TORQUE CONVERTER

How torque converters work

Torque converters make automatic transmission possible.

       If you know about manual transmissions, you would know that an engine is connected to a transmission by way of a clutch. Without this connection, a car would not be able to come to a complete stop without killing the engine. But cars with an automatic transmission have no clutch that disconnects the transmission from the engine. Instead, they use an amazing device called a torque converter. It may not look like much, but there are some very interesting things going on inside. What I hope to show you is why automatic transmission cars need a torque converter, how a torque converter works and what some of its benefits and shortcomings are.

       Just like manual transmission cars, cars with automatic transmissions need a way to let the engine turn while the wheels and gears in the transmission come to a stop. Manual transmission cars use a clutch, which completely disconnects the engine from the transmission. Automatic transmission cars use a torque converter.

       A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal. If you were to step on the gas pedal while the car is stopped, you would have to press harder on the brake to keep the car from moving. This is because when you step on the gas, the engine speeds up and pumps more fluid into the torque converter, causing more torque to be transmitted to the wheels.

       As shown in the figure below, there are five components inside the very strong housing of the torque converter:

• PUMP
• TURBINE
• STATOR
• FLUID
• CONVERTER CLUTCH

The parts of a torque converter (left to right): turbine, stator, pump

       The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the engine is running at. The fins that make up the pump of the torque converter are attached to the housing, so they also turn at the same speed as the engine. The cutaway below shows how everything is connected inside the torque converter.

How the parts of the torque converter connect to the transmission and engine

       The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside, much as the spin cycle of a washing machine flings water and clothes to the outside of the wash tub. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center.

The pump section of the torque converter
is attached to the housing.

       The fluid then enters the blades of the turbine, which is connected to the transmission. The turbine causes the transmission to spin, which basically moves your car. You can see in the graphic below that the blades of the turbine are curved. This means that the fluid, which enters the turbine from the outside, has to change direction before it exits the center of the turbine. It is this directional change that causes the turbine to spin.

The torque converter turbine: Note the spline in the middle. This is where it connects to the transmission.

       In order to change the direction of a moving object, you must apply a force to that object -- it doesn't matter if the object is a car or a drop of fluid. And whatever applies the force that causes the object to turn must also feel that force, but in the opposite direction. So as the turbine causes the fluid to change direction, the fluid causes the turbine to spin.

      The fluid exits the turbine at the center, moving in a different direction than when it entered. If you look at the arrows in the figure above, you can see that the fluid exits the turbine moving opposite the direction that the pump (and engine) are turning. If the fluid were allowed to hit the pump, it would slow the engine down, wasting power. This is why a torque converter has a stator.

The Stator
The stator resides in the very center of the torque converter. Its job is to redirect the fluid returning from the turbine before it hits the pump again. This dramatically increases the efficiency of the torque converter.

The stator sends the fluid returning from the turbine to the pump. This improves the efficiency of the torque converter. Note the spline, which is connected to a one-way clutch inside the stator.

      The stator has a very aggressive blade design that 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 (the direction that the clutch allows the stator to spin is noted in the figure above). Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades.

       Something a little bit tricky happens when the car gets moving. There is a point, around 40 mph (64 kph), at which both the pump and the turbine are spinning at almost the same speed (the pump always spins slightly faster). At this point, the fluid returns from the turbine, entering the pump already moving in the same direction as the pump, so the stator is not needed.

      Even though the turbine changes the direction of the fluid and flings it out the back, the fluid still ends up moving in the direction that the turbine is spinning because the turbine is spinning faster in one direction than the fluid is being pumped in the other direction. If you were standing in the back of a pickup moving at 60 mph, and you threw a ball out the back of that pickup at 40 mph, the ball would still be going forward at 20 mph. This is similar to what happens in the turbine: The fluid is being flung out the back in one direction, but not as fast as it was going to start with in the other direction.

      At these speeds, the fluid actually strikes the back sides of the stator blades, causing the stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving through it.

Benefits and Weak Points

      In addition to the very important job of allowing your car come to a complete stop without stalling the engine, the torque converter actually gives your car more torque when you accelerate out of a stop. Modern torque converters can multiply the torque of the engine by two to three times. This effect only happens when the engine is turning much faster than the transmission.

