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.
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.
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.
For good reason. On paper, the engine should have pulled strongly to 6,000 rpm; on the track, it nosed over at about 4,800 rpm. That being the case, we had "too much" converter-the stall speed (and amount of slippage) was too high for the engine's torque output and rpm capability. The 10-inch Super Street Fighter was still slipping excessively after the transmission was shifted into Second and Third gears, so the vehicle acceleration rate was reduced.
After reviewing our test results, Friend and Winstead thought a Breakaway converter (11-inch diameter) would be a much better choice, given the engine's true rpm range. Friend noted, "This is a perfect example of why accurate specifications are so important. On paper, the engine should have pulled to 6,000 or 6,500 rpm, and that was way off the mark. Considering vehicle weight and the relatively low rpm at which the transmission had to be shifted, the Super Street Fighter did a reasonably good job, but it's definitely not the right converter for this particular engine/chassis combination. The Breakaway is considerably tighter than the Super Street Fighter, so I'd expect all-around performance to be better, even though 60-foot times may not be quite as good."
Upon our return to Silver Dollar Raceway, Friend's predictions proved to be right on the money. Trap speed jumped almost 3 mph-from 103.73 to 106.68, while elapsed time dropped just over a tenth of a second to 12.752. This improvement was achieved in spite of the fact that 60-foot times remained virtually unchanged.
In addition to illustrating the performance shortfall that can result from a torque converter that isn't properly matched to a particular engine, the test results indicate a different type of torque converter is required for various Corvette intake systems. With its lower stall speed and reduced slippage, a Breakaway would be the better choice for a Tuned Port or a Cross-Fire engine. On the other hand, a Street Fighter or Super Street Fighter is better suited to LT1 and LS1 engines. In older Corvettes with carbureted engines, a camshaft profile can be used to ease the torque-converter selection process. Stock-type engines with standard hydraulic-lifter cams will obviously respond better to a "tighter" converter, while high-performance hydraulic- and mechanical-lifter profiles indicate a "looser" converter is required.
Obviously, making a selection is never as cut-and-dried as looking at a cam spec card or engine type. Vehicle weight and gearing also enter the picture and, irrespective of the actual specifications, an engine's performance capabilities are still the governing factor-especially if you're aiming for optimum performance in a car you race on Sunday and drive on Monday.
Stall Speed: Loose Or Tight?Stall speed is about as misunderstood as torque converters. It is simply the maximum rpm an engine can reach when the throttle is wide open but the vehicle is held at a standstill. Stall speed is relevant to vehicle acceleration for a number of reasons, but primarily because it allows an engine to reach a particular rpm before attempting to move a vehicle.
As stall speed is increased, so is a converter's overall slippage; all other things being equal, a "loose" converter will typically have a higher stall speed, more slippage, and more advantageous torque multiplication than a "tight" one. Unfortunately, all other things aren't always equal, so there's no guarantee that a "loose" converter will offer a performance advantage. Slippage can be the result of nothing more than poor design and, even worse, quality control.
The specifications for most high-performance torque converters relate dragstrip performance potential, which doesn't help much when selecting a converter for autocross use. Although many of the same considerations apply, autocrossing calls for a somewhat lower stall speed than drag racing. A Corvette set up for autocrossing won't have a drag racing-oriented Corvette's launch capability (due to tires and suspension) so it's usually best to keep stall speed well below an engine's torque peak. Autocrossing also involves acceleration and deceleration, and a tighter converter offers quicker response during throttle transitions.
By The Numbers
If you're not familiar with dragstrip elapsed times and trap speeds, you'll understandably view improvements of a few tenths of a second and 2-3 mph as insignificant. But if you've ever tried to improve your Corvette's dragstrip performance, you know it takes a good bit of effort to cut e.t. by a tenth and increase trap speed by 1 mph. An improvement of 0.30 second and 6 mph is huge; and, as the accompanying chart demonstrates, that's just what the right converter can deliver.