What Does Stall Speed of a Torque Converter Mean?

Torque converters allow a car to sit still while the engine is running.

Torque converters allow an engine to idle without engaging the transmission. The revolutions per minute (RPM) at which a converter begins to transfer power from engine to transmission is called the stall speed.

Converter Function

The converter uses an engine-mounted turbine (fan) to push fluid through another turbine connected to the transmission input shaft.

Engagement

When the engine-mounted turbine spins fast enough to overcome the transmission-turbine's ability to pass fluid through its tubes, the transmission-side turbine is forced to spin with the engine turbine. The engine RPM at which this power transfer happens is the stall speed.

Engine torque

Most engines only produce enough torque to move the car above about 800 RPM. The torque converter turbines are designed so that the engine is not able to drive the transmission input shaft below this point; otherwise, stalling would occur.

Gearing

A torque converter with a low stall speed can be compensated for by installing numerically higher rear-end gears. These gears give an engine greater mechanical advantage and require less torque to move the vehicle.

Stall testing

The stall speed of a converter can be determined by applying throttle while the vehicle is in gear and the brake pedal is fully depressed. The RPM at which the engine stalls or overcomes the brakes is the stall speed.

References

• Rod and Custom magazine: Understanding Torque Converters
• Chevyhighperformance.com: Torque Converter Basics
• Car Craft magazine: What's Up With Stall

Resources

• Video: How a torque converter works