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Transmission Components: Torque Coverters and Stall Speed

Essentially speaking the torque converter in an automatic transmission takes the place of a clutch in a manual transmission. An explanation of stall speed will help you understand how it works. Read it carefully.

Stall speed — The rpm that a given torque converter (impeller) has to spin in order for it to overcome a given amount of load and begin moving the turbine. When referring to “how much stall will I get from this torque converter”, it means how fast (rpm) must the torque converter spin to generate enough fluid force on the turbine to overcome the resting inertia of the vehicle at wide open throttle. Load originates from two places (1) From the torque imparted on the torque converter by the engine via the crankshaft. (This load varies over rpm, i.e. torque curve, and is directly affected by atmosphere, fuel and engine conditions.) (2) From inertia, the resistance of the vehicle to acceleration, which places a load on the torque converter through the drive train. This can be thought of as how difficult the drive train is to rotate with the vehicle at rest, and is affected by car weight, amount of gear reduction and tire size, ability of tire to stay adhered to ground and stiffness of chassis. (Does the car move as one entity or does it flex so much that not all the weight is transferred during initial motion?)

Another point concerning engine torque is that we are only concerned with what we’ll call the “relevant range” of the engine torque curve when discussing initial stall speed. This means if our particular torque converter chosen has a design that should produce a stall speed in a range of say 2000 to 2600 rpm given the application then we would refer to this as the relevant range of our interest in the engine’s torque curve for this particular torque converter. In other words, only the torque characteristics of the engine torque in this rpm range will affect the amount of stall speed we actually observe. If we are using a high horsepower/high rpm engine that does not make much torque before 3000 rpm, it does not matter that the engine makes excellent torque over 3000 rpm if we are trying to use the torque converter in this example because its relevant range is 2000-2600 rpm and we would expect to see poor stall (2000 rpm or less) due to the poor torque produced by the engine in this range.

A torque converter does not function in a void by itself. The torque converter is an integral part of the total vehicle combination. While many vehicle combination’s and applications are very similar and it may seem obvious what the best torque converter selection is, it is normally a wise step to take a look at the intended application and choose the best torque converter for the particular application.

That is about the simplest terms I could put stall speed into. I hope you understand it, you may have to read it a few times. Realistically speaking, it took me several years before it even made any difference to me how a converter works. Now that I understand, it is not so complicated.

Remember, if you buy a rebuilt transmission from, it will have a rebuilt torque converter with it.
Call 866-320-1182.

See Part-1 on Transmissions and Torque converters if you have not read that first..

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