When was the last time you had to sit down and decide what torque converter your project needed? For many people, selecting the right torque converter is akin to knowing some sort of black magic. Let’s face it, unless you have a lot of experience with torque converters, it’s nearly impossible to choose the correct one for any given application and predict with certainty how it will work in the car. When looking at all of the options available, how can you narrow all the choices down to the right one for your ride?
Do you need a converter that stalls at 2,000, 2,500, or 3,000 rpm? Will an 11-inch converter work the same as a 10-inch converter if rated at the same stall speed? And, what is stall speed anyway? These are many of the questions that get asked, so we decided to talk to a few professionals in the field, and get their input. Both ATI Performance Products Inc. and TCI Auto have been building torque converters for a long time, so getting the correct information from them was simple. But before we get into choosing a torque converter, you should at least understand how they work.
When using an automatic transmission, it’s the torque converter’s job to transmit and multiply the engine torque, and deliver it to the transmission. If you cut open a race torque converter, inside you will find: the turbine, stator with sprag, and impeller pump. The converter’s turbine is attached to the drivetrain via the input shaft of the transmission. When the turbine moves, the car moves.
The stator is the fluid director and changes fluid flow between the turbine and pump. It is what makes a torque converter a torque multiplier, and not strictly a fluid coupler. Remove the stator, and a torque converter will retain none of its torque multiplying capabilities. For the stator to properly function, the sprag (a mechanical, one-way clutch mounted on bearing races) must hold the stator in place while the converter is in stall mode (at engine idle) and then allow the stator to spin with the rest of the converter after the turbine speed approaches the pump speed (as engine RPM increase).
The impeller pump is the outside half of the converter, on the transmission side of the weld line. Inside the impeller pump are a series of longitudinal fins that push the fluid around its outside diameter, and into the turbine. This component is welded to the cover, which is then bolted to the flywheel, and spins with engine RPM. The size of the torque converter, impeller pump, and the number and shape of the fins, all affect how the converter works.
There are basic factors and variables that can dramatically sway the amount of footbrake stall someone can see from a converter. – Dallas Moss, TCI Automotive
When someone talks about stall speed, they are talking about how many RPM the engine can attain with the transmission in gear and the car’s brakes applied — before the drive wheel(s) spin. When discussing street-use torque converters, stall speed is typically listed as a range (example: 2,500 to 3,500 rpm). The range listed will tell you what you can expect from the converter. If a 2,500 to 3,500 stall speed is selected, you should be able to hold the brakes (footbrake), and the converter should allow the engine to rev to around 2,700 to 3,200 rpm. Of course, this is dependent on the vehicle’s setup. Dallas Moss at TCI Automotive helps us understand how.
There are many factors that determine what RPM the converter will flash stall after it is installed in your car. When discussing converters that are race car specific, they are not built with such a wide range of stall speeds. According to Dallas Moss of TCI Automotive, “In most cases, manufacturers will rate the stall of a converter at flash stall, which is slightly different than footbrake stall.
In essence, a torque converter will never be able to footbrake stall up to where it is actually flash stalling. It’s very difficult to tell customers what they can look for as a footbrake stall, simply because of the variables that can cause it to either hit the mark, or way undershoot it.” Moss continued, “There are basic factors and variables that can dramatically sway the amount of footbrake stall someone can see from a converter.”
To an extent, the overall diameter and the internal construction of a converter can cause a substantial change in footbrake stall. For example, In most cases, one of TCI’s 10-inch competition-model converters will footbrake much higher than one of their 8-inch converters. This is because the internal setup of the 8-inch unit generates massive amounts of torque multiplication, and causes the vehicle to want to push forward before it gets remotely close to the overall flash stall of the converter. Typically, the 8-inch converter (depending on internal fin angle and the style of stator), will most likely flash stall at a much higher RPM than the 10-inch competition unit.
The tires on a vehicle play a large part in footbrake-stall capabilities as well. For instance, a car with a set of wider tires at every corner will hold the car better because of the large contact patch of the tires holding the vehicle. This will allow you to get higher in the RPM range than with the same or equivalent vehicle equipped with a set of big and little tires.
The only true way to determine a converter’s stall speed is at the racetrack with the car in low gear when you launch. – JC Beattie, ATI Performance Products, Inc.
The vehicle’s weight affects footbrake stall, by giving the converter more load to move. The heavier the vehicle, the higher it will push the stall of the converter. Of course, the opposite is true of lighter-weight vehicles. Since they are lighter, they are easier to move and cause the converter to react with less stall, because it doesn’t have to work as hard to move the vehicle. Finding how much your car weighs is as easy as driving to a local truck stop.
Rear Gear Ratio
This one is relatively straightforward, the lower numerical-value gears, such as a 2.73 or 3.08 gear, the higher the converter will stall because it’s taking more force to move the vehicle.
