Torque Converters

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working principle


Torque Converters

PurposeAllow the vehicle to come to a complete stop without stalling the engine Provide torque multiplication to allow smooth acceleration from a stop House a torque converter clutch which will eliminate torque converter slippage at highway speeds

Parts of a Torque Converter


HubSplit ring guide Split ring guide Stator one-way clutch Apply piston




Torque converter clutch

Parts of a Torque Converter

Principles of Operation

A torque converter is a type of fluid coupling There is no direct mechanical link between the input (engine flywheel) and the output (transmission input shaft) The impeller (pump of the torque converter) forces fluid through the turbine, which forces the turbine to turn

The turbine is splined to the transmission input shaft

Principles of OperationImpeller Turbine

Fluid Coupling Problems

When there is a large difference in RPM between the impeller and the turbine in a fluid coupling, the fluid coming off the turbine strikes the impeller opposite the direction of rotation, thus slowing the impeller down (robbing power)

Fluid Coupling Problems

Fluid Coupling Solutions

By incorporating a stator into a fluid coupling we can overcome the problem of turbine discharge oil slowing down the impellerStator

Fluid Coupling Solutions

Split Ring Guide

Guides the fluid flow during vortex flow conditions

Phases of Operation

Torque multiplication

Relatively low impeller (engine) RPMs Stator is locked into place by its one-way clutch Vortex fluid flow within the converterOccurs at approx. 35-40 MPH under normal driving conditions No torque multiplication Stator is freewheeling Turbine is spinning at approx 90% of impeller speed Rotary flow within the converter

Coupling phase

Torque Multiplication

Because the turbine discharge oil is redirected so that it hits the impeller in the direction of impeller rotation, it helps the engine turn the impeller.

This is what causes torque multiplication

Torque Multiplication

Torque converters can multiply torque at a 2:1 to 3:1 ratio

Exact amount depends on the design of the impeller, stator, and turbine and impeller RPM The point at which maximum torque multiplication occurs is near the stall speed of the converter

During the torque multiplication phase, turbine speed is significantly lower than impeller speed A torque converter attached to an engine producing 200 ft/lbs of torque would deliver 500 ft/lbs of torque to the input shaft of the transmission (with a 2.5:1 torque multiplication ratio) Fluid flow is vortex

Vortex Flow

During vortex flow the fluid is circulating from the impeller to the stator to the turbine and then back to the impeller

Cross-Section of Torque Converter

Vortex Flow

Torque Multiplication600 3 500 2.5









0 1000












0 4000

Engine RPM Engine Torque Input Shaft Torque Torque Multiplication

Torque Multiplication Ratio

Ft/Lbs Torque

Stator Operation

Coupling Phase

As turbine (vehicle) speed increases and approaches the speed of the impeller the turbine discharge oil is accelerated to the point that it no longer strikes the front side of the stator blades, instead it strikes the backside of the stator blade causing the stator one-way clutch to unlock and the stator to freewheel

Since the stator is unlocked, fluid is not redirected and no torque multiplication occurs Fluid flow is rotary

Rotary Flow

As the speed of the turbine approaches the speed of the impeller fluid flow switches from vortex to rotary After the fluid is discharged from the turbine it is not redirected by the stator, instead it rotates with torque converterFront-View of Torque Converter

Rotary Flow

Stator Operation

Comparison Of Flow Directions

Converter Phase Tradeoffs

Stall Speed

Stall speed is the engine RPM at which the torque converter has coupled enough that with the wheels locked the engine is not able to increase RPM any further

Stall Speed

Types of stall Speeds True stall This is the maximum rpm the engine can attain with the driveline completely locked Generally can only be attained with a trans-brake Brake stall This is the maximum rpm the engine can attain with the brakes applied The brakes generally will not have enough holding power to allow the engine to reach true stall speed Flash stall This is the rpm at which, when you accelerate at full throttle from a dead stop the engine RPM flashes to TCIs recommended method of testing stall speed

Stall Tests CAUTION


not brake stall a converter for more than 10 seconds at a time. During a brake stall 100% of the power developed by the engine is converted in heat in the torque converter. Wait at least 2 minutes between brake stall tests

Factors Affecting Stall Speed

Vehicle Weight Vane/Fin Angle Horsepower Impeller to Turbine Clearance Powerband/Camshaft Stator Design Torque Rate Converter Diameter Gear Ratio

Vane DesignDepending on the design and pitch of the blades, the impeller will scoop the most oil at a specific RPM, thus altering the stall speed

Stator Design

By altering the stator design, stall speed and torque multiplication ratios can be alteredAftermarket Stock

Stator Design

Selecting the Proper Stall Speed ConverterFor non-stock engine-vehicle combinations the stock torque converter may not be optimal Work with a torque converter company to choose the correct torque converter Remember Torque converter design involves trade-offs

Torque Converter Design TradeoffsLow Stall Speed Converters High Stall Speed Converters

Torque Multiplication Efficiency Heat Generated Best Application

Low (2.0-2.5) High Not as Much Engines producing a Lot of torque at low RPMs

High (2.5-3.0) Low A lot Engines producing power at high RPMs