Turbo 350 or Turbo 400 for my GS?

Discussion in 'The "Juice Box"' started by Canadian GS 350, Dec 15, 2019.

  1. Canadian GS 350

    Canadian GS 350 Well-Known Member

    Completed the rear end back in April - new posi, 30 spline moser axles, control arms, bushings, sway bar, air bags ........ thanks Monza, now onto the tranny.

    As car is a 350 GS, it has a non numbers turbo 350. I have a turbo 400 and the gas pedal with switch and a 3.5 inch aluminum drive shaft for the turbo 400. I plan on having Jim from TSP put a 9.5 stall together for the car as well.

    I need a tranny to live behind the future 455 build....approx 550 hp, the torque that comes with that.

    So build the turbo 350? Or build the true tried and tested turbo 400?

    Part of this decision is cost......the turbo 400 will be reasonable.......the turbo 350....I have no experience in having a turbo 350 built to be reliable for this level of power, torque.

    The car will be 95% street use, likely 6-10 trips to mission a year max.

    Value members opinions as drag racing is relatively new for me, I want a reliable package I can beat on for many years.

    Thanks, Brad

    BQUICK Well-Known Member

    In my experience you have to put a bit more money into a T350 to get it to hold up at 500-550hp....such as heavy duty drum. I had sprag failures until I went with HD drum whereas a T400 just needs HD sprag and a good build to hold up.
    T350 will be up to a tenth quicker in the quarter.....
  3. Matt69olds

    Matt69olds Well-Known Member

    Machine the direct piston the equivalent of one friction and steel, then assemble with 5 clutches instead of the factory 4. Install a wide direct drum bushing. Dual feed the direct clutch, either with a shift kit, or internally. Leave the wave plate in the intermediate clutch, use a orange accumulator spring in the intermediate accumulator on the side of the trans. You can delete the 2-3 accumulator by blocking the 3rd clutch feed in the valve body. Remove the 2-3 accumulator piston from the valve body, install a 3/8 freeze plug in the feed hole under the piston. If the piston is plastic, replace it with a earlier aluminum piston. Drill the 1-2 and 2-3 clutch feed holes to .110 to a maximum of 3/16. You can use a low roller clutch from a 4L60, the rollers are larger. You can also use the turbulator steels ( they have small oval/shaped holes) in the low/reverse clutch. The purpose of the turbulator holes is to remove extra oil from the friction, and to prevent the clutch and steel from sticking together. Lastly, you can speed up the 1-2 shift by eliminating excessive clutch pack clearance by taking a old low/reverse friction, scrapping all the old friction material, and making a single cut in the steel clutch backing. The disassemble the pump, put the cut steel backer under the piston, and then reassemble. The steel backer will act as a shim, taking up some of the excess clearance.

    I have built many 350 that have lived in low 11 second cars. The key is to not make them shift really hard, and to keep end play within specs. If/when the intermediate clutch breaks, with upgrade to a better part, or swap to s400
  4. 1969RIVI

    1969RIVI Well-Known Member

    Build the TH400, it can take a sh*t kickin' for a long time for less money than the 350 would cost you to be as strong as the 400. I did my th400 myself for around $2500 CDN and that's with 9.5" JW converter, transgo 1-2 kit, a bunch of mods to the drums and valvebody as per Ben's mod thread,upgraded 34 element sprag, high performance complete rebuild kit with kolene steels and red alto frictions and bands.
  5. Canadian GS 350

    Canadian GS 350 Well-Known Member

    If I was doing a stock 455, I think I would just do the turbo 350......given that’s not the case, I think I will go tested and true abuse friendly turbo 400. And I have a sweet drive shaft for the turbo 400.
  6. Matt69olds

    Matt69olds Well-Known Member

    It’s hard to beat a 400. If your rebuilding the trans I wouldn’t bother with the shift kit, it’s cheaper and much more effective to do the dual feed by either welding the reverse feed hole in the spacer plate closed, or installing a freeze plug in the case. Drill a couple holes in the plate bigger, firmer shifts for the cost of basically free.

    The TH350 can be built to handle some power, but it isn’t nearly as durable as a 400. I would love to try a 400/350 swap at the track just to see if there really is the .010-.015 difference in ET between the two.

    BQUICK Well-Known Member

    T350C lockup would be the ticket....never did one but wanted to.....have seen them in 10sec cars
    1973gs likes this.
  8. 1973gs

    1973gs Well-Known Member

    I believe that they were only used for a few years. I would like to find one.
  9. brent earp

    brent earp Well-Known Member

    have one if your interested
  10. Brandon Cocola

    Brandon Cocola Well-Known Member

    How is the 350c the best? I'm not sure what happened but I had a rebuilt one behind my 550hp 454 and lost 2nd after 100 miles.
  11. BQUICK

    BQUICK Well-Known Member

    Didn't say "best".
    Has to be built right. Having lockup converter is the attraction. If you can get one to live, it would be fastest IMO and good mpg.
    I believe Jim Haas had one in Big Red years ago.
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

  13. Canadian GS 350

    Canadian GS 350 Well-Known Member

    As I will be going turbo 400 route, I have the gas pedal with switch, I have found the wiring to the switch in the stock harness under the dash. As my car came with a turbo 350, is there a turbo 400 kick down wire in the engine harness? If so.....where is it?
  14. Canadian GS 350

    Canadian GS 350 Well-Known Member

    My craigslist special - Pontiac turbo 400- 1970 455 unit, turned out to be a rebuilt unit, never had tranni oil in it! Has a Transgo2 kit, “red” clutches, 34 element sprag. So the builder is going to clean everything, add a few mods he likes. Best $100 I ever spent. Ordered my 9.5 inch converter from TSP. Should be ready in a week. Now I need a potent 455!

    Attached Files:

  15. 1969RIVI

    1969RIVI Well-Known Member

    SWEET SCORE!!!:D:) ahh man your miles ahead all for a couple Benjamins :cool:
  16. Bens99gtp

    Bens99gtp Well-Known Member

    Nice score, even has the hd case
  17. telriv

    telriv Well-Known Member

    How can you tell it has a HD case??? Never heard of that ever being mentioned.
  18. Bens99gtp

    Bens99gtp Well-Known Member

    I can see the the extra support rib, it will also have extra webbing in the bell housing, thicker bellhousing flange, and thicker pan rail.

    See red circle

    Normal case dont have the support rib here. See second pic .

    Attached Files:

    Last edited: Jan 13, 2020
  19. Bens99gtp

    Bens99gtp Well-Known Member

    Also here is the easiest way to tell from the outside the extra webbing in the bellhousing

    Attached Files:

  20. '72 Skylark Custom 4bbl

    '72 Skylark Custom 4bbl Well-Known Member

    Look under the dash in the area above gas pedal. That's where mine is. A yellow wire with a black plug on it.

    I also have a TH350 & will be swapping to a TH400 in the future.


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