Transmission Change: Project Red Dragon Gets A TH400 Transmission

You can build an engine that will make all the horsepower in the world, but if the transmission behind it isn’t up to the task your project car isn’t going anywhere. We’re transforming Project Red Dragon into a wicked street/strip car and needed a stout transmission, so we turned to the folks at ATI Performance Products and BMR Suspension to assist us in our transition to a TH400. In this article, we’ll outline why you want to use a TH400 for a street/strip car, and what it takes to bolt one in a fourth-generation F-body.

Project Red Dragon’s built 4L60E transmission worked flawlessly when we were cracking off runs in the 10.80 range and making just over 500 horsepower. The new turbocharged combination will easily make double that amount of horsepower, so moving to a TH400 transmission was the most logical choice, since the car is still driven on the street. We worked with ATI Performance Products to get a flexplate for this conversion, and had them build a custom torque converter for our combination. The team at BMR Suspension helped us with getting the right transmission crossmember, and beefing up the torque arm so we can put the maximum amount of power down to the track.

We need Project Red Dragon to apply all the turbocharged horsepower it can to the track’s surface. To make this happen, we’re moving to a TH400 transmission for strength and a BMR Xtreme Torque Arm for traction.

Making The Switch

The TH400 transmission is the Swiss Army knife of transmissions — it can be used in numerous applications that cover all forms of motorsports. Those who want to make a lot of horsepower and drive their cars on the street like to use the TH400 because it be built with three forward speeds, versus a Powerglide that’s limited to two. Since Project Red Dragon is going to see some action in True Street-style classes and possibly even drag-and-drive events, it made sense to use a TH400 to keep the engine RPM levels down when we’re cruising on the streets.

ATI’s John Lane is a racer himself, so he knows what works at the track — he believes that the TH400 is the best option for a high horsepower street/strip build like ours.

“Your transmission choice needs to be based on the power you plan to make. The TH400 is a versatile transmission and is one of the top choices for drag racing. You can build one of these transmissions with all aftermarket parts, and it will work with virtually any engine combination. A TH400 has a huge number of gear ratio options that help it act like two-speed transmission if you need it to based on your combination. It can be set up like a close-ratio three-speed, or a wide-ratio three-speed transmission depending on what your power and torque curve looks like.”

The TH400 we're using was recently rebuilt with heavy-duty internals and is ready for action.

Streetcars that are carrying extra weight like our Trans Am will benefit from using a TH400. The robust nature of a TH400 helps it move all that extra mass, and you can upgrade every part of the transmission. A TH400 can be built without even having an OEM core since there’s such a wide range of aftermarket parts available.

Frank Steadman from BMR adds his thoughts on why the TH400 is the right choice for a build like this.

“The TH400 transmission is strong and affordable, it’s a proven commodity in the racing world, and you don’t have to break the bank to swap one into a fourth-generation F-body. There are a lot of options out there for these transmissions and you can still use them on the street without any issues.”

Not only are we using a custom-built ATI torque convverter, we're also using ATI's Super F transmission fluid. This will ensure the converter works right and will help to battle all the transmission heat that comes with spooling a turbo.


Torque converter selection is so important with a swap like this, and adding boost to the equation makes it tricky. If you don’t have the right torque converter in front of the transmission, your car is going to struggle both on the street and the track. When you call a company like ATI to have a converter made, they need to know about every part of your combination and the car to make sure they can build a converter that will work. The torque converter is load-sensitive, so there’s a lot that goes into making sure it will work within the operating range of your engine.

When you’re building a dual-purpose car like ours, that adds to the complexity of the converter selection.

“Street car stuff is a lot harder than racecar stuff when it comes to building a converter. A racecar you only have to deal with the vehicle leaving off the line, going down the track, the flashpoint of the converter at the gear change, and it’s done working. The problem with a street car is the vehicle has to idle in gear at a red light without any issues, not burn up the transmission, and still perform at the racetrack. When you’re looking at a turbocharged application, you have to have it drive like a stock car in full vacuum, but also get into boost quickly without putting too much heat in the transmission,” John explains.

Your transmission choice needs to be based on the power you plan to make. – John Lane, ATI Performance Products

The fourth-generation F-body was never available from the factory with a TH400 transmission, so there are some special parts required for this conversion. Thankfully, ATI makes things easy with its flexplate conversion kit for LSX-based engines that use a TH400. The kit comes with an SFI-approved flexplate, flexplate bolts, torque converter bolts, and crank adaptor flange, if you need it.

The ATI flexplate conversion is designed to be a direct-fit part for an LS engine.

“A factory-style flexplate for an LS-based engine is dished, while our flexplate is flat. We have a crank adapter that takes up the distance that is missing from the flexplate not being dished. The ring gear lands in the OEM spot because you can’t move that, but the crank pilot spacer adapter takes up the distance and allows you to use a converter that doesn’t have an extended pilot or a specific LS converter. If you have a 6.0 truck engine with an extended flange, you don’t use the crank adaptor. You can use an OEM GM or SFI case and won’t have any issues with this flexplate,” John explains.

