Having a bunch of horsepower on tap is fun until it starts to break stuff. It’s easy to get lost in building a powerful engine combination, but you need to upgrade the rest of your driveline to match what it can do. If you’re using a TH400 transmission, there are some upgrades you need to think about when you start climbing the horsepower ladder.

We’ve all heard the tall tales at the track or on the internet about how people have made crazy horsepower and sent it through a stock or “lightly built” TH400. The reality of the situation is there are hard limits on what a TH400 can take horsepower-wise, especially when you start getting close to, or exceeding, the 1,000 horsepower mark.

We knew the TH400 in Project Red Dragon would need to be upgraded based on how much power the car made on the dyno. After talking with JC at ATI, we put a plan together to create a transmission that would be perfect for our application.

Our Project Red Dragon Trans Am got a major TH400 rebuild with parts from ATI Performance Products and was assembled by the team at 501 Performance. We got a big education about how these transmissions work and what you need to do to beef them up when you’re trying to make a lot of power.

When You Make Big Power, You Need To Upgrade A TH400

Transmissions are subjected to all of the horsepower-driven abuse an engine generates. The OEM parts that came inside a TH400 weren’t meant to hold much more than 500 horsepower, let alone double that, and transbrake launches.

Parts like input shafts, intermediate shafts, clutch hubs, and drums all need to be replaced when you hit the 1,000 horsepower mark. Most of these parts aren’t made of strong enough materials in their OEM form to withstand high levels of horsepower. The aftermarket versions are made of better materials and have design upgrades to deal with increased horsepower.

Logan Sigman, the master TH400 transmission builder at 501 Performance, built us a total unit for Project Red Dragon. This transmission is filled with the right parts from ATI that can deal with the power our turbo LS combination makes and then some.

You also need to consider how you plan on using your vehicle and how much it weighs. A heavy vehicle that makes 600 horsepower, and uses a transbrake is going to put a lot of stress on the transmission. A lightweight car that makes big power is going to put stress on that same transmission, but in a different way. The bottom line is, if you want to make a TH400 live and be reliable when you make power, you need to have a professional shop build it with good parts.

What You Need To Upgrade And Why

We knew that the “built” transmission was a little suspect for Project Red Dragon, but we had no idea how bad it really was. 501 Performance tore our TH400 down and discovered it used OEM parts, the clutches weren’t rated for big horsepower, and the case was cracked. We talked with ATI to get a parts list together based on our horsepower goals, and had 501 Performance start the upgrade process.

A transmission’s input shaft is what couples with the torque converter and gets the parts moving inside the transmission when everything starts to spin. The input shaft is exposed to a lot of twisting forces.

Bubby Rafferty from 501 Performance explains why it’s so important to upgrade the input shaft of a TH400.

“This is the part of the transmission that eats the initial shock of the power transfer to the transmission. It absolutely needs to be upgraded in a high-horsepower TH400 build. It needs to have the right amount of torsional strength for the application it’s being used for.”

There are a few different options for input shafts out there. According to ATI Performance Products owner J.C. Beattie Jr., you need to look at the material and size of the shaft for your TH400-based specific application.

“For most applications, you’ll want to get an input shaft that’s made of 300m material and is standard size. If you’re running something a little more extreme, a Vasco shaft is what you’ll want to use. Now, for the most extreme applications, you’ll also need to increase the size of the input shaft, but there’s a caveat there. Once you increase the size of the input shaft, you’ll have to also move to a forward drum and pump that will work with the larger diameter shaft.”

The intermediate shaft connects everything front-to-rear inside the transmission. This is a critical upgrade for a high-horsepower TH400.

The intermediate shaft’s job is to connect the front of the transmission to the rear of the transmission in a TH400. This is an important job in the power transfer process, and the intermediate shaft sees a lot of stress. Just like the spine in your body, you want a strong intermediate shaft connecting everything together inside the transmission.

“They say that horsepower will find and break the weakest link, that’s why you need to upgrade the intermediate shaft. This shaft connects the front and rear of the transmission. You need to use one that’s going to hold the horsepower you’re shooting for, while being able to handle the weight of the vehicle,” Rafferty says.

Just like the input shaft, the intermediate shaft’s material and size are important.

“Your intermediate shaft needs to be made of at least 300m, or Vasco steel if a lot of horsepower is being made. This shaft takes a lot of abuse during the One-Two shift, so it’s really important that it gets upgraded. Once you push past the 1,500 horsepower mark and get closer to 2,000 horsepower, that’s when you need to look at upgrading the size of this shaft, and that will change everything from the direct drum all the way through the gear set,” Beattie states.

The forward clutch hub (front and center) is another failure point inside a TH400 that needs to be addressed.

A TH400’s forward clutch hub is another important part of the power transfer process. This hub is mated to the direct drum and main shaft. The clutches and steel plates that are part of the forward clutch pack are directly connected to the forward clutch hub.

