Stall speed: it’s a number that’s 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 in order for the torque converter to do its job properly. To break this down, we’ve taken a look at the basics of stall speed with Dave Caine from ATI Performance Products to see how their Super Street Master series converter works with our Project Red Dragon Pontiac Trans Am.
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 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 can’t 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.
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’re 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,” Caine says.
In general, the larger the converter, typically the tighter and more efficient it will be. – Dave Caine
“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,” Caine explains.
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.
Torque Converter Operation Explained – Video courtesy of Engineering Explained
“The stator can have more or fewer fins changing the size of the windows that let fluid through to the turbine. The different angles on the stator’s fins are used to increase or decrease the velocity at which the fluid hits the turbine,” Caine says.
The use of specific materials for the stator in a stall converter has changed over the years. Racers used to worry about the use of steel stators in a torque converter, specifically in turbo applications where issues would arise relating to the heat they generated during long spooling times. Now, since EFI and other technology has progressed allowing cars to spool or build power quicker, aluminum stators are more frequently used in converters.
“The more torque your engine makes, the more holding capacity you need from your torque converter. You also have to be aware of that capacity, because you can’t have too much or it will hinder performance at the track. If you have too little holding capacity, you will start to blow through your converter and experience slippage. The RPM range of the torque and horsepower curves are most important in configuring a converter correctly for a given engine combination.”
If you’re planning on bringing a power-adder into the mix with your car, that will absolutely require a change in your torque converter’s stall speed. Changing where the car has power being brought in and the amount will cause the converter to hit its stall point at a different place overall. We will get into how different power-adders do this in a moment, but Caine gives a very simple explanation of how a power-adder factors into the equation.
A converter reacts to two things to multiple torque: force applied from the torque of the engine, and resistance, which is the car. When the vehicle has a power-adder, that will need to be factored into the choice of torque converters. The reason for that is because no matter what power-adder you opt to use, it will change not only how much power the engine makes, but where it makes it. Where that power comes in will drastically change what the converter does, so a nitrous converter won’t be effective in a boosted application.
As mentioned earlier, besides the engine’s ability to produce power, the weight, gear ratio, and tire height all play a role in what the converter’s stall speed needs to be. The heavier a car is, the more load it will put on the torque converter when power is applied, causing it to flash at a higher speed. Gearing plays a part because as you add more gear, it will cause the load on the converter to become lower, while taking away the gear will make the load grow. Tire height falls into the same function zone as the gearing of the car: as you increase the height of the rear tire, you will increase the load on the torque converter and change where the torque converter will flash.
The Stall Speed Range
There can be some confusion that the stall speed of a torque converter is an absolute thing. The reality is that a torque converter has a range of speed that the stall can engage in, and that will change based on the previously mentioned factors.
“Stall speed is a relative term. It only reacts to a situation; it does not dictate anything. The same converter will flash differently when put behind different powerplants or in different cars. It will even change with the weather, so there can be no exact stall speed,” Caine points out.
You’re able to determine the stall speed range of your torque converter after it has been installed in your car. Checking for a stall speed on the footbrake isn’t the most accurate way because you’re only at part throttle and not making a large amount of torque, so the converter will not see much stall speed.
Stall speed is a relative term. It only reacts to a situation; it does not dictate anything. – Dave Caine
“Going to wide open throttle on a transbrake, you will make more torque, so the converter will flash much higher. From there, checking actual flash is best done via data log, but in lieu of that, once you’ve made a pass, you can let the car roll in high gear at idle at around five mph and stab the throttle long enough to watch where the tach jumps.”
Going a bit deeper, Caine explains when it’s best to do this, and what not to do when trying to find your stall speed range.
“Doing the test on the return road after a pass is good because everything is at temperature, and you have the weight of the car working in the equation, as well. Doing the no-chip, two-step test in the pits is bad because it stresses the sprag in the converter and doesn’t allow it to unwrap as designed. These types of tests can lead to premature failure of the converter and are not a recommended practice.”
How To Select The Correct Stall Speed For Your Car
Before you start calling every torque converter manufacturer trying to gain access to the secrets of stall speed and what your car needs, you first need to figure out what the primary purpose of the car will be. If you’re looking for something that’s going to be used at the track only, the process of selecting a converter simpler for the manufacturer, but when you start creeping into the realm of street/strip use or daily driving, that’s where things get tricky. Knowing the combination and its use is the best way for a torque converter company to build a converter to fit your needs.
