How does a stall torque converter work

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?)

We will try to keep this short because high stall torque converters is a pretty involved subject. Stalling is when the torque converter locks up during a load. For instance, if you have a car that has a 2,500 RPM stall converter, and then you hold both gas and brake at the same time, like you were going to power brake, then general RPM that the tires will break loose at is going to be 2,500 RPM. If the converter has a stall of 3,500 RPM, then it would break loose at 3,500 RPM. You should keep in mind that if the same type of converter is in a big block that is in a car that is quite heavy or has serious traction, then the torque will drive the stall speed to a higher RPM, while an engine that has less power will do the opposite and it won’t stall the converter out as high. It is all due to torque, traction, rear gearing, and weight. It isn’t an exact science as far as finding the exact RPM stall.

Stock Torque Converters

Most stock cars will have between 1,000 – 1,400 RPM stalls. The reason that a stall converter works is easy. An engine that has 1200 RPM will only have about 80 horsepower, but at 2500 RPM it could be up to 200HP; obviously a car will launch easier at 200 HP than one at 80HP. Many performance engines will not have good horsepower until around 3000 RPM, which is why when you have higher HP engine with a large cam, you will need a higher stall speed so that it will be closer to the power band when it takes off, otherwise it will fail and start out slow.

What High Stall Really Means

There are some that believe that stall means that the car won’t move until the engine hits a certain RPM, that isn’t true. A car that has a 6000 RPM stall converter in gear and idling will roll along at about 10 MPH, just like any other car that is in gear with the brake off. Normally cruising into the pits, is done with the car idling and in gear. If a 6000 RPM converter means that the car doesn’t start moving until that RPM then the engine would need to be wound out just to hit 15 MPH, and that isn’t how it works. Stalling means that whenever the car is on the start line, and the trans brake is on, and you hit full throttle, the engine will rev up to 4500 RPM with the car sitting there, so whenever you slip that finger off the trans brake, and the tranny slips out of reverse, the car will launch at 4500 RPM, and a race motor at that RPM is close to hitting the peak torque curve when it leaves the line, which is why you get big wheelies or low 60ft times whenever the traction is good. Race engines will have no power at a low RPM, and will be shy on torque, so the nastier that the engine is, the higher the stall speed will need to be. Gear ratio, cubic inches, weight, and torque have a factor in this. This won’t get the flash effect whenever the stall converter will flash to when your finger comes off the trans brake. A normal 3000 RPM converter can flash up to around 5000 RPM once you have launched the car. It is a pretty complicated process, so I will keep it simple.

Vehicles That Need High Stall Torque Converters

A 6000 RPM converter that is behind a stock engine may not go to 6000 RPM because the engine won’t make enough torque to get up to that point because of the load converter. The car will roll at a lower RPM and may never reach 6000 RPM. This is true for a 3000 RPM converter. Most mild race cars will use between 3500 to 4500 stall speeds on average. Faster cars will normally have a higher stall speed, especially in a smaller cubic inch engine that doesn’t have a lot of torque displacement. Plus, the higher the stall speed, the more heat the converter will make with hydraulic fluid inside of the converter. Normally a high stall converter will be able to make enough heat to destroy a transmission if you are holding the line too long at full RPM if the stall speed is higher than 3500RPM. Getting a tranny cooler is important for any high stall converter.

Many street performance cars will be running normal performance cam which should have a stall converter of 2800 to 3000 RPM.For the hotter cars that have bigger cams, then a 3500 RPM converter is better. If the car has a 350HP engine, then a 2400 RPM stall is best. The rule of thumb is if the engine is coming alive at 3500 RPM then you want a converter with about 3500 stall to it. Many people will tend to over cam and carburet their cars, and those people under stall which sucks. If you are needed a 3000 RPM stall, then they will get around 2000 RPM, which doesn’t get it off that starting line. You need to remember, 80% of the race is going to be the first 100ft. If you aren’t able to get off the line, then you will lose the race.

Buying High Stall Torque Converters

To choose the right torque converter for your vehicle the stock parts and any alterations and additions should be considered. This will help pair up the power, transmission, and all variables with the right torque converter that has the proper stall speed for your set up. We specialize in building custom highly specialized torque converters for all applications. We manufacture them for race, towing, and even higher MPG applications. Call us today to find out which torque converter will help you tap into the highest performance, most solid towing, or best gas efficiency possible. You can reach us at 602-437-2301 or contact us online.

What is a good stall for a torque converter?

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 don't 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.

How does a stall converter affect daily driving?

However, a stall converter will affect your daily driving by lowering your fuel economy substantially. This makes it a poor option if you're using the car for commuting. The reason that a stall converter boosts your acceleration is that it doesn't allow the tires to break loose until you reach a higher RPM.

What is the purpose of a high stall torque converter?

A higher stall torque converter will let your car accelerate better because the car will be taking off at the rpm range where it is making the most power.

What does a 3500 stall converter do?

If the converter was a 3,500 RPM stall, then it would be at approximately 3,500 RPM in which it forces the tires to break loose.