Life is filled with mysteries:

Are we alone in the universe? What really happened to the dinosaurs? And what exactly is Lady GaGa?

Although we can’t provide definitive answers to any of those dark mysteries, we can help you solve one of the automotive world’s biggest mysteries: the torque converter.

In this installment of our Summit Racing Quick Flicks videos, we talk about the fundamentals of the torque converter, including:

  • How does a torque converter work?
  • What is torque converter stall speed?
  • How to choose the right torque converter for your vehicle

The torque converter doesn’t have to be one of the great wonders of the automotive world. Learn the basics—watch our video now!

Selecting the right torque converter for a vehicle can be critical to a vehicle’s performance and overall acceleration as an end result. It is difficult for a lot of customers to select a proper torque converter because they don’t understand how a torque converter operates. What we are going to talk about today is the function of a torque converter, what it does, and how to select the proper torque converter for an application.

So before we go about selecting a torque converter for an application, it is important to understand how the torque converter operates and what a torque converter actually is. A torque converter is a fluid coupling device that transfers power from the engine to the transmission and it is also going to multiply the torque being created by the engine. What this does it is it gives us the ability to leave the vehicle in gear and come to a complete stop at the same time.

Now what makes this possible in the torque converter is the three components that it is made up of. The torque converter as a whole is going to look like a fairly simple device and internally it is actually fairly simple as well. What you are going to see is an outer shell that is going to attach at the back side to the flexplate assembly, but as we dissect this torque converter, you’re going to come to find that a torque converter mainly operates off of fluid motion and centrifugal force.

What we are going to start with explaining is the fluid impeller, and that is going to be the front shell of torque converter assembly. This is the only part of the torque converter that is physically attached to the engine and is in constant motion with the engine because it is because it is welded via this seam to the compete assembly. This is going to operate off of flexplate motion as the engine is rotating.

The impeller’s job is to create fluid motion inside the torque converter to create that transfer of power. These fins will catch the fluid that the converter is filled up with, then It will go ahead and send that fluid through the torque converter assembly itself.

The next part of the torque converter we are going to discuss is the stator. The stator is a part that goes ahead and redirects fluid flow towards the impeller for a quicker reaction of the torque converter for better torque reaction in general in the torque converter itself, followed up by the turbine assembly.

The turbine assembly is the part inside the torque converter that actually transfers the motion between the engine and the transmission. This is the part that engages with the input shaft via the spline shaft internally, and what it’s doing is catching the fluid motion created by the impeller that is being forced against the turbine, and then transferring that power and torque to the transmission assembly. We can go ahead and alter torque converter stall by changing the fin angles the size of the torque converter which is going to go ahead and affect the amount of fluid flow throughout the torque converter assembly itself.  This in turn affects how high a torque converter stalls, how quickly it reacts, or how slowly it reacts, and this is what makes it such a critical component when making engine changes, when changing camshafts in an engine, and determining how the vehicle is going to go ahead and accelerate from a dead stop.

So now that we understand how the parts inside a torque converter actually operates and what it actually does, we have to understand what stall speed really is. Stall speed is basically the term that refers to the rpm in which the torque converter is transmitting all of its torque throughout the transmission assembly.

There is point in which the converter is going to be slipping prior to this that’s going to go ahead and let the engine reach its rpm band at a quicker rate of speed to bring it up to point in which it makes power. That is why a torque converter must have stall in order for it to operate correctly in an automatic transmission application.

Now torque converters do have two different types of stalls they can produce, a lot of times there is a little confusion between two and the accuracy of one compared to the other. The first type of stall is the one that most people tend to measure a converter by at home. Let’s say they have their vehicle at home and they put a new converter in, and they want to test the stall range rating of the converter. They’re going to go ahead and test it via foot brake stall and here’s what this is essentially is: They’re going to put the converter in the vehicle, go out start the car up, get in the driveway, hold the brake pedal and the gas to the floor at the same time. Now one of two things is going to happen at this point: either one, the engine is going to quit making rpm way before its max rpm point, or two, it’s going to break the tires loose and override the brakes at some point in time in the rpm band. This is what’s known as foot brake stall. Foot brake stall is actually a very inaccurate way to measure torque converter stall–it’s always going to happen much sooner than the other type of stall that a torque converter is going to produce. It is actually harmful and dangerous because what it does is overload the torque converter creates a lot of excessive heat and a lot of excessive wear and tear on the torque converter itself and will actually shorten the torque converter life. So when you do something like a torque brake on a vehicle with an automatic transmission, what’s probably going to end up happening is damaging the torque converter in the long run.

