2026-08-20
A torque converter is the fluid-filled donut that sits between the engine and the transmission in every automatic-transmission vehicle. It does two jobs a mechanical clutch can't: it lets the engine idle while the wheels are stopped, and it multiplies torque during acceleration — sometimes by 2× or more.
Inside the housing, three bladed wheels spin in transmission fluid:
When the turbine is slow relative to the impeller (launching from a stop), the stator is locked stationary and redirects fluid to add energy to the impeller's flow. The torque at the turbine can exceed engine torque by a stall torque ratio of typically 1.8–2.5. As the turbine catches up to the impeller (cruising), fluid hits the back side of the stator's blades, the sprag clutch releases, and the stator freewheels. Torque ratio drops to 1:1 and the converter behaves like a simple fluid coupling.
The efficiency problem: a torque converter always has slip — the turbine never quite matches impeller speed. That slip becomes heat. At highway speeds this wastes fuel, so every modern converter includes a lockup clutch that mechanically bolts the turbine to the housing above ~40 mph, giving a direct 1:1 mechanical connection.
Rule of thumb — stall speed: the RPM at which the converter would hold the engine if the output were locked. A stock passenger car runs 1800–2200 RPM stall. A drag car with a "high-stall" converter (3500+ RPM) lets the engine reach its torque peak before the car moves — great for launches, terrible for fuel economy.
Concrete example: A 4.0L Jeep Wrangler engine makes 235 lb-ft. Its torque converter has a 2.2:1 stall ratio. At launch, the transmission input sees up to 235 × 2.2 = 517 lb-ft — before the first gear (2.84:1) and axle (3.73:1) even multiply it further. That's how a modest engine yanks a 4,500 lb vehicle off the line.