Torque converters are probably the most misunderstood component in an automatic transmission, yet they’re the simplest in both theory and function. Think of a torque converter like a water wheel in an old saw mill: the waterwheel is driven by fluid in motion. A torque converter works on the same principle—a fluid coupling or clutch that slips when the vehicle is stopped and transfers power as engine RPM increases and gets fluid moving. A torque converter, by its very nature as a fluid coupling, also dampens engine combustion pulses to achieve smoother operation.
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A Bit of History
The use of torque converters dates to the early 1900s. The Germans were among the first to use torque converters in automobiles, trains, and industrial machinery. The first US automaker to use a torque converter was Chrysler in the 1939 Imperial, known as Fluid Drive. General Motors followed that act in the 1940 Oldsmobile. Ford then followed suit in 1942 with a BorgWarner derivative in Lincoln and Mercury automobiles.
These early uses of torque converters didn’t work very well on start-out because there was no torque multiplication in those days. In fact, torque converters were called “fluid couplings” at the time because they didn’t multiply torque. General Motors was first with a true torque converter in the 1949 Buick Dynaflow transmission. Ford followed GM’s lead in 1950 with the first Ford automatic designed and manufactured by BorgWarner. GM’s legendary Powerglide 2-speed automatic came along in the mid 1950s and became a favorite with drag racers as time went on.
Torque Converter Function
Thanks to the basic principles of hydraulics, a torque converter puts fluid in motion to do our work. Fluid is thrust into motion to drive components in a process known as hydraulics. The same principle that stops your car in the braking system or operates your power steering is what gets it going in an automatic transmission. And if everything is working properly, the work is done smoothly and efficiently. A torque converter consists of four main components: • Impeller, which is tied to the crankshaft and puts fluid in motion
- Stator, which directs fluid under pressure to the turbine
- Turbine, which is tied to the transmission input shaft, driven by fluid in motion from the impeller and stator
- Cover or shell, which is welded to the impeller
The cover/shell and impeller are welded together to form the main torque converter shell, which drives the transmission’s front pump to provide hydraulic pressure for operation and lubrication. The impeller drives fluid through the stator to the turbine, which is tied to the transmission’s input shaft. As engine speed increases, fluid flow is directed through the stator to the turbine, which drives the turbine and transmission input shaft to get you moving.
The point at which the impeller begins to drive the turbine is known as stall speed. Most stock torque converters “stall” at around 1,500 to 1,900 rpm of engine speed. Highperformance torque converters stall at higher engine speeds because you want the engine well into its power band when the converter stalls (begins to move the turbine and vehicle). For example, a 2,400-rpmstall torque converter doesn’t begin to move the vehicle until engine speed reaches 2,400 rpm. The same can be said for a racing converter with a stall speed of 3,600 rpm. You want the engine making power when it hooks up (stalls) with the transmission’s input shaft.
Stator and Clutch
Stall speed is determined mostly by stator design. The stator is the “brains” of a torque converter because it manages fluid flow from the impeller to the turbine. This is what makes a torque converter a torque multiplier. The engine’s torque output is multiplied at least twice over, thanks to the stator. Most torque converters multiply torque in a 2.5:1 ratio over actual engine torque at stall speed. Within the stator is the one-way clutch splined onto the transmission’s stator support shaft. The one-way clutch allows the stator to rotate in one direction only with the engine’s crankshaft and converter impeller/shell. Torque conversion or multiplication happens at stall speed with the stator stationary before the turbine begins to move. When the turbine gets underway with the vehicle in motion, the stator moves at the speed as the turbine.
You can actually feel this process happen as you step on the gas and feel the vehicle accelerate. During hard acceleration, you can feel torque multiplication (stator stationary or slower than turbine speed). As the vehicle gets up to speed, the stator slowly begins to rotate to crankshaft speed. Lean on the gas and stator speed falls behind and torque multiplication comes into play, which is when you feel gut acceleration.
There are two basic types of flow: rotary (circular) and vortex (roundy-round circular). When impeller and turbine speed areuniform, you have rotary flow in a circle around the converter’s circumference. If there’s a difference in impeller and turbine speed, flow becomes more vortex (tornadic) in nature.
