What Is a Stator in a Torque Converter?

The stator is a bladed component mounted on a one-way clutch between the impeller (pump) and turbine inside an automatic transmission’s torque converter; it locks at low speeds to redirect oil flow and multiply torque, then freewheels at higher speeds to reduce drag. In practical terms, the stator is why an automatic can launch a vehicle strongly from a stop yet cruise efficiently once up to speed.

Definition and Where It Sits

Position in the Converter

Located at the center of the torque converter between the impeller (driven by the engine) and the turbine (which drives the transmission input shaft), the stator sits on a fixed support shaft. It is sometimes called the “reactor” because it reacts to fluid forces in the converter.

One-Way Clutch Action

The stator is mounted on a one-way clutch (sprag or roller clutch) that allows it to lock in one direction and freewheel in the other. When there’s a strong difference in speed between impeller and turbine—such as at a stoplight launch—the clutch locks the stator. As the turbine speed approaches the impeller speed, the reaction torque on the stator drops to near zero and the one-way clutch overruns, letting the stator spin freely.

Flow Redirection and Torque Multiplication

During launch, oil exiting the turbine flows back toward the impeller in a direction that would oppose the impeller’s rotation. The locked stator intercepts and redirects this return flow so it re-enters the impeller in the direction of its rotation. This redirection converts fluid energy into additional impeller torque, multiplying engine torque to the transmission—typically by about 1.8:1 to 2.5:1 in many OEM converters and up to roughly 3:1 in some performance or heavy-duty units.

Freewheeling at Speed

As vehicle speed rises and the turbine nearly matches the impeller speed, the return flow no longer strikes the stator blades in a way that creates reaction torque. The stator’s one-way clutch then overruns, letting it freewheel and minimizing hydraulic drag. In modern converters, a lock-up clutch may also engage to provide a direct mechanical link for greater efficiency; the stator’s role remains to control fluid dynamics during all non-locked conditions.

Why the Stator Matters

The stator’s behavior underpins how a torque converter balances launch performance and cruising efficiency. The following points highlight its core contributions.

• Torque multiplication at low speed for stronger launches and better drivability.
• Improved efficiency by redirecting fluid to support impeller rotation rather than oppose it.
• Reduced heat generation once freewheeling, cutting losses during cruise.
• Influence on stall speed, affecting how quickly the engine reaches its powerband at launch.
• Smoother power transfer by managing fluid momentum between turbine and impeller.
• Compatibility with lock-up clutches to further boost fuel economy at steady speeds.

Together, these functions allow a wide operating envelope: high multiplication when needed and low drag when it isn’t, making automatics both responsive and efficient.

Operating Phases and the Stator’s Behavior

The stator’s locking and overrunning behavior changes across driving phases. Understanding these phases explains why symptoms of stator issues vary with speed and load.

1. Stall/Launch: With the vehicle stopped and throttle applied, the turbine is slow, the impeller is fast, and the stator locks. Torque multiplication is highest and heat generation is greatest.
2. Acceleration: As the vehicle moves, turbine speed rises. The stator may still lock intermittently, sustaining multiplication while reducing slip.
3. Coupling/Cruise: Turbine and impeller speeds converge; reaction torque on the stator fades and its one-way clutch overruns, minimizing drag.
4. Lock-Up Engaged: At light-to-moderate steady speeds, the converter clutch may lock the impeller and turbine together. The stator is typically freewheeling because little to no slip exists.
5. Deceleration: Fluid forces reverse patterns; the stator generally freewheels to prevent unwanted drag and heat.

These phases ensure that the converter can adapt to varying demands, delivering torque when needed and efficiency when possible.

Design Details That Affect Performance

Different stator designs tailor a converter’s character for economy, towing, or performance. Key design choices shape both torque multiplication and drivability.

• Blade count and angle: More aggressive angles increase multiplication and stall speed but can add heat; milder angles favor efficiency.
• Stator diameter and throat area: Larger or tighter passages reshape flow energy and response.
• One-way clutch type: Sprag clutches engage smoothly; roller clutches can be more robust in some applications but vary in feel.
• Clearances and hub geometry: Precision reduces leakage and turbulence, improving consistency and durability.
• Materials and brazing: Performance converters often use brazed fins and stronger hubs to withstand high torque and heat.
• Application-specific “aggressive” stators: Selected for towing or racing to raise multiplication and alter stall characteristics.

In practice, the stator’s tune is matched to the engine’s torque curve, vehicle weight, gearing, and intended use to achieve the desired launch and cruise behavior.

