TH350 vs TH400: I have a couple questions and need help

Discussion in 'Transmission & Driveline Topics' started by LDBrock87, Dec 2, 2017.

  1. LDBrock87

    LDBrock87 New Member

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    Dec 2, 2017
    I’m still kinda new to the car scene, so my knowledge is pretty limited and I definitely get lost on some of the lingo, so please don’t be upset if I have no clue what you mean about certain things.

    Backstory: So I’ve had an ‘81 Z for a few years as a project car. It was being built as a drag car by the previous owner, but they got about halfway there and basically gave up(what I’m getting at here is the body, though straight, doesn’t look the best and the interior is bad, but the engine, trans, suspension, and basically everything performance-wise is spot on). For a Christmas/birthday present this year, my parents are helping me pay for a custom-built Stroker 383 to put in. It’s all chrome, never been fired, the works. My questions are mainly about the transmission though. It comes with a TH400 reverse shift trans, built to the same degree as the motor.

    Finally getting to questions: According to the builder, the stroker is gonna put out about 800 horsepower. My first question is if the TH350 currently in my Z will be able to handle that kind of power, and if so, what modifications I may need to do(or make sure it has) in order to to be able to push it? My second question is if I do need to go ahead and put in the TH400, will I still be able to drive it normally as a cruiser without tearing it up?

    Quick note: both my TH350 and TH400 are automatics. I feel like that’d make a difference.
     
  2. SRGN

    SRGN Veteran Member

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    Feb 20, 2009
    Central NJ
    If you're pushing over 500 hp, you're going to need a performance rebuilt transmission regardless. Both have their advantages. If the th400 is already converted to a reverse manual valve body, its not going to shift automatically. Might be better off with a street/strip style th400 build. Driveshaft and crossmember are different for TH400 and TH350 as well. I prefer a 400 myself.
     
  3. LDBrock87

    LDBrock87 New Member

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    Dec 2, 2017
    Thank you! Both of the ones I have are performance built specifically, and getting a different driveshaft and crossmember shouldn’t be a problem around here. Do you think I should stick with the 400 I got, or convert it back to a normal auto for now? Cause it’s mostly going to be a cruiser, maybe see the track every few weeks(if that)
     
  4. SRGN

    SRGN Veteran Member

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    Feb 20, 2009
    Central NJ
    400 is inherently stronger to begin with. Torque converter choice is critical to getting the performance out of the transmission, as well as rear end ratio. You need at least cam specs on the new engine. Then pick the rest of the combination to match the engine. A well matched combo can get great performance without a lot of power. The wrong combo, and the car will be a dog no matter how much power the engine puts out.
     
  5. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

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    San Jose, CA, USA
    I'm curious - how much is this 800 Horsepower 383 costing you?
     
  6. junkyardZ28find

    junkyardZ28find Veteran Member

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    Mar 4, 2012
    des moines, iowa
    I'm not going to question your 383 being an 800 HP motor even though unless you're using an aftermarket Bowtie or Mowtown block you're asking for trouble-I'm more curious about what's been done to the transmissions as far as internal mods. Personally I'd choose the TH400-and to live behind an 800 HP 383 it better have:

    32 element sprag
    Extra clutch discs between 1-2 and 2-3
    Extreme shift kit mods
    Extra capacity pan
    HD 3000-3600 converter
    An external cooler


    And a reverse pattern manual valvebodied trans is fun at the track but a pita on the street
     
  7. camaro71/holland

    camaro71/holland Veteran Member

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    Aug 27, 2003
    The Netherlands
    Unless the 383 is like 13:1 compression with 230+ sized heads and top of the bill parts..... but hey, let us know and surprise us. :bowtie:

    Assuming the mule has 800hp, th400 all the way and expect to upgrade it as well.

    The basics are covered above, but there is more to be done, like rollerizing a couple of washers, and a stronger forward clutch hub. Might also reconsider a stronger input shaft/drum.

    If you don't want to shift it manually, check how its been done. If it's through a Transgo kit, it might be a 15 min job to have it shifting automatically again.

    Good luck!
     
  8. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

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    Mar 25, 1999
    San Jose, CA, USA
    I'm wondering if this is a reputable engine builder. A naturally aspirated 383 making that kind of power is a rare bird... certainly not a bird that will survive long in a street car.

    If this engine is actually making 800 HP, I'd expect the budget to be $15-$25,000 (example). If the engine is making that much power, you're probably spending $3-$5K on a transmission to make it live.

    We haven't even spoken about driveshaft or differential yet.
     
    Last edited: Dec 3, 2017
  9. LDBrock87

    LDBrock87 New Member

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    Dec 2, 2017
    I just got it in today and haven’t had much time to really look it over, but the guy who built it is a good friend of ours and there’s about $20-$25k in the motor itself, and he said around $4k in the trans so your estimates were dead on. I know there’s still gonna be a ton to do to get the rest of the car worthy of the engine, but as I said it’s a project car so I’ve got plenty of time for it. And since you asked, we’re paying $10k for the engine because a) were really good friends with the guy and b) he hasn’t been able to do anything with it since he finished it and needed it gone
     
  10. LDBrock87

    LDBrock87 New Member

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    Dec 2, 2017
    Thank you! I’m definitely going to do a check of the Trans when I get the chance and make sure it’s up to par
     

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