As said earlier, the stator is what helps the impeller and turbine multiply torque. During acceleration, the stator turns at a slower speed than the impeller and turbine, which directs fluid flow more aggressively against the turbine blades. As the vehicle speed catches up with turbine speed, the impeller, stator, and turbine are all whirling around at the same speed. Any time you step on the gas, stator speed slows momentarily to help direct fluid and multiply torque.
Choosing a Torque Converter
Most manufacturers categorize torque converters by size and stall speed. Performance Automatic, for example, makes it easy for you to choose a torque converter for your street or race application because, on its website, it explains the differences. As the diameter of a torque converter decreases, stall speed goes higher, which is why race converters are generally smaller than street converters.
It is a good idea to discuss your performance needs and expectations with a sales/tech professional before ordering a torque converter. Transmission parts supply houses generally sell stock torque converters with 1,500- to 1,900-rpm stall speeds. These converters are off-the-shelf dead-stock pieces that are not always designed and constructed for performance purposes.
If you’re seeking performance, it is wise to deal with aftermarket performance transmission companies like Performance Automatic, B&M, and TCI Automotive, whose products are all available from Summit Racing Equipment.
Aftermarket high-performance torque converters are designed and constructed to take additional punishment, with features such as:
- Furnace-brazed fins for solid integrity (stock fins are slotted in place, but not brazed)
- Dynamic balancing for high- RPM use
- Needle bearings instead of thrust washers
- Heavy-duty stator and sprag/oneway clutch
- 400- to 600-rpm-over-stock stall Speed
Converter Diameter and Stall Speed
Stock torque converters come in sizes around 11 to 13 inches in diameter with stall speeds around 1,500 to 1,900 rpm. This RPM range is where you want a street engine to begin applying torque. When you slip the transmission into gear, a stock converter provides a gentle nudge as engine torque is applied to the transmission’s input shaft and forward clutch. When you have a higher stall speed, that nudge doesn’t happen until the engine is closer to stall speed.
You want a higher stall speed on a street engine when the application of power is expected to be in the 2,400- to 2,600-rpm range. Weekend racers like having a high-stall torque converter that takes hold in this range because that’s where the power is.
For example, if you have a hot cam and an aggressive induction system along with a rough idle around 1,000 to 1,200 rpm, you want a higher stall speed for better traffic light idle, higher in-gear quality, and proper application of power as RPM increases. You want the torque converter to take hold (stall) at 2,400 to 2,600 rpm as the engine begins to make power. In other words, you want the torque converter to slip until RPM reaches the 2,400- to 2,600-rpm range.
The type of torque converter you choose depends upon how you intend to drive the vehicle. Street cruisers do not need high-performance, high-stall torque converters. They don’t even need a high-performance converter with all of the features mentioned above. If you’re going racing on Saturday night, you probably need a higher stall speed to get your engine into its powerband for a blistering holeshot and solid hook-up off the line.
Stock engines normally make peak torque around 2,000 to 3,000 rpm, with peak horsepower coming in around 5,500 rpm. Highperformance engines normally make peak torque around 3,500 rpm, with peak horsepower rolling in around 6,000 to 6,500 rpm. A stall speed of 1,500 to 1,900 rpm is perfect for street use with a mild engine because you want the converter to take hold at the beginning of the engine’s offidle rise in power.
High-performance engines begin to make power at a higher RPM, which is where you want a torque converter to take hold with a higher stall speed. If you run a high-stall converter with a stock engine, slippage occurs until your engine reaches the high stall speed. This makes normal driving difficult. This means your engine revs and doesn’t begin to transfer power until the higher stall speed is reached.
Slippage and high stall speeds affect upshifts. At 5,200 rpm, the engine speed drops by 3,500 rpm with each upshift. If the converter isn’t fully stalled at that point, you lose performance, which is wasted via slippage. This costs you precious time on the quarter-mile or at the traffic light.
Torque converter performance isn’t just about stall speed; it’s also about how firm a converter hooks up when it does stall. This is known as a tight or loose converter. Torque converter manufacturers like B&M, TCI Automotive, and Performance Automatic employ techniques that make torque converters more efficient with less slippage. Much of the general technology is rooted in fluid dynamics and how fluid behaves under given conditions. The greatest factor in converter construction is stator design, meaning blade/fin shape and angle, which determines stall speed and slippage. And this fact alone helps determine your quarter-mile times and the way your Ford behaves on the open road.
Written by George Reid and Republished with Permission of CarTech Inc