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.