Making a lot of horsepower is what we all live for, but all of that power is useless unless you can get it to the ground. Part of the magical equation of applying power to the track efficiently and improving the performance of a car is the torque converter. In this article, we’ll be taking a ground-floor look at torque converters, at the components that comprise them, how they function, and how they affect your vehicle at the track.

Many racers don’t put much thought into the exact parts inside their torque converter, they’re just concerned with how it performs on race day inside their driveline. To help explain exactly what’s inside a torque converter and how it works, we assembled a panel of experts to provide some knowledge: Dave Caine from ATI Performance Products, Marty Chance from Neal Chance Racing Converters, and Kevin Winsted from TCI Automotive. All of these gentlemen have in-depth knowledge about torque converters and have provided some great information about how they put the power your engine produces to the racing surface.

Torque Converter Basics

A torque converter is a circular metal object surrounded by mystery in how it functions; to some, at least. By definition, a torque converter functions by transferring power from the engine to the transmission and the rest of the car. As the torque converter spins, it multiplies the torque produced to move the vehicle with extra force. This is accomplished by the fluid dynamics produced inside the converter. As this hydraulic power transfer continues, it couples up as the car moves down the track.

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The fluid dynamics and hydraulic power transfer process that goes on inside the converter is a process that’s kicked off by the engine itself. As the RPM levels rise when the engine turns over, it begins to rotate the impeller pump of the converter, sending fluid through the stator. Now, the stator is the “brains” of the converter and it is what really multiplies the torque as it directs the fluid inside the converter. The stator is held in place while the engine is at idle by a one-way clutch, known as a sprag. That sprag also allows the stator to move with the rest of the converter when the turbine speed reaches the pump speed as RPM increases.

As all of this rotation happens, the stator aims and moves the transmission fluid toward the inner fins on the converter’s turbine. The turbine itself is what moves the input shaft of the transmission and begins the power transmission process in the driveline. When the fluid from the stator begins to spin the turbine, it starts to rotate the input shaft and the car begins to move as power is transferred to the tires.

No matter what type of converter you use or how entry level or all-out race-minded it is, there's a lot of thought and a lot of parts that go into its construction.

The most common feature that every racer looks at with their converter is what the stall speed is; that’s the bread and butter of how the converter reacts with the vehicle. The stall of the converter is the slippage that occurs based on the load versus the resistance being put on the driveline. Most people look at it as how many RPM the engine generates with the car in gear before the tires begin to rotate. That stall speed can have a range with your average off-the-shelf converter, or it can be more precise with a custom converter.

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Converter size will usually determine efficiency and the amount of power it is able to hold. -Dave Caine, ATI Performance

How you determine where your converter’s stall speed is can be done a few different ways, according to Chance.

“Some say that when you put your foot on the brake and bring the RPM to where it pushes the car or spins the tires, that’s the stall speed. Others will apply the transbrake and mash the gas until it stops the engine to find the stall. Another way is to look at the engine RPM when you shift, and what it drops down to when the shift occurs is the stall. None of them are wrong; there is no one stall speed for a converter because it’s infinitely variable.”

Besides the stall speed of the converter, the size of the converter is crucial in how it will work with the car overall. Every car will require a different size converter based on the power output of the engine and style of racing. Caine is able to explain in fairly simple terms how the size of the converter plays into the overall driveline picture.

Converter size will usually determine efficiency and the amount of power it’s able to hold. The larger the converter, the tighter it will typically be and the more power it can harness.”

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The impeller fin angles and stator design are the primary components used to change torque converter stall speed and torque multiplication. – Kevin Winstead, TCI Automotive

Chance adds some additional details on how the converter size interacts with the car.

The size of the converter you need, whether it’s an 8-, 9-, or 10-inch is determined by the load sensitivity of your engine. For example, a motor that makes a lot of torque and makes it at a lower RPM needs a much larger diameter converter because it has a much faster rate of efficiency. It has the ability to load the motor and lock the converter up at a faster rate with less engine speed. A naturally aspirated motor that turns 10,000 rpm needs a smaller diameter converter so it can get the engine speed up and not load the motor up too fast.”

