Understanding Stall Speed

By Aaron Gold

With the introduction of the Banks Billet Torque Converter, you'll hear a lot of talk about stall speed. What exactly is stall speed, and how does it affect your vehicle?

This article assumes you have a basic knowledge of how torque converters work. If you'd like more in-depth detail, see "Understanding Torque Converters" elsewhere on this site.

Let's start by illustrating how the stall speed works. Even under light loads, a vehicle with an automatic transmission will start moving as soon as you take your foot off the brake. The stall speed comes into play under all load conditions. When we talk about stall speed, we're referring to engine RPM. If the vehicle isn't moving by the time the impeller reaches the stall speed, either it will start to move, or the engine RPM will no longer increase. In other words, stall speed is the engine RPM at which the torque converter transfers the power of the engine to the transmission.

In the real world, the torque converter's stall speed roughly equates to the clutch engagement point on a manual transmission. Let's say you're driving your stick-shift car around town. Normally, you'd give the car a little gas and ease off the clutch pedal gently enough to get a smooth start. Likewise, under most driving conditions the torque converter will start delivering power to the transmission at relatively low engine RPM.

Now, let's say you need lots of power, either to make a fast getaway or to start with a heavy load. You'd rev the engine up to a point where it delivers more power before letting up on the clutch pedal. It's under those same circumstances that the stall speed becomes important. The torque converter will allow the engine to build RPM without turning the output shaft (the turbine) until the stall speed is reached.

Unlike gas engines, diesels develop massive amounts of torque (pulling power) at low RPM – enough that it's possible (and, in fact, desirable) to get a heavily-loaded diesel truck rolling by simply easing off the clutch without touching the accelerator. Banks' dynamometer tests show that Ford's 7.3 liter Power Stroke hits peak torque at 1,600 RPM and begins to drop off at around 1,850 RPM, while the Dodge's 5.9L Cummins engine peaks at 1,400 RPM and drops off at 1,800 RPM. As RPM continues to rise, torque decreases even further.

How would you translate this to a torque converter? With a low stall speed. But both the Ford and Dodge torque converters stall between 2,000 and 2,500 RPM – so with a heavy load, the torque converter won’t start turning the rear wheels until well beyond the engine's torque peak. In this case, the stall speed is too high - it is literally impossible to get the engine's full power to the rear wheels! In order to access all of the engine's potential power, the stall speed must be lowered.

Lowering the stall speed has another advantage: It reduces the transmission's temperature. Let's go inside a high-stall torque converter under heavy load. The impeller (input side) of the torque converter is spinning quickly, while the turbine (output side) is spinning slowly or not at all. The motion energy of the impeller is being converted into heat energy, most of which is passed on to the transmission fluid. The higher the stall speed, the more heat will be generated. Heat is the enemy of a transmission. You want to keep the fluid temperature as low as possible. With a lower stall speed, less time elapses before the motion energy of the impeller is converted to motion energy to drive the turbine, so the transmission runs cooler and lives longer.

What many people don't know is that the torque converter is a tunable device. Stall speed is determined by several factors, including the distance between the impeller and the turbine and the design of the stator. By properly modifying the converter's internal components, it's possible to alter the stall speed and create a torque converter that is tuned for a particular engine.

Enter the Banks Billet Torque Converter. Our new torque converter is designed to stall at a speed to match Banks Power systems – precisely the point where the applicable truck or SUV diesel engine puts out maximum pulling power. And because the Banks Billet Torque Converters "hook up" sooner, they run cooler, even under high loads. You don't need a dynamometer to see the results – with a Banks Billet Torque Converter, the truck will accelerate faster. Fuel economy goes up, transmission temperature goes down, and your transmission will live longer. It's a match made in heaven! (Or is it Azusa?)