Just like the foot bone's connected to the leg bone, it's only natural to think that a car's engine is connected to its rear wheels. This seemingly reasonable assumption would be wrong—at least in cars equipped with automatic transmissions—because there is no real physical link between the flexplate and the input shaft. Instead, all automatics rely on hydraulic fluid swirling around inside the torque converter to transfer energy from the crankshaft to the tires. In other words, it's the basic principles of fluid dynamics that enable a car to move forward in the first place. The primary component controlling the flow of trans fluid is the torque converter, and consequently, it plays an astoundingly vital role in overall vehicle performance. Dialing in the converter just right can be the difference between taking home a trophy or loading up the trailer after the first round. To get a better handle on how converters work, and what's involved with setting one up for the rigors of racing, we chatted with Brian Reese of TCI Automotive. As we learned, converter setup is often a balancing act among slippage, efficiency, max performance, durability, and driveability, so knowledge is the key to getting it right.
How Torque Converters Work
At its core, a torque converter transmits torque from an engine's crankshaft to the transmission input shaft using hydraulic fluid. To conceptualize how this works, imagine placing two electric fans with their blades facing each other. If one of the fans is turned on, the resulting airflow will spin the other fan even though the two aren't physically connected. A torque converter works on a similar principle, but uses hydraulic fluid instead of air to transfer energy. The torque converter mounting cover bolts to the flexplate, and the impeller pump is welded to the cover to create a single sealed housing. Inside the converter, there are three sets of fins: the impeller, turbine, and stator. As engine rpm increases, the centrifugal force created by the spinning impeller results in increased fluid velocity. This transfers energy to the turbine, which is connected to the transmission input shaft. Since the impeller is moving faster than the turbine, the resulting flow strikes the turbine veins, which transmits torque from the engine to the transmission input shaft. A stator positioned between the impeller and the turbine determines stall speed and torque multiplication characteristics by redirecting fluid from the turbine back to the pump impeller.
The design of the torque converter cover, impeller, stator, and turbine have a profound affect on its performance characteristics. The impeller and turbine can have varying fin angles as well as a varying number of fins. The combination of fin angle and number of fins is what dictates the performance of a given torque converter while in coupling mode. Furthermore, the stator influences converter behavior from peak stall up to the point of coupling. Different stator designs allow for more or less torque multiplication. The angle of the inlet and exit of the blades relative to the pump and turbine blades can influence the torque multiplication behavior in addition to determining when the torque converter starts to couple. Likewise, in addition to serving as a housing for the converter's internals, the front cover reduces deflection and ballooning as well.
Slippage and Multiplication
A torque converter's stall speed is one of the most important elements of a car's off-the-line acceleration. In essence, stall speed is the engine rpm at which the torque converter begins to transmit sufficient torque to where any additional rpm will move the turbine, and thereby accelerate a car forward. By raising the stall speed of a converter, the converter will allow the engine to operate at a higher rpm before moving the car. Consequently, simply increasing the stall speed can dramatically improve acceleration. Another vital characteristic of a torque converter is its multiplication ratio. Simply put, this is the torque ratio between the engine and the transmission input shaft. As the torque converter stator redirects fluid from the turbine exit toward the impeller's inlet veins, it pushes on the impeller and transfers torque back to the engine side of the system, which multiplies torque. It's important to note that torque multiplication is a trade-off between rpm and increased torque. Any gains in torque are offset by an increase in slippage. This is why maximum torque multiplication occurs at peak stall, and why accurately gauging efficiency requires taking into a