A torque converter is the fluid coupling that sits between your engine and automatic transmission. It does two jobs: it lets the engine idle while the car sits still, and it multiplies torque when you launch. Here's how.
The core parts
- Impeller (pump): bolted to the engine side, it flings transmission fluid outward as the engine spins.
- Turbine: connected to the transmission input, it catches that moving fluid and starts to turn.
- Stator: the piece in the middle that redirects fluid coming off the turbine back into the impeller — this is what actually multiplies torque at low speed.
- Lockup clutch (on lockup converters): mechanically links the engine and transmission at cruise so there's no slip.
The three phases
Stall — at a stop, the engine spins the impeller but the turbine is held still by the brakes; fluid slips and the car stays put. Coupling — as you accelerate, fluid drives the turbine and the converter multiplies torque, helping the car get moving. Lockup phase — at speed, engine and transmission spin at nearly the same rate; a lockup clutch (if fitted) removes the last bit of slip.
Why it matters for performance
The converter's "stall speed" and torque-multiplication characteristics decide how your combination feels and performs. Get it right and the engine launches into its powerband; get it wrong and you either bog or slip away power and generate heat. That's why a converter should be matched to your specific engine, weight, gearing and use.