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
Granted, a high-stall converter can knock several tenths off of your quarter-mile times, but they also produce lots of heat in the process. As a result of the trans fluid getting sheared by the turbine blades during torque converter operation, heat is created. Furthermore, any energy that is not used to move the car forward is converted into heat. This heat can be detrimental to the transmission fluid, and as the transmission fluid breaks down, all the transmission's components are exposed to broken down fluid. Serious damage can occur when the transmission fluid has reached a point where it no longer provides sufficient lubrication for its moving parts. This is why it's imperative to run a transmission cooler in all performance applications. OE trans coolers that are built into the factory radiator can only reduce the trans fluid temperature down to the same level as the coolant temperature. Running a separate, dedicated cooler for the trans fluid will result in fluid temperatures that are 20-30 degrees cooler.
Another downside of a loose torque converter is that their increased slippage adversely affects fuel mileage. As such, most late-model vehicles feature lockup-style torque converters. A lockup torque converter has all the torque-multiplying benefits of a non-lockup converter, but has the added benefit of clutch lockup once the torque converter reaches coupling speed. Once a car reaches cruising speed, a clutch assembly locks the turbine assembly to the front cover, thereby eliminating converter slippage. Locking up the converter at coupling speed offers unparalleled efficiency over a non-lockup converter. With late-model vehicles, the car's computer determines when the lockup circuit is activated by looking at a number of parameters, including throttle position, vehicle speed, and engine load. The good news is that lockup functionality can be retained when swapping a late-model trans like a 4L80E or 6L80E into an older muscle car. The aftermarket has developed stand-alone transmission controllers, such as TCI Automotive's EZ-TCU, to allow hot rodders to utilize this same technology without the need for a complex computer-controlled powertrain.
Stock Converter Shortcomings
A factory torque converter is designed with driveability and fuel mileage as top priorities. As engine performance is increased, their shortcomings become more detrimental. Perhaps the biggest drawback of a stock converter is that it's designed to couple at low rpm. This is very undesirable in high-performance application since they generally produce power higher in the rpm band. Higher stall speeds allow a performance torque converter to launch a car at an engine rpm where much more torque is present, enabling more power to be put to the ground. In aggressive applications, TCI Automotive has found that roller sprags are particularly prone to failure due to the inherent radial loading characteristics of their design. For this reason, TCI Automotive has created a mechanical diode, which allows for a true, one-way engagement clutch that can handle the most punishing applications.
N/A vs. Power Adder Converters
Since a converter must be optimized to match the power curve of the engine it's bolted behind, naturally aspirated, forced induction, and nitrous combos each require different converter setups. Naturally aspirated converters tend to be set up on the tight side and have slightly more aggressive stators to account for the low-rpm torque that they produce. Converters for turbo applications are set up more loosely, allowing them to sufficiently spool the turbo and create some boost prior to launch. Converters in supercharged and nitrous motors are set up similarly to naturally aspirated combos, but with slightly more slip due to their increased torque output.
Popping the transbrake button and pulling the front wheels is great fun for the driver, but not so much for the torque converter. Transbrakes put particularly high stress on the converter turbine. That's because when the transbrake is holding the car stationary during throttle input, the turbine is still getting pushed by all the fluid flow from the impeller pump, creating tremendous stress. To combat this stress, furnace brazing and TIG reinforcing the turbine's fins greatly enhances strength. The furnace brazing process involves loading the turbine and the pump impeller with powder brazing material, and then running the parts through an oven. This melts the powder as it conforms along the entire length of the fins. Since the fins have complex contours, the furnace brazing process is very effective. Once the brazing cools, it bonds to the fins and creates one solid and uniform component. The final product is substantially stronger than a converter that isn't furnace brazed.
Slippage vs. Efficiency
Loose converters get you out of the gate in a hurry, but a converter that's too loose will suck up horsepower and sacrifice trap speed at the end of the track. Dialing in a converter just right is a trade-off between slippage and efficiency, and the more information you have the better. Since it isn't practical or effective to stare at the tach during the entire duration of a quarter-mile pass, serious racers are increasingly relying on data acquisition systems to help dial their converters in to perfection. By monitoring both engine and driveshaft rpm, data acquisition systems allow racers to verify that the converter is efficiently transmitting power from the engine to the transmission. After logging a run, racers can clearly see how much the torque converter is slipping off the line as well as at the far end of the track. With a tight converter, the difference between engine and driveshaft rpm will be minimal while with a loose converter, the difference between engine and driveshaft rpm will be much greater. If a converter is working well off the line, but is creating too much slippage later on in the run, this information can be relayed back to the converter manufacturer so they can modify the converter accordingly.
