The torque converter in an automatic transmission serves the same purpose as the clutch in a manual transmission. The engine needs to be connected to the rear wheels so the vehicle will move, and disconnected so the engine can continue to run when the vehicle is stopped. One way to do this is to use a device that physically connects and disconnects the engine and the transmission – a clutch. Another method is to use some type of fluid coupling, such as a torque converter.

Imagine you have two fans facing each other. Turn one fan on, and it will blow air over the blades of the second fan, causing it to spin. But if you hold the second fan still, the first fan will keep right on spinning.

That's exactly how a torque converter works. One "fan," called the impeller, is connected to the engine (together with the front cover, it forms the outer shell of the converter). The other fan, the turbine, is connected to the transmission input shaft. Unless the transmission is in neutral or park, any motion of the turbine will move the vehicle.

Instead of using air, the torque converter uses a liquid medium, which cannot be compressed – oil, otherwise known as transmission fluid. The spinning impeller pushes the oil against the turbine, causing it to spin. But if the turbine is held still (the car is stopped with the brakes applied) the impeller can keep right on spinning. Release the brakes, and the turbine is free to turn. Step on the accelerator and the impeller will spin faster, pushing more oil against the blades of the turbine and making it spin faster.

Once the oil has been pushed against the turbine blades, it needs to get back to the impeller so it can be used again. (Unlike our fan analogy, where we have a room full of air, the transmission is a sealed vessel that only holds so much oil.) That's where the stator comes in.

The stator is a small finned wheel that sits between the impeller and the turbine. The stator is not attached to either the turbine or the impeller – it freewheels, but only in the same direction as the other parts of the converter (a one-way clutch ensures that it can only spin in one direction). When the impeller spins, the moving oil pushes against the fins of the stator. The one-way clutch keeps the stator still, and the fins redirect the oil back to the impeller. As the turbine speeds up, oil begins to flow back to the impeller on its own (a combination of the turbine's design and centrifugal force). The oil now pushes on the back side of the stator's fins, and the one-way clutch allows it to spin. It's job now done, the stator spins freely and doesn't affect oil flow.

Because there is no direct connection in the torque converter, the impeller will always spin faster than the turbine – a factor known as "slippage." Slippage needs to be controlled, otherwise the vehicle might never move. That's where the stall speed comes in. Let's say a torque converter has a stall speed of 2,500 RPM. If the vehicle isn't moving by the time the engine (and therefore the impeller) reaches 2,500 RPM, one of two things will happen: either the vehicle will start to move, or the engine RPM will stop increasing. (If the vehicle won't move by the time the converter reaches the stall speed, either it's overloaded or the driver is holding it with the brakes.)

The stall speed is a key factor, because it determines how and when power will be delivered to the transmission under all conditions. Drag racing engines produce power at high RPM, so drag racers will often use a converter with a high stall speed, which will slip until the engine is producing maximum power. Diesel trucks put out most of their power at low RPM, so a torque converter with a low stall speed is the best way to get moving with a heavy load.

And now we get to one of the best-kept performance secrets: by altering the design of the torque converter, it is possible to tune the stall speed to match an engine's power curve. The Torque Converter is tuned to provide a stall speed that is optimal for your power system.

Torque converter slippage is important during acceleration, but it becomes a liability once the vehicle reaches cruising speed. That's why virtually all modern torque converters use a lock-up clutch.

What other ways are there to improve a torque converter? We've already discussed the use of a tuned stall speed. Another area that can be improved is the front cover, which is the side of the converter that faces (and is attached to) the engine's flywheel or flexplate.

Since the front cover connects directly to the engine, it is subject to incredible amounts of stress. Many stock torque converters use a stamped steel front cover because they cost less, but under high power loads they can bend or deform. The solution is to use a billet front cover.

Technically speaking, a billet part is something that is machined from a solid chunk of material. Some torque converter manufacturers use a solid disc and weld it to the sidewall, while others simply weld a reinforcement ring into the stock stamped-steel cover. This compromises the cover's strength and can cause it to warp under load. The strongest covers are precision-machined from a single piece of forged steel, which is then welded to the impeller to form the outer shell.
  – The turbine is what connects to the input shaft of the transmission via a splined turbine hub. Once the turbine starts to move then the vehicle will move.
  – The impeller is the outside half of the converter that is welded to the cover on the transmission side. The impeller is turned by the engines flexplate and fluid flow is started by centrifugally generating fluid flow inside the converter.
– The stator resides between the impeller and turbine. The stators job is to redirect the fluid back into the impeller after leaving the turbine. The stator houses a mechanical one way clutch commonly called a Sprague. This allow the stator to stay stationary while multiplying torque and will free spin once turbine speed reaches roughly 40% of impeller speed.
  – when the fluid enters the converter it is sent to the outside of the impeller centrifugally. Once the fluid leaves the impeller it feeds the outside fins of the turbine. This makes the input shaft move and therefore the car will drive. When the fluid leaves the turbine it is redirected back to the impeller via the stator. This is when torque multiplication occurs hence the name torque converter. Impeller blade angle and stator blade angle and blade count all denote how much torque will be multiplied in the converter.

Converter Stall

Let's start by illustrating how the stall speed works. Even under light loads, a vehicle with an automatic transmission will start moving as soon as you take your foot off the brake. The stall speed comes into play under all load conditions. When we talk about stall speed, we're referring to engine RPM. If the vehicle isn't moving by the time the impeller reaches the stall speed, either it will start to move, or the engine RPM will no longer increase. In other words, stall speed is the engine RPM at which the torque converter transfers the power of the engine to the transmission.

In the real world, the torque converter's stall speed roughly equates to the clutch engagement point on a manual transmission. Let's say you're driving your stick-shift car around town. Normally, you'd give the car a little gas and ease off the clutch pedal gently enough to get a smooth start. Likewise, under most driving conditions the torque converter will start delivering power to the transmission at relatively low engine RPM.

Now, let's say you need lots of power, either to make a fast getaway or to start with a heavy load. You'd rev the engine up to a point where it delivers more power before letting up on the clutch pedal. It's under those same circumstances that the stall speed becomes important. The torque converter will allow the engine to build RPM without turning the output shaft (the turbine) until the stall speed is reached.

How would you translate this to a torque converter? With a low stall speed.  Stall between 2,000 and 2,500 RPM – so with a heavy load, the torque converter won’t start turning the rear wheels until well beyond the engine's torque peak. In this case, the stall speed is too high - it is literally impossible to get the engine's full power to the rear wheels! In order to access all of the engine's potential power, the stall speed must be lowered.

Lowering the stall speed has another advantage: It reduces the transmission's temperature. Let's go inside a high-stall torque converter under heavy load. The impeller (input side) of the torque converter is spinning quickly, while the turbine (output side) is spinning slowly or not at all. The motion energy of the impeller is being converted into heat energy, most of which is passed on to the transmission fluid. The higher the stall speed, the more heat will be generated. Heat is the enemy of a transmission. You want to keep the fluid temperature as low as possible. With a lower stall speed, less time elapses before the motion energy of the impeller is converted to motion energy to drive the turbine, so the transmission runs cooler and lives longer.

What many people don't know is that the torque converter is a tunable device. Stall speed is determined by several factors, including the distance between the impeller and the turbine and the design of the stator. By properly modifying the converter's internal components, it's possible to alter the stall speed and create a torque converter that is tuned for a particular engine.



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