To get a converter to behave a certain way or have a specific stall speed range, the internal parts can be manipulated to achieve the desired result. Which parts are addressed will determine the final performance of the converter. “The impeller fin angles and stator design are the primary components used to change torque converter stall speed and torque multiplication,” Winstead says about how you can change a converter’s profile.

Here you can see all the different parts of a converter from TCI and how they fit together when assembled.

Because the stator is the real conductor of the orchestra that is a torque converter, the changes to it are the most important.

The stator can have any number of blades and the angles of the blades themselves contribute to its function. The larger the window, the more fluid passes through and the tighter the stator will act. ‘Cutting’ or ‘back-cutting’ a stator can also make it tighter or looser, as well. The angle at which the fluid is directed at the turbine can also affect the torque multiplication,” Caine says.

The Parts Of A Torque Converter

A torque converter is a simple yet complex device that harnesses the power of fluid dynamics to make your automatic-equipped hot rod move. The converter is made of four main parts; the impeller pump, stator, turbine, and cover that all work together to accomplish this goal. You could easily write a book about each part, but we will take a high-level look at each one to help build a basic understanding of what they do.

The impeller pump has the job of beginning the fluid flow and pumping the standing fluid through the converter. “The impeller is the pump side of the torque converter, and it typically has a hub in the center that engages the transmission pump. The impeller rotates at engine speed and is responsible for pumping fluid throughout the converter,”  Winstead explains.

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Since the impeller pump is the starting point for all the converter’s actions, it can be modified to help change how it applies the power the engine provides. By making small adjustments to the impeller, a converter can be modified to fit a change in an engine combination or if you make a big weight reduction in your vehicle. “The angles of the pump’s blades can promote or hinder the velocity of the flow based on the angle. A positive angle will create more velocity, which creates a tighter converter. A negative blade angle slows down the charge, resulting in a looser converter,” Caine says.

The stator is the brains of the converter; it controls the flow of fluid inside the converter and is the backbone of the torque amplification for the converter. The stator works with the sprag, a one-way clutch inside the stator, to move the fluid through the converter and move the car when power is applied.

The stator controls the flow of fluid inside the converter and helps multiply the torque.

When the vortex flow hits the stator and the stator is locked, that’s where the fluid is forced to change direction. – Marty Chance Neal Chance Racing Converters

Chance gives a great breakdown of how the stator works inside the converter. “When the sprag and stator are locked, it forces the fluid in the converter between the pump impeller and turbine impeller to create a vortex flow. When the vortex flow hits the stator and the stator is locked, that’s where the fluid is forced to change direction. It will increase velocity as it goes through the stator, hitting the pump impeller, and that’s where you get the torque multiplication. When the turbine gets around 80 percent of the driving impeller speed, the stator loses the mechanical advantage that keeps it locked, so it begins to freewheel. Now the stator is turning at whatever speed the fluid will most easily flow through it to create a more efficient coupling or efficiency mode in a non-lockup converter.”

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If the stator is the brains of the converter, the turbine is the brawn of the operation. As the fluid flows from the stator, it hits the turbine and rotates it. Since the turbine is connected to the input shaft of the transmission, it results in the car moving as it rotates. Caine points out that the turbine must be designed correctly to function in the converter. “There is no real magic in the turbine other than using quality material and furnace brazing the fins for longevity,” he says.

The final part of the converter, the front cover, is what secures the entire unit to the flexplate of the engine. At first glance, the cover may not seem like it plays a big role in how the converter performs, but there are some advantages that can be found in it depending on the application.

A racer who isn’t working with a lot of power, or is footbraking the car will want a cover that is light to cut down on rotating weight. If the racer is using a transbrake, heat and pressure will build quickly, and at higher power levels, will fatigue the metal so a cover made of cast steel is an option. For those looking for the ultimate performance, a billet cover is the way to go because it has both strength and a much lower weight.

