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Torque converters are the essential link that allow automatic transmissions to work and thus make the auto trans the most popular transmission choice in America. Modern torque converters are extremely efficient and good at their job, but there is always room for improvement in specific applications such as drag racing.
Before we discuss upgrading a torque converter, let’s talk about how they work. A torque converter looks a bit like a big metallic donut with a shaft coming out of the center (where the hole would be in a real donut). The torque converter bolts to a flat plate called a flex plate which in turn is bolted to the rear end of the engine’s crankshaft. That shaft coming out of the center goes into the transmission. The torque converter’s job is to take the place of the clutch that a manual transmission would have and pass power from the engine to the transmission yet allow for the car to sit stopped with the engine still running.
Conceptually, a torque converter can be thought of as two fans that point at one another. One fan is driven by the engine and the other fan is connected to the transmission. If you set two household fans up facing each other and turned one of them on, the other one would rotate as the driven fan blew fluid (air) at it. On a basic level, this is what the torque converter does. The part of the converter that is driven by the engine is called the impeller and the part that is rotated by the impeller is called the turbine. And instead of air the fluid of choice is a liquid, the same ATF that is used by the transmission. There is a third major component of a torque converter, the stator which is sandwiched between the impeller and the turbine, we will cover it in a moment.
In action, when the engine is running the impeller pumps fluid into the turbine which spins the turbine. If the car is idling at a stop, then the brakes prevent the turbine (which is connected to the transmission’s input shaft) from moving and the ATF fluid just circulates without imparting energy to the turbine. When the brakes are released and the throttle applied, the force of the pumped fluid begins to spin the turbine and the car moves. After spinning the turbine, the ATF fluid circles back to the impeller and the cycle starts over.
At steady state speeds such as on a highway, the impeller and turbine will turn at almost the same rate. But under acceleration the impeller spins much fast than the turbine. And here’s where the stator comes into play. When the impeller is spinning faster then the turbine the stator redirects fluid returning from the turbine into the impeller at an angle that increases the force of the fluid. It essentially captures energy that wasn’t used by the turbine and sort of recycles that energy into the impeller, thus increasing the torque multiplication of the whole device. So the stator is responsible for creating the torque multiplication of which a converter is capable.
Late model transmissions have a fourth component, the lock up clutch. Note that we said at steady state speeds the impeller and turbine “almost” turn at the same rate. That slight difference in rotation speed is lost efficiency. So a late model torque converter has a mechanical clutch that can be activated to lock the impeller to the turbine and maximize efficiency and thus improve fuel mileage.