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Tech Answer

What does it mean when a torque converter "locks up"?

Lockup converters contain another part: a torque converter clutch. When the clutch engages, it allows the converter to lock the engine to the transmission input shaft, providing a direct 1:1 communication from the motor to the transmission.

Why is this necessary, or even desirable? In many ways, it comes down to fuel economy. Lockup torque converters have become popular since automakers have been stuck in a sort of Catch-22: Car buyers overwhelmingly prefer automatic transmissions, but automatic transmissions are not as fuel efficient as manual transmissions and automakers must meet government-mandated corporate average fuel economy targets. Overdrive transmissions have been one step along the path toward increased fuel economy from an automatic transmission. Overdrive transmissions allow the motor to spin at a lower rpm during cruising speeds. A higher final drive ratio (numerically lower) does the same thing.
Lock-up torque converters contain a torque converter clutch. When the clutch engages, it allows the converter to "lock" the engine to the transmission input shaft, providing a direct 1:1 engagement between the engine and transmission. Why is this desirable? Fuel economy. Automatic transmissions are not as fuel efficient as manual transmissions. Overdrive has been one step along the path toward increased fuel economy from an automatic because it allows the engine to spin at a lower rpm during cruising speeds. However, when the engine spins more slowly it causes increased slippage inside the torque converter, which in turn creates more heat. Heat within the converter reduces fuel economy and can harm both the converter and the transmission. The solution is to allow the converter to lock up at a 1:1 ratio. Lockup eliminates the slippage, which reduces heat and improves fuel economy.

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