Torque Converter Stall Speeds Basics

After reading this post, you should have a basic idea of what stall speeds are and how to select the right stall speed for your vehicle. Stall speed and stall speed selection are terms that are frequently misused and misunderstood when shopping for a torque converter. Most of the time they are confused with other elements of a torque converter or not measured properly. Even seasoned car enthusiasts are guilty.

Stall speed: it is a number that is attached to every torque converter and is one of the key elements in making a vehicle perform at its highest level. There are many factors that go into choosing the correct stall speed for your car, and each must be examined for the torque converter to do its job properly.

Besides the engine between the fenders, the stall speed of your torque converter can have the greatest impact on your car’s performance. Just selecting a random converter based on a few calculations from a website will not allow your car to perform its best and could actually hurt your ability to get the best ET’s possible when at the track; it’s important to ensure you have the right configuration for your application. When you have a converter that is sized correctly and has the right stall speed for your combination, the results will speak for themselves on your time slip.

The above chart is for Dodge and Plymouth cars

What is Stall Speed?

Stall speed is the speed at which the converter holds back or limits the engine speed if the transmission output is prohibited. When your torque converter prevents the power transfer from your engine to your transmission, it increases the engine’s RPM stalls. For example, when you press down on your gas pedal, the stall speed is the gap between where your vehicle ideals and begins to move. The more stall speed you have, the more pressure you must put on the gas pedal before your car begins to move.

Basics Of Stall Speeds

A torque converter’s job is to take the horsepower an engine produces and multiply that to make the car accelerate. The torque converter is a hydraulic coupling which takes the engine’s mechanical force and converts it into hydraulic pressure. Its impeller turns at engine speed; the more torque the engine makes, the higher in the RPM range the converter will “stall” or lock up. The stall speed is the level where the engine’s force cannot overcome the hydraulic pressure and varies for a number of reasons. The action of the converter’s stall is what helps the car accelerate at a peak level once the driver applies the power to the driveline.

The main factors that go into calculating the stall speed for any car include the weight of the car, the circumference of the rear tires, the gear ratio, and of course, the engine’s performance level. With the engine, even the camshaft must be taken into consideration to make sure the stall speed is matched up when the engine comes alive in the power-band. Making the correct choice in stall speed is important because it allows the car to launch harder and have improved 60-foot times — both critical factors in laying down fast and consistent elapsed times.

Each part of the torque converter plays a significant role in making sure the stall speed is correct.

In a previous article, we covered the basics of what makes up a torque converter and how it functions within the driveline of your car. The hard parts of the torque converter that really affect the stall speed are the overall size of the converter, the pump blade angle, and the stator. By making changes to each of these parts, a torque converter manufacturer can dial-in where the converter will begin to stall the engine.

Torque converters come in a range of sizes, and that size will impact how the converter will behave with an engine. Depending on the engine combination you are using, choosing a converter that is sized to what its output capability is will vary.

The overall size of the torque converter has to do with its efficiency and holding power. In general, the larger the converter, typically the tighter and more efficient it will be.

The pump blade angles inside the torque converter can be set in what we call a negative, neutral, or positive angle– the negative angle being more on the loose side of things, and a positive angle will make the converter tighter.

The biggest part of the torque converter that will change the stall speed is the stator. A torque converter’s stator is what sends the fluid to the turbine, which in turn drives the input shaft and moves the car.

Why is Torque Converter Stall Speed Important?

High performance torque converters can do a lot for your vehicle when the proper stall speed is selected. For racing, the proper stall speed will increase your launch and get you off the line quicker. In towing, the proper stall speed will help your engine pass more power to your transmission, causing your tow capacity to increase.

How To Choose the Right Stall Speed

When you are selecting the right stall speed for your vehicle, be sure to match the engine’s peak torque, torque curve and vehicle weight.

Peak Torque – The maximum torque your engine can exert at a certain RPM.
Torque Curve – The place where the engine has the maximum amount of power at a certain RPM.
Vehicle Weight – The weight of your vehicle has a significant impact on the measurement, and do not forget to factor in aftermarket modifications.

If you do not know these exact specifications, we recommend giving a conservative estimate. If you overestimate, your stall speed will be lower than intended, which will reduce your torque converter’s performance.

In general, the desired stall speed should be 500-700 RPM below the engine RPM at peak torque. This ensures a margin for application for the torque converter to take off. You do not want to have to floor your gas pedal and wait a few seconds for your vehicle to move. Think of it as a balancing act.

Finally

All advertised stall speed ratings of all torque converters are nominal ratings regardless of the manufacturer or converter design. These advertised ratings are not indicative of the actual stall characteristics of a torque converter. Advertised stall speed ratings are published simply as a general guideline to help in the selection of the correct torque converter for a customer’s application. A torque converter will provide a range of stall speeds based on the amount of load that is placed on the torque converter. Many factors influence torque converter stall speed including output torque of the engine, vehicle weight, rear axle ratio, rear tire height, torque converter diameter, impeller fin angle design, stator design, impeller-to-turbine clearance, stator-to-impeller clearance, stator-to-turbine clearance, and brake bias.

When considering torque output of the engine, you must not only consider peak torque but average torque and must consider all components and design characteristics of an engine that will influence torque production. Several key factors to consider in engine design that will have a dramatic impact on torque production and the overall shape of the torque curve include but are not limited to, stroke length, static and dynamic compression ratios, camshaft specifications, cylinder head intake runner volume, intake manifold design, header primary tube size and length, etc. Something as seemingly simple as an incorrectly tuned idle and/or part throttle circuit in a carburetor or incorrectly tuned ignition timing can have a dramatic negative impact on the stall speed characteristics of a torque converter.

When shopping for a torque converter and determining which stall speed is best suited for you, we suggest you ask an expert at one of the manufacturers.