Failure Symptoms and Diagnosis

Because the stator’s one-way clutch must lock and overrun correctly, faults show up most clearly at launch or during sustained high-load use. The following are common indicators and considerations.

• Poor off-the-line acceleration: A stator that freewheels when it should lock eliminates torque multiplication, making launches sluggish.
• Excessive transmission heat: A stator locked when it should freewheel, or damaged fins, can create drag and overheat fluid.
• Shudder or vibration on takeoff: Irregular stator engagement or damaged blades disturb flow.
• High RPM with little vehicle movement at launch: Can indicate a failed sprag or internal converter damage (also consider clutch or band issues elsewhere).
• Metallic debris in pan or fluid: May point to one-way clutch or fin damage; requires teardown to confirm.
• No direct diagnostic trouble codes: Stator issues rarely trigger specific OBD codes; temperature and slip-related codes may appear indirectly.

Confirming stator faults typically requires professional testing or converter inspection, as symptoms can overlap with other transmission problems.

Care and Upgrade Tips

Proper maintenance and thoughtful component choices extend converter life and optimize performance, especially when power levels or vehicle use change.

• Maintain fresh, correct-spec ATF and ensure cooling capacity; heat is the enemy of the stator’s clutch and brazed joints.
• Match converter to engine torque, vehicle weight, and gearing; choose appropriate stall speed and stator aggressiveness.
• Upgrade to strengthened or “billet” stator hubs and brazed fins for towing, performance, or high-heat use.
• Replace the one-way clutch during rebuilds; sprags are wear items and critical for proper multiplication.
• Consider auxiliary coolers if towing or driving in hot climates to protect converter internals.
• Work with reputable builders; small geometry changes can dramatically affect driveability and heat.

These steps help preserve the stator’s ability to lock when needed and freewheel when not, keeping performance and efficiency in balance.

Summary

The stator is the torque converter’s traffic cop for oil flow: it locks on a one-way clutch at low speeds to redirect fluid and multiply torque, then freewheels at higher speeds to reduce drag. Its design and condition determine how strongly a vehicle launches, how efficiently it cruises, and how much heat the transmission must manage. Proper stator function is essential to the automatic transmission’s blend of performance and efficiency.

What is the purpose of a stator?

The primary purpose of a stator is to be the stationary component in an electric motor or generator that creates a magnetic field to either produce mechanical motion in a motor or generate electrical current from a rotating magnetic field in a generator. In a motor, it creates a rotating magnetic field to drive the rotor, converting electrical energy to mechanical energy. In a generator, it converts the mechanical energy of the rotor into electrical energy through electromagnetic induction.
 
In Electric Motors

• Creates a rotating magnetic field: Opens in new tabWhen electricity flows through the coils (windings) in the stator, it generates a magnetic field that rotates. 
• Drives the rotor: Opens in new tabThis rotating magnetic field interacts with the rotor (the moving part of the motor), inducing forces and torque that cause the rotor to spin. 
• Converts electrical to mechanical energy: Opens in new tabThe result is the conversion of electrical power into the mechanical power needed to drive various devices. 

In Generators

• Converts mechanical to electrical energy: Opens in new tabIn a generator, the stator receives the mechanical energy from the rotating rotor and converts it into electrical energy. 
• Induces electrical current: Opens in new tabAs the rotor spins, its changing magnetic field induces an electric current in the coils of the stator through electromagnetic induction. 
• Generates power: Opens in new tabThis generated electricity then serves to power other components or recharge batteries. 

Key Characteristics

• Stationary: The stator is the part of the device that remains fixed, in contrast to the rotor, which is the rotating component. 
• Components: It consists of a laminated core and coils of insulated wire (windings) designed to optimize magnetic flux and efficiency. 

What are the 4 parts of a torque converter?

The torque converter is what transmits that torque from the engine to a rotating driven load. In an automatic transmission car, the torque converter connects the power source to the load. Torque converters are comprised of five main components: the impeller, the turbine, the stator, a clutch, and the fluid.

What are the symptoms of a bad torque converter stator?

Symptom: A bad torque converter can cause a noticeable decrease in acceleration, making the vehicle feel sluggish and unresponsive. This symptom is often more pronounced when climbing hills or during heavy acceleration.

What does a sprag do in a torque converter?

Sprag clutches are designed to transfer torque in one relative direction, referred to as the “engaged direction,” and to disengage—or “freewheel”—in the other direction. That means in the engaged direction, there is no relative rotation allowed between the two components connected by the sprag clutch.