Determining the stall speed needs for an all-out racecar or a street/strip car that is looking for the best elapsed time will usually start on the looser end of the converter spectrum. The first thing that you will need to know is approximately where the engine starts to make its power. The key is not to have the torque converter flash its stall speed too far below that level or it won’t work properly, as it will bog and delay the acceleration of the car.
Caine expands upon this theory and what works best for a dedicated racecar and a car that is more geared towards street/strip use.
“For a race-only application, the basic rule is to get the converter to flash just above peak torque. For a car that is setup for street/strip use, you will need to be a little more conservative, but you still need to ensure it gets past what we call ‘the stumble zone.’ The object is to get the engine into an area of the power-band where it’s happy and can accelerate easily without stumbling.”
Previously, we had touched on the concept that bringing a power-adder into the mix will change how a torque converter behaves when it comes to the stall speed. According to Caine, the stall speed will be different for each, but how you determine it remains the same.
“They’re all different when it comes to what the stall speed requirement is. The basic rule applies: you should get a torque converter built to your specific combination and intentions so the stall speed is correct. An N/A torque converter will typically want to be lightweight, whereas a power-adder converter will most likely want a heavy cover and anti-ballooning plates, and each will have a different stall speed range, too.”
Stalling The Dragon: What Went Into Choosing The Converter
The previous torque converter we were using in the Red Dragon wasn’t a bad unit by any stretch of the imagination. It just didn’t fit the combination of the car. The torque converter had entirely too high of a stall speed, and it wasn’t allowing the Red Dragon to reach its full potential with our goals, so it was time for a change. After doing some research, we found the ATI StreetMaster torque converter with a single disk lock-up, and it fit the bill perfectly, so we made the call to pick one up.
When you’re looking at buying a torque converter for street/strip use, there are some tradeoffs that you will need to consider when it comes to stall speed. If you go into a higher stall speed range, the car has the ability to produce better elapsed times, however, it will start to generate a lot more heat on the street and lose some drivability. If your car has a bigger camshaft and rearend gear, that will only amplify the drivability issues for street use.
Caine further explains what can happen with too big of a stall speed on the street for a torque converter.
“Heat is the biggest issue. Too much stall and people tend to short shift, leaving them in high gear at 20 mph and making the converter do all the work; this is a bad scenario. If you choose to run a loose converter on the street, you must wind up each gear and let the converter couple before shifting. This can make a world of difference on the temperature gauge.”
It’s also possible to go too tight with a converter in your street/strip car, and that will cause its own set of issues. The converter and transmission might not be seeing nuclear levels of heat and the driveability might be better, but the performance at the track will suffer. The car will seem very lazy when you launch, and your 60-foot times will be poor, causing your elapsed times to rise.
The ATI SuperStreet Master we used is a billet-faced torque converter that’s a direct-fit for the 4L60E & 4L65E transmission that came in GM cars and trucks. The great part about this converter is that ATI includes the pilot necessary to get the torque converter to fit on the input shaft with no additional shims. All you need to do is fill it with fluid and make sure you turn it until you hear the three distinct “clunks” and the converter is seated on the pump and ready to be bolted to the flexplate.
The old converter in the Red Dragon was flashing at nearly 4,000 rpm on motor and a whopping 5,200 rpm on nitrous. This was causing the converter to not be as efficient on the top end of the track and going through the traps nearly on the rev limiter. The new ATI converter is flashing around 3,500 on motor and a more manageable 4,000 on nitrous. In testing, the Red Dragon ran a best of 10.94 at 123 mph with a 1.53 60-foot time. Now, this is slightly slower than its best of 10.90, but this converter is built for the other changes we will be making to the car in the near future. As we add power and sort out some other items, the car will respond better down low while still being efficient at the top end.
Selecting the correct stall speed for your vehicle’s torque converter is more than just throwing a dart at a dartboard with a blindfold on; it requires some research and thought. Going to a reputable torque converter manufacturer like ATI with your vehicle’s information will allow them to build you a unit that will have a stall speed catered to your vehicle and its goals. Having a converter with the correct stall speed will make your car fun to drive at the track or on the street, and will give you those nice, consistent elapsed times on your ticket, pass after pass.