Torque Converters In Racing

Understanding how your converter works and the pieces that go into it are just part of the equation of choosing the correct unit for your car. It’s important to work with a good torque converter company to ensure you get the best converter for your setup. Key information like your camshaft profile, engine power, tire size, and vehicle weight are just a few of the items a converter specialist will need to help you get a converter that will fit your performance goals.

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The ultimate goal is to get a converter that will put the engine in its “sweet spot” of the powerband quickly, and hold it within that range during the entire pass. If you don’t have the converter matched to your entire setup correctly, it could have trouble getting off the line, crushing your elapsed times, or fall out of its power range, robbing the car of consistency and horsepower.

Torque Converters For Power Adders

Every power-adder is going to have a different level of load sensitivity, so it’s important to be sure you account for that when picking a converter. “Any power adder converter should have an investment cast or billet cover and an anti-ballooning plate for strength. Turbos are the trickiest, as they need to be loose enough to allow the turbo to spool, but then tight enough to harness the power once boost is built. In many cases, a ‘fluid dump’ is required in turbo applications to achieve this balance,” Caine says.

According to Winstead, the style of racing plays a big role in the converter size and type right off the bat.

“Bracket racers will go for an 8-inch converter for applications that are less than 1,000 horsepower. In those cases where the racer is making over 1,000 horsepower, a 9-inch converter would typically be used. Heads-up racers who are making more than 1,000 horsepower will need a specialized converter with exact specifications, usually in the 9-inch to 11-inch range, depending on the application.”

When it comes to converter size, the smaller units tend to be more loose, while the bigger converters are tighter to help them with efficiency.

“The role the size plays is the larger the diameter the converter is, the faster it’s able to harness the power and how efficient it can become at a lower speed,” Chance explains. “Larger or smaller diameters are based on load sensitivity. When combined with where we need to load the motor, it will set the size of converter you will use.”

Stall speed plays an important role in converter selection since you want to make sure you’re getting the optimal amount of efficiency to make the motor happy. Stall speed is the same as slippage, so you want to get a converter that will keep the engine within its powerband that has the best rate of acceleration. “You want a converter that stalls just above peak torque, so you can make the most of the engine’s effective powerband,” Caine says.

A converter needs to be built to fit your exact combination to function correctly. Companies like Neal Chance Racing Converters offer a wide range of converters to work with different applications.

Picking the incorrect converter for your racecar is something that can be easily avoided as long as you do your research. There’s a laundry list of mistakes that you can make, so making sure you understand what not to do is critical. Basic issues can be the easiest to avoid from the start, according to Winstead. “Try not to pick a torque converter with too low a stall speed. When in doubt, it’s best to choose one that’s a little on the looser side that has a higher stall speed.”

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Caine says a big mistake that racers make is making assumptions based on their car being the same as another at the track. “A race car has a thousand different moving parts, figuratively speaking, and there is just no way everything is going to be identical between two cars. Most guys don’t realize how some simple differences can affect the way a torque converter reacts. Some racers don’t understand the difference between footbrake stall and flash, or they want to buy a converter based on a stall number. Many don’t understand the dynamics of how a converter works and the fact that it is the biggest variable in the drivetrain.”

A bolt-together converter like this one from ATI is designed for high-horsepower racecars, or for racers who want to make their own stator swaps to change the converter's feel.

Another thing that racers will overlook is how to correctly read the data their car is providing. “One of the biggest mistakes racers make when it concerns the converter is when they have data-logging, they pull their data up and they go right to the finish line data to grade their torque converter. You don’t tune anything on the racecar from the finish line back, so this makes no sense to do. The back half of the track is always a product of the front half, so make the front half right and the back half will follow,” Chance explains.

Now that you have a better understanding of what goes into a racing torque converter and how it works, picking the right one for your car should be easier. The knowledge of how a converter works can be added to the tool chest of tuning tricks and help any racer win more at the track. With the basics out of the way, we’ll take a closer look at stall speeds and the value of bolt-together converters in upcoming installments, so keep an eye out for those.