Automatic Transmission Torque Converters

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Automatic Transmission Torque Converter
Automatic Transmission Torque Converter

In a vehicle with an automatic transmission, there is no clutch like in a manual transmission. Since there is no clutch, a torque converter is used to allow the vehicle to stay in gear, while stopped. This is accomplished by allowing the engine to turn, but the amount of torque is low enough that it will not pump enough fluid through the torque converter, and move the vehicle. Like a manual transmission vehicle, there is a flywheel attached to the engine. The torque converter cover, is attached to the flywheel,and it spins at the same speed as the engine. Inside the housing are fins, which make up the pump. These fins are attached to the housing but are spinning at a slightly faster speed than the turbine. The pump inside the torque converter works by pulling the transmission fluid through the center of the torque converter, inside of it and pushing it to the outside. This process creates a vacuum, which keeps the fluid constantly moving.

Once the pump starts pushing the fluid to the outside of the torque converter, the fluid enters the blades of the turbine, which is connected to the transmission, through an output shaft. The turbine is what causes the transmission to spin, allowing the vehicle to move. This is all accomplished by forcing the fluid to change direction, before it exits the center of the turbine. When the fluid exits the turbine, it is moving in a different direction than when it enters. In order to prevent fluid from coming in contact with the pump, a stator is used, if fluid came into contact with the pump as it is leaving the turbine, it would cause the engine to slow down, and decrease the efficiency of the process.

A stator is placed in the center of the torque converter. It redirects the fluid leaving the turbine, before it goes through the pump again. The stator blades design is very aggressive, which almost completely reverses the direction of the fluid. A one-way clutch (inside the stator) connects the stator to a fixed shaft in the transmission. Because of this arrangement, the stator cannot spin with the fluid -- it can only spin in the opposite direction, forcing the fluid to change direction as it hits the stator blades.

Common Questions

What stall speed that is right for my application

The advertised stall speed will typically need to be at least 500RPM higher than the beginning of the powerband of your camshaft. Almost all aftermarket camshafts state the RPM range. If your camshaft states an RPM range of 1500-6500 then a torque converter with a minimum of 2,000 RPM stall would be the one to use.

What is Flash Stall and Foot-Brake Stall

Of the two measurements Flash Stall is the most accurate. Foot-Brake stall is dependent upon too many variables. To determine the Flash Stall either one of the following process can be used.

Process 1 With the vehicle sitting still and idling in low gear, apply full throttle. As the vehicle begins its motion forward, notice the RPM on the tachometer. That is your Flash Stall. The engine should be very responsive from idle. If its not, camshaft timing and/or carburetor adjustments may need to be made in order for engine to be crisp from idle.

Process 2 With the vehicle in forward motion in high or drive gear and at its lowest mph where it will not kick back to a lower gear, apply full throttle while noticing RPM on the tachometer. This measurement of flash stall is best achieved with a full manual transmission.

What does the term Lock-Up Torque Converter mean

This term refers to a converter that contains an internal lock-up piston or device, either friction or mechanical. Transmissions such as TH350C, 2004R, 4L60 (700R4), 4L60E, 4L80E, AOD, AODE/4R70W and others, use these methods of eliminating slippage for an increase in fuel economy. Older transmissions such as the TH400, TH350, C6, C4 and others did not incorporate these methods of lock-up. The only way to increase fuel efficiency in these types of converters is to change clearances, redirect fin angles and usually lower the actual stall speed.

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