      At higher speeds, the transmission catches up to the engine, eventually moving at almost the same speed. Ideally, though, the transmission would move at exactly the same speed as the engine, because this difference in speed wastes power. This is part of the reason why cars with automatic transmissions get worse gas mileage than cars with manual transmissions.

       To counter this effect, some cars have a torque converter with a lockup clutch. When the two halves of the torque converter get up to speed, this clutch locks them together, eliminating the slippage and improving efficiency and also reducing HEAT.

STALL SPEED

       Torque converter stall is a commonly used term and is commonly misunderstood. Stall is the speed at which the converter will hold the engine speed and not allow further gain (i.e., the engine "stalls"). The key word here is engine. The speed at which stall occurs with a given converter is a function of engine peak torque. It is clear that the stall speed on a given converter will not be the same coupled to a tame small block engine when compared to a big block with all of the muscle features added. When comparing stall speeds it is important to account for the engine that drives it. True converter stall can best be determined when a Transbrake is used. Testing for stall value by locking the wheel brakes generally does not produce a true stall value because the engine power can often cause wheel turn by overpowering the brakes. Stall speed determined by this method should be identified as such when discussing stall speed determination. Flash stall is determined by launching at full throttle and observing the peak speed attained at launch. Selection of the right stall speed for your vehicle should be matched to the engine peak torque, engine torque curve shape and vehicle weight. In general, the stall speed selected for your converter would be 500 to 700 rpm below the peak torque. This speed allows the margin for application of the torque reserve on takeoff. When selecting stall speed without having prior experience to go by, it is better to conservatively estimate the engine torque than it is to over estimate it. If you over estimate the torque output you will have a converter with a stall speed too low, making your car slow off the line and have slow ET. A properly selected stall speed will give you better launch and better ET. You can see why it is important to consult with professionals prior to making a stall speed selection. Within the converter, stall speed is balanced off against inefficiency after launch. Getting desired stall at the expense of performance after launch is just as costly as improper stall speed to begin. The optimum converter has careful selection and design of changes to the impeller, turbine and stator.

HOW CAN I DETERMINE THE STALL SPEED OF MY CONVERTER?

Stall speed is very difficult to determine unless your car is equipped with a Transbrake to lock your drive train. Testing stall speed by holding the wheel brakes and running the engine against the locked brakes will usually result in wheel rotation before true stall speed is reached. The engine simply overpowers the ability of the brakes to hold the car. When rotation starts you are no longer at stall. For this reason people talk about brake stall which is not a true stall at all. An alternative method of measurement is to launch at wide open throttle and observe engine RPM reached at launch. This is flash stall.

WHY IS PROPER STALL SPEED IMPORTANT?

Stall speed should be matched to engine performance, the car weight, tire size and gear ratio. One should consult their camshaft grind for the specs to the camshaft they are using as that will help determine the RPM's of the power band. Knowing when your engine starts into it's power band helps in selection of the "stall speed". Also do not forget that a 3000 stall in one car doesn't mean it will stall 3000 in another. Everything works together in determining the stall. Proper selection of stall spec will make for quicker launch, better 60 ft. time and better ET. It is very important that these parameters be specified when ordering a converter to assure satisfaction.

IS STALL SPEED THE ONLY CONSIDERATION IN SELECTING A CONVERTER?

While stall speed is very important it is by no means the only consideration when selecting a converter. Torque multiplication at launch and high end efficiency are equally important. Stall speed can be attained in many ways that cripple the converter in other ways. Stall speed can be obtained at the expense of looseness at low speeds and loss of performance at higher speeds after launch. You want a converter that produces the right stall without sacrificing performance down the street or down the strip.

DOES STALL SPEED AFFECT NORMAL STREET DRIVING?

Normal driving is not affected by stall speeds up to approximately 3000 RPM, particularly with a good converter. A car will begin to roll normally when a higher stall speed converter is used. Quick acceleration will be favorably influenced by stall speed. A very high stall speed (above 3000 RPM) would not be satisfactory for street use.