Unlike a manual transmission, most cars that are equipped with an automatic transmission have what is called a Torque Converter. A Torque Converter is one of the most complicated and misunderstood parts of the vehicle’s drivetrain. While not all modern automatic cars have a torque converter, they are the most important component to the cars that do have them!
In short, a torque converter is a centrifugal pump that allows the engine to stay running while the wheels are not moving. Similar to how a manual transmission utilizes a flywheel and a clutch assembly, most automatic transmissions have a flywheel and a Torque Converter.
The torque converter is located between the engine and the transmission. Unlike a manual transmission that requires a mechanical operation of the driver letting their foot off the accelerator pedal and pressing the clutch pedal to shift gears and come to a stop, all of these actions take place in the torque converter automatically for the driver by transferring transmission fluid contained in the converter itself.
The first part of the torque converter is the Impeller. It is welded to the outside housing of the Torque Converter. The Impeller has curved blades and, as the Impeller spins, the fluid is thrown outward by centrifugal force caused by the rotation of the impeller blades. The faster the Impeller rotates, the higher the centrifugal force.
Since the Torque Converter is bolted directly to the transmission’s flexplate, which is bolted to the engine, the impeller spins at the exact same RPM as the engine spins. As the engine speeds increase, more fluid is forced out of the Impeller. The fluid passes from the Impeller to the next part of the Torque Converter called the Turbine.
The Turbine or pump (as it is also known) is positioned opposite of the Impeller and is connected directly to the transmission by the Input shaft by splines. The Turbine is what causes the transmission to spin resulting in the movement of the vehicle. Both the Impeller and the Turbine move independently of each other.
Fluid flows from the Impeller and is directed over the blades of the Turbine resulting in its rotation because the fluid moves in the opposite direction. The turbine is designed so the fluid enters from the outside and flows to its inner portion back to the Impeller. From there, the cycle repeats itself.
However, if the fluid was to flow directly back to the Impeller, it would slow the engine and lose power. To prevent this power loss, Torque Converter has another part called the Stator.
To increase the efficiency of the Torque Converter, the Stator is placed between the Impeller and the Turbine; in the center of the Torque Converter. As the fluid passes back to the Impeller from the Turbine, it passes through the Stator and reverses the flow of the fluid. Because the Stator is splined to the Pump Housing of the Transmission, it will not spin with the passing fluid.
It is designed to spin only in the opposite direction but lock up in the other. This happens because of an internal one-way or “Spray” clutch. This will force the fluid also to change direction as it flows back to the Impeller.
Without the stator, the Torque converter would be a simple fluid coupling. When the Impeller begins to spin the Turbine, it will take time to move the Turbine because of the “static inertia” or weight of the vehicle. The Impeller is trying to work against the vehicle’s weight.
When it locks up, the Stator fins stop in place and cause the fluid that is passing from the Impeller to the Turbine to change direction. As a result, the torque output of the engine is increased, doubled, and even sometimes tripled! This greatly assists the vehicle’s acceleration and overcomes the “static inertia” of the 3,000 plus pound vehicle and forces the car to move off the line quicker!
Because the fluid changes direction, it multiplies torque so when the car begins to move, the stator will “Free Wheel” and spin. Keep in mind that the Internal Clutch will only work from a Dead Stop.
The Torque Converter Clutch, if equipped, is attached to the back of the Turbine. The back of the clutch assembly has friction material that, at highway speeds, will lock up against the outside of the Torque Converter housing.
When locked, as the engine is spinning, the whole assembly will spin as one unit to increase fuel efficiency. Sometimes by the assembly spinning, it can feel like an additional “shift”. For example, if you have a 5-speed auto, it may slightly feel like the transmission shifts once more when in reality, this is the Torque Converter Clutch or Lock-up clutch engaging.
Through Fluid pressure, this causes the clutch to lock against the outside housing of the Torque Converter. So, instead of having power loss between the Impeller that spins at engine RPM and the Turbine which will be spinning at roughly 90% of the Impeller, all the components will spin as one unit resulting in no power loss!
If this Torque Converter clutch does not disengage when coming to a stop, your engine will stall since the Torque Converter is acting as one unit. So, if the car is having stalling issues when coming to a stop, it may be the TCC is not functioning properly.
The Turbine itself will spin slower than the Impeller. This is because energy losses and torque conversion losses take place in the transfer of energy.
Think of the Turbine and Impeller as two fans facing each other whereas one fan is plugged into the wall and the other fan does not have a power source. The Fan plugged into the wall will act as the Impeller receiving power from the engine and the fan that is not plugged in would be the Turbine connected to the transmission.
As the first fan receives its power and pushes air to the turbine, the turbine will also begin to spin but at a slower speed. This is where the energy or power losses come from.
With this configuration, if the transmission comes to a sudden stop, it will not affect the rotation of the engine. The Impeller and the engine can continue to spin while the Turbine, transmission, and drive wheels stop moving. Without the Torque converter, the engine would stall in the same manner as a manual transmission if you come to a stop without pressing the clutch pedal.
When the car takes off from a stop, the vanes in the Stator cause the fluid going from the Impeller to the Turbine to change direction. This results in an increase in output and, as previously mentioned, will Multiply the Torque by double or even triple. Remember, this function will only occur from a dead stop.
Stall speed occurs when the Stator locks up and causes the car to accelerate. In performance applications, you want the stall speed to match the maximum torque output of the engine. This way, when you take off from a launch, the engine is producing maximum torque so the car pulls hardest off the line for peak acceleration.
The higher the stall speed, the smaller the converters will be. Big Torque converters have relatively low stall speeds in comparison.
Many factors that determine the best stall speed include but are not limited to: Peak Torque, Torque Curve, and Vehicle Weight. If you do not know the exact specifications of your engine and vehicle, experts recommend being conservative with your estimations. If you overestimate, the stall can be lower than desired resulting in your torque converter underperforming.
There is the Impeller that is welding to the outside of the Torque Converter housing and spins with the engine. If the engine is running, the Impeller is ALWAYS spinning.
Next on the list, we have the Stator. This has the one-way internal roller clutch “aka” the spray-clutch. When taking off from a dead stop, the clutch locks up, causes the fluid to change direction, and results in the amount of torque output being multiplied from the engine.
The turbine is driven by the Impeller. There is some power loss between these two components. To overcome that power loss, some Torque Converters have the TCC that fastened to the Turbine. At highway speeds, the TCC will lock up and result in the Turbine now being able to spin at the same speed as the case and Impeller.
With modern technology, automatic transmissions are not the “slush boxes” of yesteryear. In many ways, like 2018 to current Mustangs with the 10-speed automatic transmissions, they are faster than their manual transmission counterparts! More performance automatic transmissions like the one found in the 2020 Shelby GT500 have dual-clutch technology that does not need a torque converter and is able to shift incredibly fast.
The decision is not as clear-cut anymore when choosing the gearbox for your car. Today, you can have your cake and eat it too when it comes to gearbox options!