The growing popularity of turbo cars has brought their unique converter needs to the forefront. They need lots of slippage early in a run to help spool the turbos, but need the converter to tighten up at the far end of the track to prevent excessive power loss. To accomplish this, converter manufacturers manipulate the impeller fin configuration, core selection, and stator design. These tricks result in a converter that yields both high slip just off the line, and couples tightly as a car makes its way down the track. TCI Automotive uses larger-diameter cores to obtain desirable fluid velocity in addition to employing specific clearances to yield an effective converter setup for turbo combinations.
Tune it Yourself
Most torque converters must be cut in half to access their internals. This can be a major hassle for track cars that may need their converters opened up several times before the stall characteristics are perfectly dialed in. To address this issue, TCI Automotive's Pro-X billet aluminum torque converters feature bolt-together covers for easy servicing. The ability to simply unbolt and disassemble a torque converter has revolutionized the drag race scene. Now a racer can simply open up his torque converter and experiment with different stator designs or even replace the impeller pump while at the track. This ability to easily manipulate the behavior of the torque converter while at the track is truly invaluable for racers, allowing them to adjust their converters to suit specific conditions they face. In Outlaw 10.5 classes, a 200- to 400-rpm change in stall speed can be the difference between making a great pass and blowing off the tires. Changing the stall speed is a simple process that involves disassembling and shimming the stator closer to the turbine or closer to the pump. That means you can fine-tune your setup without the downtime involved with sending your converter back to TCI Automotive for adjustment. Furthermore, Pro-X billet converters can handle up to 3,000 hp.
The diameter of a torque converter plays an important role in its performance characteristics. The torque converter diameter establishes the moment arm, or the distance of the outer fin openings from the center of rotation. With a smaller torque converter, the fluid applies less torque to the turbine because the openings are closer to the center of rotation, thus allowing the engine to operate at a higher rpm for a given load. When the openings are moved outward—as in the case of a stock converter—the moment arm is longer, which applies more torque to the turbine for the same amount of force. This results in a lower rpm for an engine operating at a given load. Also, as the diameter of a torque converter increases, greater fluid velocities are achieved as a result of a longer fluid path and the natural effects of centrifugal force. All of these conditions explain why stock converters are typically larger in diameter and stall at lower rpm, and why racing converters tend to be smaller and stall at a higher rpm.
The density of the transmission fluid plays a key role in determining how much force is transmitted when the fluid strikes against a surface of the converter fins. Fluids with different densities will alter the power transmission capacity and efficiency of a torque converter. At the same time, a balance must be maintained in density and heat dissipating qualities of a fluid. One vital role of the fluid is to continually dissipate heat created by the torque converter and a certain dissipating capacity is required. Likewise, the fluid must also maintain lubricity without adversely affecting transmission clutch performance. TCI Automotive recommends changing the trans fluid every 40 to 50 passes in a typical drag application. More extreme applications may require an increased frequency in fluid changes. Monitoring the appearance of your fluid between runs is a good way to gauge its health as well. TCI Automotive offers a full line of Max Shift synthetic and conventional fluids, as well as break-in fluids.
Ever since its introduction, the GM 6L80 transmission has given converter companies and enthusiasts a less-than-desirable experience. The technology used in this transmission can prove challenging and elude even the best of aftermarket tuning experts. The big news for TCI Automotive this year is the introduction of a new torque converter for the GM 6L80 transmission found in '10-and-up Camaros. TCI Automotive's new converter is equipped with several industry-exclusive features, including a high-stall, triple-disc, bolt-together lockup capabilities that do not require any tuning at all. It's a drop-in replacement for the stock converter and has all the road manners of the stock converter. Unlike the stock converter, however, it offers a much higher stall speed and significantly increased torque capacity. Furthermore, it can be completely disassembled, serviced, and even reconfigured to a new stall speed all by simply unbolting it.