Torque converters appear to be a bunch of mysterious whirlwind motion inside a spinning shell that transmits an engine’s power to the transmission and rear wheels. Although they seem complex, torque converters are actually very simple in scope. What’s more, it really is easy to choose the right one because ATI Performance Products makes it all simple to understand.
What does it all mean? What is stall speed? What is the difference between stall speed and flash? How does a locking torque converter work? How does a torque converter multiply torque? What makes torque converters fail? The questions are endless. However, the answers are simple.
The humble torque converter is little more than a fluid coupling, a hydromechanical connection where we enlist fluid to do our work via an impeller, stator, and turbine whirling around inside a shell. Think of a torque converter like you would a water wheel in a saw mill. Fluid in motion moves a turbine (water wheel), which drives a shaft that drives a series of machines and equipment. Where the torque converter differs from a water wheel is in how it transfers power. The engine’s crankshaft spins the finned impeller inside the shell, getting transmission fluid in motion around the inside of the shell. Fluid in motion passes through the stator (stationary fins mounted on a one-way clutch) to the turbine, which is connected to the transmission’s input shaft and forward clutch assembly. There is a roller clutch (also called a one-way clutch) on which the stator is mounted, which allows stator rotation one way, but not the other.
Torque multiplication happens when we vector fluid from the impeller through the stator in the middle to the turbine at the transmission’s input shaft. We’re taking transmission fluid and whirling it around the perimeter of the shell, which creates velocity (speed), fluid in motion, and channeling it through the finned stator. As fluid passes aggressively through the stator it picks up even more velocity as it flows through the turbine. What you get with this interaction is roughly two and half times the torque your engine makes. This means 300 lb-ft of torque at the crankshaft becomes two- to two and a half times that number under acceleration.
While the engine is driving the torque converter shell, it is also driving a hollow output shaft tied to the transmission’s front pump, which provides the hydraulic pressure necessary to both control shift function and provide lubrication throughout the transmission. Hydraulic pressure serves to engage clutches and bands as a part of shift function. Not all automatic transmissions have bands. Some are entirely comprised of clutches.
Where torque converters become more complex is when you have a lockup feature with a clutch inside the converter to improve efficiency and eliminate the kind of slippage we experience with a torque converter. When the clutch engages, torque multiplication ends and there’s a direct connection between the engine’s crankshaft and the transmission’s input shaft. This is when an automatic transmission resembles a manual transmission. There are both three-speed automatics and overdrive automatics with locking torque converters. Most overdrive automatics go into converter lockup in overdrive while three-speed automatics can go into lockup in any gear range.
Two things we hear a lot with torque converters is stall speed and flash. Stall speed is that rpm range when your engine begins to move the vehicle. The converter “stalls” or “loads” and applies torque to the transmission’s input shaft when it reaches a given rpm range. “Flash” is a more complex answer rooted in vehicle type and weight, engine size and type, and the kind of driving you intend to do. Harvey Baker of ATI Performance Products tells us flash is an involved element that would take pages to explain. Suffice it to say, flash has a different meaning on the street than it does in racing. Your focus in choosing a torque converter needs to be stall speed.
Never put your vehicle to the test with hard power braking at wide-open-throttle sitting still to ascertain stall speed because you’re bound to do a certain amount of engine and transmission damage in the process. Stall speed is best checked with the vehicle in gear at a crawl before nailing the throttle. Stock torque converters generally have a stall speed around 1,800-2,000 rpm. Higher stall speeds become necessary when horsepower and torque happen at higher rpm ranges. You want stall speed and maximum engine torque to happen around the same rpm range. This means if maximum torque rolls in round 3,500 rpm this is where stall speed should be. When you launch, torque will segue to horsepower as your transmission navigates the gears.
Photography by Jim Smart and ATI Racing