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.