The other type of stall a torque converter is going to produce is what is known as flash stall. Flash stall is the actual stall of the torque converter that the torque converter is rated at and you are going to see advertised either on our website or in the catalog. And what you will notice about the stall of a torque converter is its always listed in range. There is no such thing as a specific 3,500 stall converter–it’s going to stall around 3,500 rpm, depending on certain attributes of that vehicle, whether it be the vehicle weight, the gear ratio, the tire size that’s used on the vehicle, the amount of power that is being produced by that engine. Those are all going to have an effect on the stall point of that converter. The flash stall rating is the only true rating of that torque converter; the only true stall rating you’re going to find on the torque converter. This is typically much higher than what the foot brake stall is going to be and it is going to be much more accurate and it is actually much harder to figure out at home after the torque converter has been installed in the vehicle. It is essentially is the point from a dead stop when you are going full throttle without the brakes engaged at the point in which the tachometer needle is going to jump and then the transmission engages and the vehicle accelerates. Catching this and figuring this out is kind of a tough task at home, and it leads to a lot of confusion when people try to go ahead and look at the stall that is being produced by the converter at home for personal use in a specific application.

As we understand flash stall we now begin to understand why it is difficult to select a torque converter for an application because of all the different variables that can have an effect on the stall range and stall rating of the converter in a specific application. There are a lot of contributing factors to determining what stall is converter is going to be right for our vehicle and selecting the right one is critical to a vehicles performance and how it accelerates, especially in a drag race application in which 60 foot times are very important and can have a considerable effect on the end result as far as quarter mile times go.

The main contributing factor determining what converter stall is right for an application is the camshaft that is installed in that engine. Every camshaft is going to have an rpm starting range in which it starts to make power. Now this can be affected by the cubic inches displacement of the engine, as well as the vehicle combination as a whole. Mainly, what we will suggest to most customers and what we suggest that most of our sales representatives do, is they look at the starting arm band of that camshaft. Then we will go ahead and select a converter that is 500 rpm higher in stall range than the starting rpm of the camshaft being used in that engine. Without this critical information about the camshaft, it becomes difficult to select a torque converter accurately for a vehicle and engine application for any customer.

One other thing that was mentioned was that the gear ratio, tire size, cubic inch displacement of the engine can also have an effect on the torque converter stall. All of these are not going to have a great effect but they will have a small or slight effect on the way a torque converter reacts in a vehicle application. Some things to kind of consider is if you’re putting that converter behind a big block in comparison to a small block, typically it will stall about 300 rpm higher behind the big block application in comparison to the small block application and this has a lot to do with the amount of torque production of a big block type engine. It produces much more low-end torque, so the converter has a harder time keeping up with that torque multiplication that’s happening so it inherently makes it stall at a higher stall speed.  It’s not going to be greatly noticeable but its significant to know. This is part of the reason that torque converters are via range instead of a specific number.

Things like gear ratio–the numerically higher the gear ratio is in that vehicle, it’s going to stall sooner because the vehicle is going to have less load on it to get it to move forward. It’s going to motivate itself to move forward much more easily. Same thing goes with tire size. Tire size is going to have an effect on the gear ratio of that vehicle. So if you had let’s say a set of 3.73 gears and you change from a 28-inch tall tire to a 26-inch tall tire now what you may not notice is the torque converter is now stalling at a sooner point than it did previously because the vehicle has an easier time moving from a dead stop.

The other consideration is that you want to make sure your torque converter stall range is going to be below the cruise rpm of vehicle on a street application. This is another area where gear ratio comes into play because if the torque converter stalls higher than what the cruise rpm is on the highway at 70 miles-per-hour what’s going to happen is it’s never going to into full engagement point or stall. What that means is that the torque converter is continually going to be slipping, not completely, it may be slipping at percentage let’s say 20 percent or so. But that 20 percent is creating friction inside the torque converter as well because the components are essentially not meshing up with one another, they are not rotating at the same speed so now what you have is a turbine that is spinning at a different rate of speed than the impeller of the torque converter. What this does is it creates excessive heat that excessive heat creates wear and what can happen is you may break a weld on the snout or you may damage the internal components because of the excessive heat which inherently is then going to damage the internal components inside the transmission because the transmission has a certain RPM operating range that it wants to operate at temperature wise which is typically anywhere from 170 to 210 degrees much like your engine.

So what are some symptoms of having the wrong torque converter for a vehicle? Stall is too low, the engine shuts off when put into gear, very poor acceleration, the vehicle lunges and tries to override the brakes essentially when put into gear.

If the stall is too high the engine will have a very small operating range. The converter never reaches stall at the cruise rpm or the transmission will over heat or burn fluid through the converter never stalling.

So in summary the general rules of thumb for selecting a torque converter for a street application are: Select a converter that’s stalls roughly 500 rpm higher than starting rpm of the camshaft, remember converters installed in big block applications typically stall 300 rpm higher than a small block application and always choose a converter that will below the highways cruise rpm. This can be determined knowing the gear ratio and tire size, if this is not done, the torque converter life will be shortened considerably.

Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.