For example, let's say your 355ci small-block makes 230 lb-ft of torque at 2,500 rpm and you bolt in a B&M Holeshot 2000 converter that creates a 2,000-stall speed (this is actually the torque standard that B&M uses to establish stall speed for its street converters). Now let's bolt the same Holeshot 2000 converter behind a larger, 400ci small-block that makes 400 lb-ft of torque at 2,400 rpm. The same converter is now going to exhibit a much higher stall speed, perhaps 2,300 to 2,400 rpm, because the engine is cranking out 170 more lb-ft of torque. So as you can see, stall speed is not an exact science due to multiple factors that affect the converter. Furthermore, if you were to add a transbrake that locks the trans in two gears at once, stall speed will generally increase another 100 to as much as 300 rpm depending upon the application.
The reason for increasing stall speed is directly related to the engine's torque curve. With a manual transmission, you are allowed the freedom to pick the starting-line rpm you want to apply when you dump the clutch. In a car with a good suspension and sticky tires, you can use extra rpm to put the engine closer to its torque peak, launching the car even more quickly. Adding stall speed to the torque converter increases the power available to accelerate the car more quickly.
Unfortunately, additional stall speed also increases the slippage in the converter. For a drag-race car, the cost of this additional slippage is worth the price, but for a street car it creates other problems. A high-stall converter like a 4,000-rpm unit is not only lazy from a part-throttle-efficiency standpoint, but it also generates much more heat. Heat is the enemy of any automatic transmission and will radically reduce its life span if not mediated by an efficient trans cooler.
The issues of slippage and heat are the main reasons that street converters should be approached with a serious amount of compromise when it comes to stall speed. A converter that might have a stall speed of 5,000 rpm for a pure drag car would be completely miserable on the street. Because of the lack of traction and limited tires, it's not critical that a street engine launch at peak-torque rpm. For most hot street engines, converters in the 2,200 to 2,600-rpm range are considered streetable but will still require a very efficient trans cooler to keep them out of the heat zone.
Size And GirthAnother important part of the torque converter and stall-speed equation is converter diameter. Larger-diameter converters are usually more efficient and deliver less slippage, while smaller-diameter converters offer more stall speed and more slippage.
Vehicle weight is another major factor that should be considered when selecting a converter. Big cars weighing over 3,300 pounds generally need a converter with more torque multiplication. A flyweight 2,800-pound car like an early Mustang or Chevy II could use less multiplication to successfully launch on their relatively smaller rear tires.
Lockup ConvertersWith the advent of overdrive automatic transmissions like the TH200-4R, the TH700-R4/4L60E, the massive 4L80E, the Ford AOD, and all the rest, the new-car companies were striving for improved fuel economy using overdrive. Overdrive ratios drive the torque converter at speeds generally below normal coupling speed, creating increased slippage and heat, which is unacceptable. This created the demand for a lockup converter employing an internal clutch inside the converter to directly connect the turbine with the input shaft of the transmission just like a manual-transmission clutch.
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