Toyota Supra mkiv

Kevin's Mkiv TH400 conversion

I decided to convert my Getrag 6 speed Supra to use a GM Turbo 400 (TH400) auto so I can be quicker and more consistent at the drags. This is also because the heavy clutches I needed to hold the 700hp at the engine were becoming unreliable and the only real clutch option was a $4000 RPS carbon-carbon triple plate clutch. As I can sell my Getrag for what the TH400 setup has cost me it is effectively free, so I'd thought I'd give it a try.

This is the final look of my conversion which looks just like a stock 6 speed, except that the eagle eyed reader will see that the shifter knob is facing backwards.

The modifications done during my extensive (and over the top) upgrades were as follows:

  • Remove rear subframe and seam weld it for added strength and do a quality powder coated repaint in gloss black.
  • Change the big rear 2 pot brakes for small rear 1 pot brakes, so I can fit the 15x10" Weld wheels with Hoosier QTP 15x11.5x26". This is to get quicker 60ft times compared to the previous heavier stock 16x9" Supra wheels with MT ET Streets 16x11.5x26".

  • Replace all rubber bushes with Titan aluminium bushes and Titan steel rear differential support.
  • Slot the rear top A-arm bolt holes by 6mm (with welded in washers for support). This allows setting positive camber which will keep the tires flat during drag launches.
  • Replace stock traction and toe rods with Megan adjustable rear traction rods and Megan adjustable rear toe rods for more adjustability.
  • Replace the 220mm B03B 3.266:1 differential with the (very rare) 220mm B04B 3.538:1 UK spec auto differential for faster launches. This is still not as good as the stock auto ratio of 3.769:1 but it is the big diff, rather than the small diff.
  • Replace the torsen LSD center with the TRD LSD center.
  • Modify the driveshaft to fit the auto and increase the diameter of the driveshaft to 3.0" to make it stronger.

  • Install the GM TH400 automatic which is rated to 1500+ hp, has a 3500rpm stall and a stronger custom (8mm thick) flex plate which I got made for it. It also has a forward pattern manual valve body installed but no transbrake. This auto that I bought already had the Toyota bell housing conversion and it just squeezes in with about 10mm to spare on either side of the tunnel.
  • Create an adapter plate to the stock auto rear gearbox brace and bolt an after market GM red urethane gearbox mount support to it.

  • Install a new B&M Quicksilver rachet shifter (with 5ft cable) in the transmission tunnel just above the driveshaft. This is because it will not fit under the gear stick console with the 6 speed tunnel (auto tunnel is fine) that I have if mounted on top. The shifter has an aluminium box bolted around it to make it water and dust proof. This gearshift works backwards to most other Supra TH400 installations with 1st at the front most position and park at the rear most position. This is because this allows for easier routing of the cable under the car. The stock 6 speed gearbox knob screwed straight on but it now faces backwards. I could rotate it to face the right way but it feels better this way round when changing gears.

  • The next 3 pics are of the old shifter and gear knob which I replaced with a new shifter and my good condition 6 speed gear knob. The new shifter required a fair bit of cutting to make it fit, but is still fully functional. These pics only show the bits used for the dummy fitting which were replaced by a very nice custom made aluminium box which the shifter is bolted to at the top and bottom.

  • Shifter in neutral position (and same position for 1st to 3rd). The shifter shaft has not been shortened at all but could be shortened an inch or so if required.

  • Shifter in park position, which I won't use much as using the handbrake and neutral works fine for me.

  • Wire up the neutral start switch and the reverse light switch from the shifter micro switches.
  • Install Dakota Digital converter to take the signal from the Dakota GM sensor-01 and convert it to the required signal for the Supra speedo. This required using Output 4 to get a correct KMH reading unlike the USA Supras which mostly use Output 1.

  • Install two B&M 70297 transmission coolers (with in-built thermostat controlled fans) up front behind the intercooler pipes and just in front of the plastic inner gaurds. These are the largest coolers that I could fit (everybody says they cannot fit in the Supra but they do) but the driver side one required removing the water squirter bottle.

  • Install a custom 600x300x120 intercooler with a 3" inlet and outlet.

  • Install a custom water bottle in the boot using a 1.7L aftermarket water bottle and the stock squirter pump.

  • Install a smaller and lighter (6kg Vertex dry cell) battery in the front to reduce weight at the front of the car. This one or a stock sized one may go in the boot at a later stage. A custom aluminium water overflow bottle will fit nicely in front of the battery so that may get done as well.

  • The eagle eyed reader may spot the TH400 dip stick.

  • I am using a MasterPower T70 (70mm) turbo at the moment, but I will upgrade to a MasterPower 76mm turbo once I get used to driving the auto.

To get faster spool, more boost off the line and higher maximum power, I will be upgrading to E85 in the near future.

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Last modified on Monday, April 9th, 2012