Difference Between Stock & Stall Torque Converters
The speed at which a torque converter for an automatic transmission couples the engine to the transmission is referred to as "stall speed." This can be known as low stall speed (stock) or high stall speed. Torque converter stall speeds can vary but are designed to fit the intended use of the vehicle.
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Automatic Transmission Design
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Unlike a manual transmission that uses gears, automatic transmissions rely on internal friction plates (small packs of clutches) to transmit rotational force from the front of the transmission (input shaft) to the rear of the transmission (output shaft). These clutches are engaged by high fluid (oil) pressure.
Clutch Elimination
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Manual transmissions require a clutch pedal to engage and disengage a transmission to start and stop motion and for gear shifts. A torque converter automatically uncouples (disengages) when engine speeds are lower so that the engine remains running while stopped at idle. This is known as converter stall.
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Torque Converter Design
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At lower speeds--depending on converter design--transmission fluid movement and pressure is low. This prevents the internal components of an converter from fully engaging and driving the input shaft and transmitting power through the transmission to the drive wheels.
Low Stall Speed Converters
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Stock--or factory-type--torque converters begin to couple and transmit power to the transmission at relatively low engine speeds. This permits them to allow the vehicle to be stationary at idle until enough engine speed is generated to drive the input shaft of the transmission.
High Stall Speed Converters
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In specialty performance applications, the internal components of the converter are changed and/or modified to allow greater torque multiplication. The engine must attain a higher rpm before the converter begins to couple to the transmission and create motion in the car.
Low and High Stall Speed Examples
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Most stock converters will typically begin applying the torque from the engine at or below 2200 rpms. Many performance converters will allow more stall until a higher engine rpm is attained, with some competition racing converters applying complete power only above 6000 or 7000 rpms.
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References
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