 behind a 454 in my '75 K5 for 3 years without any problems, including a fair bit of towing firewood-filled trailers (not light). The T350 tranny is quite strong and reliable, don't sell it short. If you want in the meantime, save your money and find the right combo for your ultimate tranny/tc combo (maybe a T400 with NP205).

    By the way, my buddy in Pa runs a T350 behind a 454 in a 10 second '69 Camaro...........that should say enough.
     
  4. shane74

    shane74 1/2 ton status

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    Ok, there is a BIG misconception about TH350's. They only reason they break is if they are not built right. I've blown up a TH400 just as fast as a TH350 (actually faster). I ran a '68 chevelle with a 454 that was running approx. 550 h.p. I had a TH400 (that I spent $800 to get "bullet proofed"). It lasted all of three runs down the track. I then decided to put in my old TH350 that I had sitting on my garage floor. I had it worked over by a friend of mine who builds them for drag racing applications and extreme duty trucks and 4x4's. It cost me all of $350 and it is still tearing up the asphalt at the drag strip. It has been in the car for close to 4 years with no issues. When I sold the car last year, the guy would not let me give him the TH400. He insisted on the Th350 that was in it. I have a TH350 in my blazer right now. I am building another 454 for it that will have gobs of monster low end torque. That is what we need to turn the bigger tires. I will NOT spend the crazy amounts of money to swap stuff out when all you need to do is have the TH350 worked a little bit. Basically, if it breaks, then it ain't built right. If you REALLY want to talk to a guy with the knowledge, PM me and I'll give you his phone #. He'll let you know over the phone what you need to do. In fact, I'll bet some of the local drag racers where ever you are will know his name. Also, if my memory serves me right, he built a TH350 for a guy that was dragging his 1st gen blazer with 33" slicks in the back and street tires in the front. He runs a 500+ h.p. 454, TH350, and NP203 t-case. And...yes he DOES race it locked in to 4-HI. It's real crowd pleaser I tell ya! (stepping down off the soap box now...)
     
  5. four_by_ken

    four_by_ken 1/2 ton status

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    Thats what I have run for about 5 years. Build it nice and the 350 will last. Plus when something does go wrong it is cheaper to fix.

    Dont go throught the hassel unles you have to.

    Ken H.
     
  6. Twiz

    Twiz 1/2 ton status

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    Torque is what breaks parts, not H.P.
    A TH350 behind a 500 H.P. drag car isn't the same as a TH350 behind a 500 Ft/Lbs 4X4.

    A 400 has larger/stonger parts through out, it's the same monster they put in motor-homes (probably the hardest life any tranny could ever have).


    Even with that said:
    The TH 350 -vs- TH400 is really a tuff call.
    But I think I'd feel better about haveing the 400 behind a built big-block.
    If it was a behind a small-block, the TH350 would be O.K. (not better/worce, just O.K). - In my opinion
     
  7. 73blazerUSMC

    73blazerUSMC Registered Member

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    ok now i realize that ya'll are gonna yell at my a@@ cuz im saying this but why not put a nice strong sm465 behind that soon to be monster now i know that it is alot of work because i had a large part in installing one in a 77' k5 (my dad's) that i used to drive and that i was giving back when i got my 73' ( which came with the sm465) and he wanted a maual too. but i say if you could find the parts it would pay off having driven bolth i really prefer the manual sm465 now well thats just my 2 cents and i realize that certin applications call for an auto or are just down right more fun with one but i sware by my 73' with the manual tranny and my father's converted 77'

    ok let the yelling and name calling begin
     
  8. Blazer79

    Blazer79 1/2 ton status

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    84 Chevy K10:

    $200 sounds tempting. I agree with you that I should work now to have a stronger transmission, BUT, it still is more than twice the money I'd put into the TH350. AND I'd still have to convince the guy to split the combo and see how much he wants or find someone else who has one for sale.

    Are you making these adapters or do you just happen to have one around? I'm interested. The only problem is I'm being offered a nice set of tires by another CK5 member. If I buy those, I'd have to wait on this adapter. Can you hold it for a few weeks?

