How Does a High Stall Converter Affect Gas Mileage?
Torque converters are fluid couplings that connect an engine's crankshaft to the input shaft of an automatic transmission. The torque converter's exact design specifications can affect acceleration, fuel economy and transmission longevity.
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Converter Function
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A torque converter works like a pair of fans inside of a housing; one fan is attached to the engine and the other to the transmission. At low rpm, the engine-side fan (impeller) pushes fluid through the gaps between the transmission-side fan (turbine). As rpm rises, the transmission-side turbine can no longer pass the fluid being pushed through by the engine-side impeller without moving.
Stall Speed
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The rpm at which the transmission-side turbine begins to move is called the "stall speed." A higher stall speed can allow the car to accelerate faster by holding off engine-transmission engagement until the engine is making adequate torque, but this acceleration comes at a cost. The higher stall speed is attained by increasing the size of the slats in the transmission-side turbine, which creates slippage and a loss of fuel economy under cruise conditions.
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Solution
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A higher stall speed won't significantly affect fuel mileage if you use a lock-up torque converter. A lock-up converter uses a number of internal clutches to positively lock the engine and transmission together under cruise, so the turbine's larger slats won't affect efficiency at all.
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References
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Comments
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if you run an efficient converter from a reputable company the slippage under part throttle won't effect mileage as much as you would think. Cruising at 3k with a 5500 stall converter or cruising at 3k with a 3500 stall converter will yield the same mileage. Converter slip with higher stall converters really only comes into play under full throttle. So yes if your driving around at full throttle your mileage may not be as good......
