What Are the Advantages of a Stall Converter?

A stall converter's turbines and impellers look similar to this water pump's, but much larger and straighter.

The great "clutch vs. torque (Stall) converter" debate has been going on at least as long and as colorfully as any concerning a carbonated beverage. Torque converters have a number of advantages over both old-style clutch couplers and modern dual-clutch automatics. With all of the advancements made in torque converter design (especially TC lock-up), it's small wonder that torque converter-equipped cars comprise more than 80 percent of vehicles on the road today.

TC Function

A torque converter is a fluid coupling that allows an engine to idle without engaging the transmission. These devices use an engine-driven turbine to push fluid through an impeller attached to the transmission input shaft. When the engine reaches a certain RPM (stall speed), its turbine overwhelms the transmission impeller's ability to pass fluid without moving. Once this critical pressure is reached, the two are essentially locked together.

Convenience

The most obvious benefit of a torque converter is that it does not require any input from the driver to operate. As anyone who's driven a stick-shift car can attest, it can sometimes be a bit difficult to coordinate hand and foot movement while trying to steer, talk and eat a bacon cheeseburger. The torque converter is the one component that makes an automatic transmission truly "automatic."

Torque Multiplication

In those few moments between full engagement and idle, the engine-side turbine of a torque converter is spinning somewhat faster than the transmission impeller. This creates a sort of "gearing" effect and greatly multiplies torque transmitted to the transmission on take-off. The effect is not dissimilar to that of a continuously variable transmission and is a big part of the reason that most professional drag racers use automatics. Rest assured that the next time your buddy brags about his VW dual-clutch sequential, he's still not getting any torque multiplication.

Idle Stabilization

As a manual's clutch can only transmit engine torque directly, it can sometimes allow the engine to fall below the range in which it can operate (bogging). Because of the torque multiplying effect, you'll probably never see a torque converter-equipped car "bogging" or stalling from a stop. This is also due in part to the torque converter's high weight (40-plus lbs. in some applications), which stabilizes the idle even more by causing the TC to act as a flywheel.

Fuel Economy

Though fuel economy used to be the stall converter's Achille's heel, it is now practically a non-issue. With the advent of lock-up stall converters, which use a series of clutches to positively lock the engine and transmission together under cruise, the manual's advantage here is all but lost.

References

• Rod and Custom magazine: Understanding Torque Converters
• Car Craft magazine: What's Up With Stall?
• Chevy High Performance magazine: TCI transmission and Torque Converter Technology Insights.

Resources

• Video: Testing Converter Stall Speed