For instance, a heavy car with numerically-low (highway) gears is more resistant to forward motion, than a lighter car with numerically-higher gears. When used in the heavier car, the converter will stall at a higher RPM than it will in the lighter car. The easier the engine can accelerate the vehicle, the lower the converter will need to stall to get the car moving. According to JC Beattie of ATI Performance Products, Inc., “I refer to this as having more mechanical advantage, which makes it easier to move the weight of the vehicle. Therefore, the converter has to do less work. The opposite is true as well.”
Of course, the power and torque curves of your engine will have a huge effect on stall speed. Typically, engines that produce more low-end torque can actually increase the stall speed of a torque converter to a higher RPM. Conversely, the same converter will stall at a lower rpm behind an engine developing less torque. If you buy a converter that is rated at 2,000 to 2,500-rpm stall, that rating is meant to span a variety of engines with different power curves.
When discussing the stall of a converter, you need to be able to distinguish between flash stall and footbrake stall. A torque converter’s flash stall is typically the most accurate rating, as a footbrake stall is dependent upon too many variables. (i.e. type of brakes — disc or drum — how well adjusted the brakes are, reared gear ratios, etc).
Beattie told us, “The only true way to determine a converter’s stall speed is at the racetrack with the car in low gear, when you launch. But, another safe way to get a reading that will be within 100 to 200 rpm, is to have a manually-shifted automatic transmission, or a way to hold it in high gear (1:1 ratio). With your car rolling at 10 to 15 mph, mash the throttle pedal to the floor, look at the tachometer, and then lift your foot from the throttle. Some cars will spin the tires, but you should have a moment just before tire spin, to see where the converter spins the tachometer. Testing a two-step with the car on jackstands, in the pits, or anywhere you don’t let go of the button or footbrake and accelerate, is very hard on the internal parts of the converter.”
Even after reading this article, how do you know which converter is right for your application? Since we know that no torque converter will react the same way when installed in different situations, does that mean that it’s almost impossible to buy an off-the-shelf converter? Not necessarily, the best advice we can give you is to contact the manufacturer and get their suggestions. As knowledgeable as your buddy might be, we still recommend getting the advice of professionals. According to Moss, “The number one suggestion I can recommend to people in the market for a stall converter, is to do some research on their setup. The more knowledgeable a person is about their specific application, and the more information that they can provide to us about their specific setup, the better we can get them dialed in with the perfect stall for their vehicle.”
When relaying the required information to the manufacturer, there are several things they will need to know. We asked Beattie, what is the single biggest mistake users make when selecting a torque converter? “Thinking one size converter fits all and buying something that says it is a “X,000” stall converter, just because it is advertised that way.” While that sounds logical, we wanted to know how to avoid making that mistake. Beattie continued, “Just knowing a few basic facts can go a long way, and always call the manufacture and ask questions. At a minimum, if talking about a street-driven car that has some power, is on the heavy side, and has a decent gear for cruising, you should know the rearend gear ratio, tire size, engine size, and what transmission is in the car.” These are the basics that will get you in the ballpark for your cruiser, but what if your car is more than a cruiser? What if you are looking for the best performance possible? For those of you wanting to really maximize your car’s potential, you really need to have a lot of information ready for the manufacturer (see sidebar).
When choosing the stall speed that is right for your application, a rule of thumb is that the advertised stall speed will need to be at least 500 rpm higher than the beginning of the camshaft’s powerband. All aftermarket camshafts are delivered with a recommended RPM operating range. If your camshaft has an operating range of 1,500 through 6,500, you would select a torque with a minimum of 2,000 rpm stall. If choosing a torque converter for use in a street car, you might want to select a torque converter with a stall speed that is below the engine RPM at 70 mph, since this is where your engine will spend a lot of its time. An exception to this theory is if the transmission/converter is equipped with a lock-up feature.
For example, let’s look at a 1968 Camaro. Let’s say it weighs in at around 3,200 pounds and is equipped with a 350 cubic-inch engine that has a 9.0:1 compression ratio, and is running a relatively-mild street camshaft like a Comp Cams 260H, and has a set 3.50 rearend gears. With a stock-type fuel pump and a Turbo 350 transmission, a good converter here would be a 2,000 to 2,200 stall converter. Let’s use that same Camaro, but swap the 350 cubic-inch engine for a 383 cubic-inch small-block, with a 10.5:1 compression ratio, a Comp Cams 292 camshaft, a 4.11 gear ratio, a high-performance fuel pump, and the same Turbo 350 transmission. It would likely use a 3,200 to 3,500 stall converter to get the best performance.
When it comes to choosing a torque converter, one size certainly doesn’t fit all. All of the major torque converter companies employ skilled personnel that have a wealth of information available. But, that information is useless if you do not provide accurate information about your car and its components. If you work with the manufacturer, you should be able to get a torque converter that is the perfect fit for your car. You won’t regret the time spent, and the cost of the telephone call(s) is minimal when you consider how poorly your car will run with the wrong torque converter.