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Torque Converters explained...
Article By: Walt Vengelis, Jr.

November 2002
images attached

Torque converters are often a mystery to those not familiar with the inner workings of automatic transmissions. Torque converter problems may be confused with transmission problems, and vice versa. So if you'd like to know more about how a torque converter works, and to diagnose and service torque converter problems, keep reading.

The torque converter is bolted to the flywheel between the engine and automatic transmission or transaxle. It takes the place of a clutch, but does essentially the same thing as a clutch without the need of a clutch pedal. It transmits power from the engine to the transmission, but also allows a certain amount of slippage when the vehicle is stopped in gear so the engine doesn't die. It also acts like a reduction gear to multiply torque when the vehicle starts to move.

If you could peek inside a torque converter, you'd see what makes it work. Inside are three wheels with curved blades the impeller, stator and turbine wheels. The torque converter is filled with automatic transmission fluid, so anytime it rotates fluid is slung around and around in a circular path between these bladed wheels. The motion of the fluid is what actually transmits torque from the engine to the transmission. Here's how the fluid coupling works (see images 2)

FLUID COUPLING

The impeller is attached to the outer torque converter housing and is the drive member in the coupling. The impeller slings fluid toward the turbine wheel when the converter housing rotates - which is anytime the engine is running, whether the transmission is in gear or not.

Facing the impeller is the turbine wheel, which is the driven member in the coupling. When fluid from the impeller strikes the turbine blades, it causes the turbine wheel to rotate. The greater the force exerted by the fluid, the greater the drive torque transferred to the turbine wheel. A splined shaft couples the turbine wheel to the transmission input shaft so when the turbine wheel turns, it drives the transmission.

Sandwiched between the impeller and turbine is the third wheel, the stator. The stator's purpose is to multiply torque by completing the fluid flow circuit between the impeller and turbine. When the fluid slings off the impeller and hits the curved blades of the turbine, it has to go somewhere. The stator's job is to redirect the fluid back toward the impeller. This gives the impeller an added boost and keeps the fluid circulating in a circular pattern called vortex flow.

HOW THE STATOR MULTIPLIES TORQUE

The stator wheel is mounted on the converter hub and floats on a set of one-way roller bearings. The one-way clutch allows the stator to rotate in one direction, but not the other. The stator's blades curve in the opposite direction of the impeller and turbine blades. At idle, fluid coming off the turbine blades hits the stator blades in such a way that the stator wants to turn the wrong way. It doesn't spin, however, because the one-way clutch holds it tight. As a result the fluid slings back toward the impeller.

As long as the impeller continues to turn faster than the turbine, torque will be multiplied and the converter will act like a giant reduction gear. The amount of torque multiplication is usually about 21, which is like slipping the clutch when starting out from a dead stop. The added torque helps get the vehicle moving without lugging down the engine.

As the vehicle starts to move, the speed of the turbine wheel starts to catch up with that of the impeller. As the speed of the turbine approaches 90 percent of the speed of the impeller, the fluid dynamics inside the converter change. The fluid flows at a much steeper angle and now strikes the stator blades from the backside. This pushes the stator in the right direction and starts it turning. As soon as the stator starts to spin, however, torque multiplication is lost and the converter locks up. The stator freewheels at the same speed as the turbine and impeller, and the three elements become a fluid coupling.

The hydraulic lockup that occurs is not the same as the mechanical lockup that occurs in a torque converter equipped with a computer-controlled mechanical clutch. Because there is no mechanical connection between the impeller and turbine wheels, an ordinary torque converter will experience up to 10 percent slippage between the engine and transmission. This hurts fuel economy and performance, so the torque converters in most late-model vehicles have a computer-controlled pressure plate that locks up against the housing when the vehicle is about a certain speed and/or in higher gears. A computer-controlled solenoid controls the flow of hydraulic pressure from the transmission to apply and release the pressure plate. When the plate is engaged, there is no slippage between the engine and transmission, and the torque converter acts like a solid member in the driveline. But when the brakes are applied or the vehicle is idling with the transmission in gear, the pressure plate is released so the torque converter can slip.

UNDERSTANDING STALL SPEED

All torque converters have a specific stall speed which can vary from one vehicle application to another, even when similar transmissions are used. So if the torque converter needs to be replaced for any reason, it's very important to make sure the replacement unit is the correct one for the application. If the stall speed is not correct, it can have an adverse effect on drivability, performance and transmission longevity.

If you're not familiar with stall speed, it is the maximum rpm at which the engine will turn before the converter will allow no more slippage. In other words, it's the maximum speed at which the impeller will turn before the turbine starts to move.

The stall speed of a torque converter is determined by its diameter and the size of the stator wheel. Generally speaking, the smaller the converter, the higher the stall speed. A 10-inch torque converter, for example, typically has a higher stall speed than an 11-inch converter.

A higher stall speed generally gives more torque multiplication, which improves acceleration. The highest amount of torque multiplication is achieved when the engine reaches its maximum stall speed just before the car starts to move. It peaks out as the transmission starts to turn, and then diminishes as the speed of the turbine wheel catches up with the impeller. Hopefully this makes it easier for you to understand how torque converters work, and the role that the converter plays in the performance of your Mustang.