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An Inside Look to How a Torque Converter is Built

Torque converters are the component that made the modern automatic transmission possible, transferring power from the engine to the rest of the drivetrain without the use of a clutch. Even though we’ve all used them, few of us understand what’s actually going on inside or how they’re built. To answer those questions, we talked to TCI, one of the country’s most popular manufacturers of performance converters, and they were gracious enough to walk us through how converters are made, and how they do their job.

Before we spoke with the professionals, we did some research of our own. We discovered that although boat-loads of research and design have been dumped into the torque converter since it’s creation, it originally began as an option in luxury daily drivers in the late 1940’s.

Buick revolutionized the automobile industry by developing the first fully automatic transmission in 1948.


Torque converters have been around for decades. Originally introduced in 1948 with Buick’s 2-speed Dyna-Flow transmission, the torque converter helped make the transmission fully automatic by eliminating the clutch necessary for manual transmissions. What makes torque converters unique is the transfer of power from the engine to the transmission with fluid being the only connecting factor, allowing the engine to idle without the threat of stalling. Along with preventing engine stall, the converter helps improve performance by nearly doubling the torque of the engine which offers a higher degree of acceleration. Although OEM torque converters are continuously improving, the high performance industry will always be ahead of the game by offering more overall power, better acceleration, and improved fuel economy for not only custom engine applications, but stock packages as well.

Though it might seem strange, thinking of a torque converter as a jelly-filled donut helps to visualize how it works.

What’s Going on Inside a Converter

In order to feel confident with selecting your high performance torque converter, you must first have an understanding of what makes them work. We know that this “jelly-filled donut” bolts up to the flex plate, and we know that the input shaft of the transmission slips into the back side. What we don’t know is actually the most crucial part: the inner workings. The casing that bolts up to the engine has a set of specially designed fins fixed to the inside. This part of the converter is known as the impeller which spins at the same rate as the crankshaft. When the impeller spins, centrifugal force pushes the transmission fluid through the angled fins toward the outside of the casing. Once the fluid reaches the outside of the casing, it hits the reflected angles at the top of the turbine, which is splined to the input shaft of the transmission, causing it to turn the transmission input shaft. The fluid is then directed back toward the center of the housing where the process repeats.

The stator keeps the fluid flowing smoothly inside the converter without turbulence. This increases performance and fuel efficiency.

The Stator: Keeping it Flowing Smoothly

The whirlpool action inside the torque converter is controlled by the stator. The stator sits on a one-way clutch in between the two rotating vanes. The extreme angled fins redirect the fluid from the turbine before it returns to the impeller. This action improves performance and fuel efficiency greatly. Although the stator is key in the torque converter’s job during idle and acceleration, it plays a smaller and smaller role as the torque converter approaches ‘lock-up’ speed where the two impellers are spinning at nearly the same rate.

With a basic understanding of the internal components under our belt, we turned to one of the world’s most popular performance torque converter manufacturers, TCI Performance in Ashland, Mississippi, to get a better grasp on how all those components can be tuned to get more power to the pavement.

TCI Automotive is based in Rural Mississippi, but they are well known all over the world.


Our insider’s look at TCI’s shop gave us a perspective most people outside the business never get. While factory converters are built by the thousands, all exactly alike, TCI takes pride in doing it by hand, one at a time. “We don’t do an assembly line. We have one or two builders, and all they do is assemble the torque converters,” TCI engineer Scott Miller explained. “They set the clearances, they set the fin angle, so it keeps everything uniform when you don’t have a bunch of people doing it.”

Unless you build them, you rarely get to see what's inside a torque converter.

Turning a Stock Core into a Aftermarket Monster

“Essentially, we start out with a stock core and we cut it open,” explained Miller. “We then send the top (impeller) and turbine out to be furnace brazed, which basically strengthens the fins and welds them solid,” he continued. “When those pieces come back, we install new hubs, bearing ride plates, heat-treated turbine splines, and a modified stator with new sprags and sprag races, and we build the cover with a new pilot and mounting pads to bolt to the flex plate.”

Once everything is installed in the refurbished converter, the unit is ready for inspection to ensure it’s all correct before it is sealed and tested.

The front cover will be welded to the impeller in the final step of assembly.


Quality control is what sets TCI apart from the competition, so it’s no surprise that so much time and effort goes into the inspection process. “Our technicians have a check-off sheet when the converter is being built. They record the hub size, turbine clearance, sprag, and the pilot size during assembly,” said Miller. “From there, the converter goes to an inspector in the Quality Control department. They inspect every converter and verify that the part number stamped on the converter matches the fin angle and stator configuration.” Once the inspector signs off the inspection card, the two halves are ready to be welded together.

One of the many tests a converter must pass before it gets TCI's approval is for runout.


After the converter is fused into a single whole, the final stage of inspection can occur. The converter is submerged in water and pressurized to ensure that it’s completely sealed. The converter is then balanced and the end play is double checked. A transmission input shaft is then inserted in the hub to ensure proper fitment. Once all of the tests are passed, the converter shell is coated with a heat dissipating paint and it’s packaged for shipment. Miller stated, “Depending on the model, sometimes we can have a converter built within that afternoon. Our average turnaround time is about one or two days.” Some of TCI’s wildest builds are for 200 mph+ customers like Todd Berry, John Kolivas, and Jason Lee.

The turbine rides inside the front cover and interacts with the impeller to provide power to the transmission.

Changing Stall Speed

When asked how a torque converter is customized for each application, Scott Miller informed us that the most important factor is the converter’s ‘stall speed.’ “There are a few different ways to change stall speeds. You can change the angle of the impeller fins by giving them a positive angle to lower the stall speed or a negative angle to increase it,” Miller stated. “You can also change the stator in the converter. Different stators have different fin angles and different amounts of blades which change the way the fluid flows through the converter. You can also change the clearance between the turbine and the impeller. If you increase the clearance, it lets it slip more and if you tighten it up, it lets it lock up, which decreases the stall speed,” Miller explained.

powerTV Projects use TCI Converters too

Many of our Dragzine.com and StreetLegalTV.com followers know that we used TCI in ‘Project 666,’ a Fox Body with a 408ci crate engine. We strapped their custom converter behind our power plant, along with a TCI C4 with an upgraded case. The small block Ford pushed out an amazing 500 horsepower on the engine alone. 666’s first time at the track was at Irwindale where it ran some 11-second soft hit passes.

Putting together the right converter for your application is TCI's primary goal.


Of course, in a sport where zero-to-hero is measured in thousandths of a second, there is a lot more to getting the right converter than just stall speed. The good thing is that TCI is there to help, and with the right information about your suspension and chassis, tires, transmission, and engine, their staff can design a torque converter that will shed seconds and add MPH. You don’t have to be an automatic transmission expert – that’s their job – but now you know the right questions to ask, and have the background information to understand a little bit of what they’re talking about.

Sources

TCI Automotive
Phone: (888) 776-9824


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