One particular advantage a torque converter has over a conventional fluid coupling — and what makes it ideal for use in automatic transmissions — is the fact that it can multiply the amount of torque it generates as the engine provides more power. A real-world example of this is the comparison of the relatively light pressure that must be applied to a brake pedal to keep a car stationary while idling, when compared to the increase in pressure needed to keep it still when gas is also applied. At very low speeds torque can be multiplied two or three times by a torque converter.
One of the major downsides to torque converters, as opposed to normal fluid couplings, is that, given how the pump and turbine never spin at exactly the same speed, some power is always wasted. This, along with its typically heavier weight, is the reason that manual transmission vehicles tend to get better fuel mileage than those with automatic transmission.
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