Considering that a torque converter doesn't actually convert torque, it's no wonder it's such a misunderstood component. After all, any device that has "converter" as part of its name should transform-or convert-whatever is input into something different and present it for output.
In reality, the "conversion" that goes on inside a torque converter is torque multiplication. As opposed to a fluid coupling, which is a simple power-transfer device, a torque converter is, under certain operating conditions, a power-enhancing device. Although the mechanics of the power increase are a bit esoteric, the effect is similar to that of a gearset. If a small gear drives a larger one, torque is multiplied at the expense of output speed. Like a fluid coupling, a torque converter has an impeller (which is driven by the engine) and a turbine (which transfers power to the transmission). But it also has a stator between the two parts, which performs much the same function as a small gear driving a larger one. However, the stator uses fluid redirection, rather than direct contact, to increase the torque applied to the turbine, which is positioned outside the converter.
Externally, one torque converter...
Externally, one torque converter of a given diameter looks pretty much like another. However, the inside configuration will make a major difference in the way a Corvette accelerates, especially from a dead stop.
Nothing is free in the land of power transfer, and inside a torque converter, slippage is "currency" for torque multiplication. Within limits, paying more slippage dollars buys more torque multiplication. But the payback for increased spending isn't always improved performance. If slippage is excessive relative to engine capabilities and requirements, torque increases at one end of the rpm spectrum will not offset slippage losses at the other.
Consequently, the process of choosing the proper torque converter for a particular Corvette requires more than a waltz through a catalog. This is especially true with multipurpose vehicles that will make excursions down a dragstrip or around an autocross course, then be required to complete daily driving chores as well.
This point is clearly demonstrated by the dragstrip results of a recent project. When purchased, our test car had a "high-performance" street/strip torque converter of unknown origin. Although it seemed to offer acceptable performance during normal driving, it slipped excessively during wide-open-throttle "drag racing" acceleration. In fact, slippage was so great, it was difficult to determine the rpm at which the engine finally ran out of breath at the end of the track.
Small, 10-inch-diameter torque...
Small, 10-inch-diameter torque converters, such as this one from TCI, are designed for modified engines with an rpm capability of 6,000 to 6,500 rpm. This one features furnace-brazed fins and a high-performance stator.
When we asked Greg Friend and Kevin Winstead of TCI Automotive to recommend a replacement torque converter, they could make only an educated guess because we could not supply enough accurate information. After reviewing the alleged engine and driveline specifications provided by the vehicle's previous owner, Friend and Winstead recommended a 10-inch Super Street Fighter converter.
During the brief checkout drive following installation, it was apparent the Super Street Fighter was delivering considerably more torque, while slipping much less than the previously installed converter. So, with a pair of Mickey Thompson ET Street DOT race tires mounted on the rear, we headed to Silver Dollar Raceway in Reynolds, Georgia, for a full dragstrip test session.
With the Super Street Fighter in place, the car left the starting line noticeably harder than it had previously, and the best run of the day stopped the clocks with a quarter-mile elapsed time of 12.872 seconds and a trap speed of 103.73 mph. The timeslip also showed a 1.772-second 60-foot time and an 8.151-second eighth-mile e.t. at 83.63 mph. Considering the previous quarter-mile performance, the improvement was impressive. But trap speed was much lower than it should have been.