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Torque Conversion

Short answer: A torque converter sits between the engine and an automatic transmission and lets the car idle in gear without stalling. The engine-driven impeller (pump) flings automatic transmission fluid outward into the turbine, which turns the transmission input shaft.

The stator sits between them on a one-way clutch. When the turbine is turning much slower than the impeller, as when pulling away from a stop, the stator turns the returning fluid so it helps the impeller instead of fighting it. That multiplies torque by up to about 2.5:1 at stall. As speeds even out, the stator freewheels and the unit acts like a plain fluid coupling. Because constant slip wastes energy as heat, virtually all modern automotive converters add a lock-up clutch that mechanically joins the impeller and turbine at cruise.

VideoWrite-up4 sources

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What should you know about Torque Conversion?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. At a standstill, a car's torque converter can multiply engine torque by about 2 to 2.5 times, thanks to a part called the stator.

    DD-075-F01 · Write-up · Exhibit A (Why automatics need one)

  2. Most modern torque converters still contain a mechanical clutch, which locks up at cruising speed to stop fluid slip.

    DD-075-F02 · Write-up · Exhibit A (Why automatics need one)

Video transcript: Torque Conversion: How an Automatic Moves Without a Clutch Pedal

DID YOU KNOW? A torque converter is a fluid coupling used in automatic transmissions that transfers and multiplies engine torque to the transmission using hydraulic force instead of a mechanical clutch. It consists of three main components: the impeller, which is connected to the engine and pushes transmission fluid outward; the turbine, which receives that fluid and spins the transmission input shaft; and the stator, which redirects returning fluid to increase efficiency and provide torque multiplication at low speeds. NOW YOU KNOW!

Burned-in captions, read frame by frame (see transcript.md) · 24.4s Reel ·

Video coming soonTorque Conversion · Instagram Reel
Torque Conversion: How an Automatic Moves Without a Clutch Pedal Instagram Reel · · 24.4s

The write-up

How an automatic moves without a clutch pedal. Spinning fluid carries the engine's torque, and a stator can roughly double it from a standstill.

  1. Exhibit A

    Why automatics need one

    The torque converter replaces the clutch in most automatic cars. Because it allows slip between input and output, the car can sit still in gear with the engine running.

  2. Exhibit B

    Pump and turbine

    The impeller, bolted to the engine, pumps fluid into the turbine, which drives the transmission input shaft. Together with the stator, these parts form the 'torus' that the fluid circulates through.

  3. Exhibit C

    What the stator adds

    A plain fluid coupling has no stator. Without one, fluid leaving the turbine would hit the impeller against its rotation and brake it. The stator reverses that flow so it helps instead.

  4. Exhibit D

    Torque multiplication

    The effect is strongest at stall, when the turbine is barely turning. Typical converters multiply torque by about 1.8 to 2.5 times there, and the effect fades as the turbine catches up.

  5. Exhibit E

    The one-way clutch

    The stator sits on an overrunning clutch. It's held still during multiplication and freewheels near the coupling point, so it doesn't get in the way.

  6. Exhibit F

    Lock-up

    Slip turns some power into heat, so modern converters include a lock-up clutch that mechanically links the impeller and turbine above roughly a 0.9 speed ratio.

Questions people ask about Torque Conversion

Which torque converter part is driven by the engine?

The impeller pumps fluid into the turbine.

Source: Write-up · Exhibit B (Pump and turbine) · Torque converters (IEEE Technology Navigator); Zhang & Mi, Torque Converters, in Automotive Power Transmission Systems (Wiley, 2018)

What does the stator do?

It turns the returning flow so it helps the impeller.

Source: Write-up · Exhibit C (What the stator adds) · Torque Converter (IDC technical reference); Torque converter (Wikipedia)

Roughly how much can a converter multiply torque at stall?

Typical stall ratios are about 1.8 to 2.5:1.

Source: Write-up · Exhibit D (Torque multiplication) · Torque converters (IEEE Technology Navigator); Torque converter (Wikipedia)

What lets the stator freewheel at cruising speeds?

The overrunning clutch holds the stator only during multiplication.

Source: Write-up · Exhibit E (The one-way clutch) · Torque converters (IEEE Technology Navigator); Torque converter (Wikipedia); Torque Converter (IDC technical reference)

Why do modern converters have a lock-up clutch?

It locks at about a 0.9 speed ratio.

Source: Write-up · Exhibit F (Lock-up) · Torque converters (IEEE Technology Navigator); Torque Converter (IDC technical reference)

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. Which torque converter part is driven by the engine?

    1. Turbine
    2. Impeller
    3. Stator
    4. Lock-up clutch
    Show the answer

    Answer: B. Impeller The impeller pumps fluid into the turbine. Source: Write-up · Exhibit B (Pump and turbine) · Torque converters (IEEE Technology Navigator); Zhang & Mi, Torque Converters, in Automotive Power Transmission Systems (Wiley, 2018)

  2. What does the stator do?

    1. Cools the oil
    2. Redirects returning fluid to multiply torque
    3. Shifts gears
    4. Starts the engine
    Show the answer

    Answer: B. Redirects returning fluid to multiply torque It turns the returning flow so it helps the impeller. Source: Write-up · Exhibit C (What the stator adds) · Torque Converter (IDC technical reference); Torque converter (Wikipedia)

  3. Roughly how much can a converter multiply torque at stall?

    1. 1.1 times
    2. About 2 to 2.5 times
    3. 10 times
    4. 100 times
    Show the answer

    Answer: B. About 2 to 2.5 times Typical stall ratios are about 1.8 to 2.5:1. Source: Write-up · Exhibit D (Torque multiplication) · Torque converters (IEEE Technology Navigator); Torque converter (Wikipedia)

  4. What lets the stator freewheel at cruising speeds?

    1. A one-way clutch
    2. A belt
    3. A spring
    4. Magnets
    Show the answer

    Answer: A. A one-way clutch The overrunning clutch holds the stator only during multiplication. Source: Write-up · Exhibit E (The one-way clutch) · Torque converters (IEEE Technology Navigator); Torque converter (Wikipedia); Torque Converter (IDC technical reference)

  5. Why do modern converters have a lock-up clutch?

    1. To reverse
    2. To eliminate slip and save fuel
    3. To start the car
    4. To cool the fluid
    Show the answer

    Answer: B. To eliminate slip and save fuel It locks at about a 0.9 speed ratio. Source: Write-up · Exhibit F (Lock-up) · Torque converters (IEEE Technology Navigator); Torque Converter (IDC technical reference)

Cite this case file

Easterling, Marcus. "Torque Conversion." The Detail Department (DetailEnjoyer), case file DD-075, last updated Feb 26, 2026. https://detailenjoyer.com/case/torque-conversion/

Permalink: https://detailenjoyer.com/case/torque-conversion/ · Last updated

Markdown version

Cite a single fact by its ID, for example DD-075-F01 → https://detailenjoyer.com/case/torque-conversion/#DD-075-F01

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