“The OEM clutch hubs are made of cast material and will split in half when you try to push them, especially in a heavy car. You have to replace these if you’re going to try to make a lot of horsepower with a TH400, preferably one with a Torrington bearing setup. The modern aftermarket clutch hubs are just so much better than the stock units, and they hold up to drag racing abuse,” Rafferty explains.

So, what exactly should you look for when trying to find an aftermarket clutch hub? Beattie breaks down what you need.

“Anything that’s making over 1,000 horsepower, an upgraded clutch hub is a requirement. You don’t want one of these hubs coming apart. The ones we use are made in-house and are very strong. They cut down on weight and help the overall performance.”

The direct drum is the high gear of the TH400, and it transfers power directly from the input shaft to the output shaft of the transmission. This is probably the most critical component you need to upgrade in a high-horsepower TH400 build.

“We always recommend moving to an aluminum direct drum for any big-power TH400 build. It cuts down on the rotational weight and adds strength. The aftermarket direct drums are going to have design improvements that make them stronger as well,” Rafferty says.

There are a lot of options out there for TH400 direct drums. Beattie explains what you should look for in an upgraded unit.

“Aluminum is the only way to go, it’s four pounds lighter. Less weight means it’s going to increase the life of the sprag. We offer a billet aluminum drum that has a 36-element sprag with other upgrades that make it stronger.”

The more clutches you can fit inside a high-horsepower TH400, the better.

Clutch packs inside a TH400 transmission are in charge of applying the power to each gear set. These packs are filled with friction discs and steel discs that get squeezed together by the hydraulic pressure inside the transmission. As they are squeezed, they engage and transmit power to make the transmission function.

“You need to upgrade the material of the clutch pack for sure. Each transmission builder has their preference on what goes into the clutch pack. As long as the parts match the application, you shouldn’t have any issues. You can’t use clutch packs that aren’t rated for your horsepower goals,” Rafferty states.

Upgrading the clutch material is a must for a TH400 that’s going to be seeing 1,000-plus horsepower. But, there’s more to making the clutch packs better than just swapping out some parts.

“You want to increase the number of clutches inside the transmission. OEM-style clutches are actually too thick, so you have to move to thinner clutches for these types of builds. The material the clutches are made of needs to be able to handle all the heat and clamping load they’ll be exposed to,” Beattie says.

A transmission’s valve body acts like the shot caller, it sends fluid to the clutch packs to make the transmission shift. Think of it as the conductor at the train station who makes sure everything is on time and working properly inside the transmission.

“A manual valve body is going to move the fluid through the channels each time you move the shifter. You need an upgraded valve body that will be able to move the right amount of fluid and maintain line pressure. A good valve body is important because it won’t get hot, and it will be made of better material,” Rafferty states.

Not all valve bodies are created equal. The quality of the machine work, the material they’re made from, and the overall design play a role in how they perform.

“Our standard steel valve body is used a lot by guys who .500 Pro Tree race. They just feel more comfortable with it when they’re running a lot of rounds of racing. Some racers need an ultra-fast transbrake release and clean neutral, we’ve got that covered with our Wicked Quick™ Billet Aluminum Valve bodies. These aftermarket valve bodies have all been modified and are different from the OEM units to make them work better for performance applications,” Beattie explains.

At a minimum, you’ll need an SFI-approved bell for your TH400 transmission. If you’re making REALLY big power, an aftermarket SFI-approved case is a must-have item.

An upgraded bellhousing for your TH400 transmission needs to be part of the budget. If you’re really going to push things past the 1,500 horsepower range, an aftermarket case is going to be a requirement as well. Since our case was junk, 501 Performance built our transmission using a quality OEM case and added an SFI-approved bellhousing as well.

“We won’t build a transmission that’s going to be used in a vehicle making more than 1,500 horsepower without an aftermarket case. It’s just not safe, no matter what the internet experts will try to tell you. An OEM case won’t stand up to that. The bellhousing is another safety item you can’t skimp on. A well-built transmission could fail if there’s a parts issue, that SFI bellhousing is there to save you from serious injuries if that happens,” Rafferty says.

There are some performance reasons for moving to an aftermarket TH400 case.

“An aftermarket case is going to be able to hold more line pressure. When you’re pushing past that 1,500 horsepower mark, you need to run around 220 pounds of line pressure. An aftermarket case will have more material in the right places to hold that pressure,” Beattie states.

Final Thoughts

A well-built TH400 will easily handle a 1,000 horsepower engine, but you need to make sure the right parts are used. Adding the best parts to your TH400 for your application will ensure your transmission will be reliable and perform at an optimal level While we all know that one person that did more with less, is it worth the risk of being a cautionary tale? Save yourself, your racecar, and the track from dealing with a grenaded transmission.