Torque Converter FAQs: What is Stall Speed? How Does a TC Operate? (And Other Common Questions)

Know what a torque converter does?

Of course you do. It converts torque, right? It’s a converter of torque.

It’s OK to admit you don’t fully understand what a torque converter does or how it works. You’re not alone, because the tech experts at Summit Racing have told us that torque converter-related questions are some of the most oft-asked.

We consulted the Summit Racing folks to compile a list of the most frequently asked questions regarding the mysterious torque converter.

What does a torque converter do?

Automotive torque converters connect an engine to an automatic transmission and multiply engine torque for increased low-speed acceleration. The converter housing bolts to the flexplate and spins at the same speed as the engine. On the output side, the torque converter’s turbine is attached to the transmission’s input shaft. Inside, the converter has a stator assembly and fluid. The stator redirects fluid flow to create the torque multiplication. A vehicle’s engine must be able to connect and disconnect from the differential, so it can stop moving while the engine is running and the transmission is in gear. In the case of an automatic transmission, it is the torque converter that performs this connect/disconnect function. By slipping internally, the torque converter allows the car to idle while it’s in gear.

How does a torque converter work?

A torque converter is a fluid pump of sorts. Inside the donut-shaped housing is an impeller, which is driven by the engine. The impeller’s vanes create centrifugal force by pumping transmission fluid toward the outside of housing. The other key component inside a torque converter is the stator. It’s located between the impeller (on the input side) and the turbine (on the output side). The stator incorporates a one-way clutch, which has the ability to redirect the fluid flow inside the converter. By redirecting fluid flow, the converter multiplies the torque produced by the engine enough to propel a vehicle down the road. Most converters multiply torque by a ratio of at least 2:1, improving vehicle acceleration substantially.

What does it mean when a torque converter locks up?

Lock-up torque converters contain a torque converter clutch. When the clutch engages, it allows the converter to “lock” the engine to the transmission input shaft, providing a direct 1:1 engagement between the engine and transmission. Why is this desirable? Fuel economy. Automatic transmissions are not as fuel efficient as manual transmissions. Overdrive has been one step along the path toward increased fuel economy from an automatic because it allows the engine to spin at a lower rpm during cruising speeds. However, when the engine spins more slowly it causes increased slippage inside the torque converter, which in turn creates more heat. Heat within the converter reduces fuel economy and can harm both the converter and the transmission. The solution is to allow the converter to lock up at a 1:1 ratio. Lockup eliminates the slippage, which reduces heat and improves fuel economy.

How do I choose the correct torque converter?

If your vehicle has been modified to enhance performance, you probably need a torque converter with a higher stall speed. If the stall speed isn’t high enough, the car won’t be making power when you launch and you’ll bog off the line. If your setup is really mismatched, the engine may want to stall whenever you attempt to shift into gear at idle. A higher-stall converter will improve acceleration by allowing the car to launch at an rpm where the engine is making the most power. To decide which converter is right for you, you’ll have to look at the car’s weight, braking system, engine displacement, and engine combination. Some converters are designed for high-revving lightweight cars. Some are designed for heavy cars that make a lot of bottom-end torque. The key is to match the converter to the engine and the entire vehicle.

What is torque converter stall speed?

According to Hughes Performance, torque converter stall speed is the maximum amount of engine rpm that can be achieved in an automatic transmission-equipped vehicle while the transmission is in a forward operating range without generating any driveshaft motion. Two common ways to measure stall speed are foot-brake stall and flash stall.

What is the difference between foot-brake stall and flash stall?

Foot-brake stall is the maximum amount of engine rpm that can be achieved in a forward operating range with the brakes fully applied to prevent the vehicle from moving forward. Foot brake stall is not accurate because it is dependent upon too many variables such as the type of braking system (disc or drum), how well adjusted the brake system is, ring and pinion ratios, engine idle characteristics, etc.

Flash stall is much more accurate. Flash stall can be determined a couple of different ways:

1.) With the vehicle idling in low gear, apply full throttle. As the vehicle begins moving forward, watch the rpm indicator on the tachometer. This is the flash stall. NOTE: The engine must be responsive from idle. If not, camshaft timing and/or carburetor adjustments may be required for the engine to run crisp from idle.

2.) With the vehicle in forward motion in high gear and at the lowest mph where it will not kick back into a lower gear, apply full throttle while watching the tachometer. This measurement of flash stall is best achieved with a manual transmission.

Why is the torque converter stall speed in stock vehicles so low compared to the 2,000-plus rpm stall speed converters recommended for performance?

Because performance engines are modified to produce additional horsepower and torque. Higher-stall speed converters allow the engine to enter the bottom range of the new power curve and provide the optimum launch rpm of the vehicle.

Is a performance flexplate required when installing an aftermarket torque converter?

Usually a performance flexplate is not required for a vehicle that is used exclusively on the street. However, if the vehicle is to be raced, the NHRA, IHRA, and other sanctioning bodies require the use of an SFI-approved flexplate. Note: The bolt circle of an aftermarket torque converter may necessitate the use of a performance flexplate. This scenario holds true for many domestic applications including the Chrysler, Dodge and Plymouth families.

What do I need to know about torque converters before ordering one?

A torque converter reacts to engine torque—the more torque you feed a converter the better it will perform. For a converter to operate properly, the engine needs to make as much low and midrange torque as possible in the same rpm range the converter is rated for. For street engines, this means limiting camshafts to 230-degree duration at .050-inch lift, advancing the cam no more than two to four degrees, and using a small cfm carburetor when possible. If the cam is rated at less than 216-degree duration at .050-inch lift, you don’t need a performance torque converter. Nothing about torque converters is more misunderstood than stall speed.

Most manufacturers list stall speeds in ranges, but those numbers are very general. That’s because true, exact stall speed is difficult to measure due to many different variables. These variables include:

• Low vehicle weight
• Engine displacement
• Low compression ratio
• Intake manifold type
• Carburetor throttle bore diameter
• Carburetor secondary linkages
• Multiple carburetion
• Long duration camshafts
• Retarded cam timing

As always, we recommend you consult the torque converter manufacturer or talk with your friendly Summit Racing sales rep before ordering your torque converter. They’ll ask you a series of questions about your engine and vehicle to help you zero in on the right converter.