    Maybe I should hold on to the TH350 for now and slowly start building my TH400 combo, don't you think? (Using your adapter)
     
  9. Blazer79

    Blazer79 1/2 ton status

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    four by ken:

    What kind of build up did you do to your TH350? It's lasted 5 years behind a 454? Do you baby it or do you treat it like a man? /forums/images/icons/wink.gif What size tires and gears are you running? Are you using your doubler frequently on the trails? That really makes life easier for the tranny, doesn't it?

    Sorry for the lots of questions.
     
  10. Blazer79

    Blazer79 1/2 ton status

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    73BlazerUSMC:

    /forums/images/icons/mad.gifWHY YOU LITTLE ... /forums/images/icons/wink.gif j/k

    I thought about going to manual, but after long drives to and from the trail plus long continuous wheeling hours, an auto really makes a difference. Besides, it's a hassle to do that swap, especially finding all the parts at the junkyards.

    As I said before, all comments are welcome, so no need to start yelling and calling names. /forums/images/icons/smile.gif
     
  11. Blazer79

    Blazer79 1/2 ton status

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    Shane74 &amp; Twiztid:

    What's the most HP AND torque you'd throw into a properly built TH350?
     
  12. four_by_ken

    four_by_ken 1/2 ton status

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    The 350 was in my Jeep with a 454 and 39.5 tires. I used it HARD. I do not baby anything I build. Thats why I build it the way I do... to be used and abused.

    That being said... it was rebulit by a buddy of mine. Just did a nice rebuild kit, he takes his time and does not put those stupid shift improver kits in for 4x4s. Not unless yoi plan on drag racing it. Just keep the fluid real cool and clean.

    Mind you, I live in MI and I do a lot of mud and trais, no rocks. Whole differant ball game there.

    Wish I could tell you more about the rebuil, but I didnt personally do it. The guy that did rabes deep pit bogs.

    If you have the money and time, the 400 would be the way to go. They are definaltly real strong, but the 350 should not be knocked. And, actaully there are a few more things that can be done to mine to make it stronger, so I Am told.

    This is my old website, but geocities has a bandwidth limit and it hits it a lot. So, maybe try it another day if it wont let you in.

    www.geocities.com/baja/dunes/1842

    Ken H.
     
  13. roostr3269

    roostr3269 1/2 ton status

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    Well, i wish i could say my 350 has held up, but i just got the 502 runnin day b4 yesterday. If it does go on me, it will be replaced w/ a 4L80E.
     
  14. BurbinOR

    BurbinOR 3/4 ton status

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    The 454 I had in my '75 was estimated at 500 lb/ft of torque - full on motor with all the goodies and my T350 survived for 3 years before I sold the truck. Didn't baby it either. Pull a few trailer loads of wet Oregon firewood through the mountains here and you'll know what torture is on a tranny.
     
  15. Twiz

    Twiz 1/2 ton status

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    I talked to some of my gear-headed bothers today. They (drag racers mostly) said that the main part that fails in the tranns is the Sprag (one-way clutch)

    The Sprag assembly is exactly what "gave-it-up" in my last TH350. I didn't think much of-it at the time, though.

    So....
    There are Heavy Duty after-market versions of the Sprag assembly, I even heard that a 400 sprag can be modified to fit a 350.

    Check with B&amp;M, ATI, ArtCarr (maybe), Dedenbear, CSI, TCI, or any-other A.M. trans company.


    I have no idea what the Torque rating of either trans would be, I usualy hear them being rated by input H.P.
     
  16. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Diddo on the input sprag.
    The local tranny guy builds TH350 for the Mud trucks around here. Nitrous BB with 38-44 boggers in 4 hi and 4 lo depending on the pit. The only word of caution is to stay away from lockup 350's (TH350C). You will tell by if it has a electronic plug harness. Other than that a good rebuild on the 350 will outlast a 400. Hope this helps.
     

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