Suspension Changes And Crossmember Conversion

Maintaining the right driveline geometry is a must when using a transmission that’s different than what came in a vehicle from the factory. A new transmission crossmember is required as a part of the TH400 conversion for a fourth-generation F-body so everything stays aligned. BMR developed its transmission crossmember to make the process of switching to a TH400 easier.

“We did all of the measurments and created a crossmember that is a direct fit. You might think you can make one yourself, but 30 hours later you’re still going to be working on something when you could have bought our crossmember that bolts right in. This crossmember has already been engineered to have the right geometry to ensure the transmission and driveline are in the right position,” Frank explains.

The BMR TH400 conversion crossmember is a simple, yet effective piece. It bolts right into the factory location under the car and won't have any issues holding the transmission in place.

The crossmember wasn’t the only thing we needed to sort out for this conversion. Project Red Dragon has been rocking a long BMR torque arm that was relocated off the stock transmission, which was perfect for our previous power level, but with our 1,000 horsepower goal, it wouldn’t be ideal.

After speaking with Frank, we decided it would be best to move to BMR’s Xtreme Torque Arm. The Xtreme Torque Arm is made of 1.25-inch chrome-moly steel tubing and uses a crossmember that’s welded into place in between the subframe connectors or roll cage.  This torque arm is significantly shorter than the stock torque arm and that helps keep the rear tires planted to the track.

“One advantage of the shorter torque arm that we didn’t think about during the design process is the fact that it makes the car more consistent. A weaker torque arm will deflect and won’t launch the same even if the track conditions are identical for each run. If a suspension part bends like the torque arm it won’t perform consistently and the shorter torque arm helps prevent that issue,” Frank states.

You can see just how much shorter the Xtreme Torque Arm is compared to a standard length torque arm from BMR.

The Xtreme Torque Arm is significantly stronger than any longer torque arm you can bolt up to a fourth-generation F-body. The strength of the shorter torque arm will make sure it doesn’t flex as much as a longer torque arm, so that means it will be more efficient in transferring horsepower to the tires. This torque arm is really designed for drag racing applications and won’t improve the handling of your car.

“The shorter torque arm makes the suspension more efficient at doing its job. You’re going to see a higher amount of anti-squat in the suspension when you switch to the shorter torque arm. The higher amount of anti-squat keeps the car from squatting on the tires too much, and it transfers the weight to the tires instead of the suspension just absorbing it,” Frank explains.

The crossmember mounting system for the Xtreme Torque Arm is designed to provide maximum strength.

“There was a lot of testing that went into this product. We worked with our own cars and customer’s cars to find a length we liked, and that responded to multiple setups on different cars. Small changes have been made over the years based on customer feedback and what we’ve learned at the track. The weld-in design was selected because it allows you to adjust the torque arm position based on your car and how you need it set up. If we just made this a bolt-in kit with our subframe connectors, it would have limited who could use the product. This way, it’s a nice fabricated part that you don’t have to fabricate yourself,” Frank says.

The shorter torque arm makes the suspension more efficient at doing its job. – Frank Steadman, BMR Suspension

If you want to get the most out of one of these torque arms you have to make sure it’s installed correctly. The directions that BMR provides are pretty straightforward, but Frank has some advice for those who are tackling the installation of the Xtreme Torque Arm.

“The most important thing you need to do is to mock up everything before you weld it in. The driveshaft, exhaust, everything needs to be installed. You have to figure out the instant center you want to use for your application, as our baseline is just a place to begin. The position of the control arms, your ride height, and your desired instant center are all going to be different. This all has to be considered before you weld anything in, because once you weld that crossmember in the front position, the torque arm is locked down. You also want to run the suspension through its full travel to make sure the torque arm isn’t hitting the tunnel, which would case the tires to unload and break traction, Frank explains.

BMR added the gussets to the Xtreme Torque Arm to make sure it could deal with the highest amounts of horsepower possible without bending.

Once the crossmember for the torque arm has been welded in, you can only adjust the pinion angle of the torque arm. According to Frank, the pinion angle can be changed to deal with vibrations and harmonics you encounter at higher speeds. If you want to adjust the instant center of the rear suspension you’ll need to adjust the position of the lower control arms and shocks after everything is welded into place.

We’re getting closer to making some laps at the track with Project Red Dragon’s new combination. The TH400 transmission, ATI converter, and BMR Xtreme Torque Arm should have no problem handling all the boosted power we plan to throw at it. You can follow the entire Project Red Dragon build right here on Dragzine.

Article Sources

About the author

Brian Wagner

Spending his childhood at different race tracks around Ohio with his family’s 1967 Nova, Brian developed a true love for drag racing. Brian enjoys anything loud, fast, and fun.
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