With a traditional manual transmission, engine output is routed through a clutch mechanism to a geardrive assembly. However, in front of every automatic transmission sits a torque converter used to provide the transfer of power from the engine to the internal gearsets in the automatic transmission. The torque converter transfers this power through pressurized fluid flow.
The torque converter housing is bolted to a flexplate or flywheel attached to the rear of the engine crankshaft. Via fluid coupling, power is transmitted from inside the torque converter to a splined input shaft on the automatic transmission. The converter also serves as a torque multiplier (by a factor of 2 or more) as you accelerate from a standstill and allows the engine to idle in gear without moving. It also allows for smooth gearshifts and drivetrain slippage momentarily while gear changes are made without affecting vehicle speed.
There are three major components within a torque converter: impeller, stator, and turbine. The housing bolts to the engine flexplate and provides an enclosed area for these parts to perform their fluid coupling function. The impeller, sometimes called the pump, is attached to one half of the housing. The turbine mates directly to the splined transmission input shaft. Inside each of these two pieces that face each other is a series of metal fins where the transmission fluid circulates. As the impeller spins, it directs fluid to the turbine, which also begins to spin and push fluid back toward the impeller.
In between the impeller and turbine lies the stator. The stator has the job of redirecting the fluid coming from the turbine such that it flows into the impeller in the same direction that the engine is rotating. This reverse operation helps assist the engine rotation. Within the stator is a one-way clutch that allows this action to complement engine rotation but freewheel as vehicle speed increases.
Stall SpeedA torque converter's stall speed is basically the maximum engine rpm that can result from having the transmission in gear with the brakes locked up. In other words, it is the torque converter speed needed to overcome the inertia of the vehicle at rest while revving the engine. Stall speed can have a great effect on how your rig performs. A quick look through any performance catalog will reveal converters with stall speeds ranging from about 1,500 to over 3,000 rpm. A stock converter will typically have a stall speed in the 1,500- to 1,800-rpm range.
As you increase performance of your engine and push the peak torque curve higher in the rpm range, it is usually best to change to a torque converter with a higher stall speed. This allows the transmission to engage at the higher engine speed to more properly match where your engine is making good torque.
Since the transmission is essentially slipping until you reach the stall speed of the converter, it is best not to install a converter with too high a stall speed. Slippage produces heat, which is the biggest enemy of transmission fluid. For instance, a trail rig that is used at slow speeds will probably best benefit from a stock-style converter with a low stall speed. However, a mud racer with a hot engine would perform better with a high-stall-speed converter that allows torque and rpm to build before hooking up power to the transmission gears. Going to a higher-stall-speed converter usually necessitates a larger cooler to keep fluid temperature under control.
Lockup ConvertersIn the late '70s, lockup torque converters started to appear in production vehicles. The function of the lockup is to provide full mechanical coupling in the torque converter when the vehicle is running in high gear under part-throttle conditions. Eliminating the fluid coupling under these conditions reduces fluid temperature and increases fuel economy because slippage is entirely eliminated.
The mechanical connection is provided by a friction pressure plate inside the converter housing. When the vehicle cruise conditions are met, a converter clutch valve is actuated, allowing fluid pressure to mechanically couple the housing to the turbine. The action is usually controlled electronically by the vehicle's computer. When the vehicle slows or the throttle is applied, the converter may be commanded to unlock and resume normal fluid operation.
Torque Converter ChoicesA wide array of converters is available. Choice involves selecting stall speed, build quality, and sometimes specialty application modifications. Picking the right one for your vehicle can be confusing; however, careful thought concerning your specific application, combined with a recommendation from a supplier, can help you find a combination that works well for you.
With regard to stall speed, there are a number of factors that can affect the actual stall speed within a given vehicle. Among these are vehicle weight, tire size, gear ratio, engine size, transmission ratio, and brake setup. Actual stall speed will vary from stated specifications in the actual application based on these factors. For instance, lightweight vehicles may find the need to use a slightly higher stall converter to compensate for lighter-than-average weight. Conversely, a heavy 4WD with massive tires would probably benefit from a lower stall speed.
We've dissected a typical torque converter to give you a detailed view of the parts that are involved with transferring the torque from your engine to your automatic transmission.