Whats What?
Aside from making your driving life easier, automatic transmissions are the performance weapons of choice for most people who like their pleasure a quarter mile at a time. The torque converter is an essential part of an automatic transmission and choosing the right one is fundamental to running fast. Understanding the basics of how they work can only help you make the correct choice, so here’s a brief overview of the inner workings of the common standard OEM torque converter.
While every manufacturer has its own take on torque converter design, they all possess four main components; impeller, turbine, stator and front cover. The impeller (or pump) consists of a circular array of radial vanes that form the rear half of the converter housing – the end closest to the gearbox. The turbine (which looks a lot like the impeller) is fixed to the spines of the gearbox’s input shaft and spins freely inside the front half of the converter housing – the end closest to the engine (Note, there is no mechanical connection between the impeller and the turbine). Sitting in between these two is the stator (or reaction member). Lastly there is the front cover, which bolts to the flex plate (or crank-mounted drive plate). Once the internals are assembled, the front cover and impeller (rear half) are welded together with a high degree of accuracy – you can’t get inside a torque converter without cutting it open along the weld.
The whole assembly is bolted to the flex plate on the back of the crankshaft so it spins as soon as the engine starts. The motion of the oil starts with the impeller. As the impeller is rotated by the engine (remember the impeller forms the rear half of the converter housing, the half facing the gearbox side), oil is drawn in through its center before inertia flings it out through its outer edges with considerable force, into the turbine’s vanes. These vanes are curved oppositely to the impeller, forcing the fluid to change direction as it enters the turbine. It’s this reversal of transmission fluid flow that forces the turbine to rotate.
Think of paddle wheels and water. Rotating a paddle wheel in a stationary body of water makes the water flow, while flowing water can make a paddle wheel turn. A torque converter works by combining these two actions. The engine drives one paddle wheel which pushed fluid onto the other paddle wheel causing it to turn.
Torque Multiplication
Torque multiplication during initial acceleration is one of the bonuses with a torque converter. 
Modern converters can multiply torque by two to three times. Torque multiplication is achieved by redirected oil exiting the turbine back into the input of the impeller – whereupon the process starts over again. Problem is, oil exiting the turbine is traveling in the wrong direction. Here’s where the stator comes in.
The stator’s blades are set at an angle which redirects the oil flow so that it acts against the trailing surfaces of the impeller blades, where its speed is converted into additional driving torque. Inside the stator is a one-way mechanical clutch which allows torque multiplication by holding the stator stationary when the turbine is spinning slower than the impeller. It also permits the stator to spin up to the speed of turbine/impeller, which improves efficiency.
The greater the difference in rotational speed between the impeller and the turbine, the greater the multiplication factor. As such, the opposite also applies.
Stall Speed
STALL SPEED is a very misunderstood term as many get it confused with converter slippage. In its purest term, if you locked the output (turbine) of the torque converter, stall speed is the RPM at which the converter places sufficient load on the engine to the point where the engine doesn’t have enough torque to further increase engine speed.
Therefore, the stall speed of a converter is not an absolute figure. A converter’s particular design will result in a range of stall speeds dependent on the amount of torque fed into it by the engine, which is also affected by the weight of the vehicle. For example, the stall speed for a given converter would be higher behind a torquey big-block than it would be with a small-block, and higher again if it’s pushing a heavy vehicle.
There is a common belief that converters always slip appreciably all the way up to stall speed. Not so, the impeller and turbine can reach similar speeds at quite low engine rpm. Throttle application and vehicle weight are the critical factors in determining torque converter stall speed RPM.
Determining Stall
In reality, you can’t determine stall speed by pushing hard on the brake and bringing the revs up, as any decent converter will multiply torque so much that it will easily overpower the brakes. If you can get to the rated stall in this way, you’ve probably got a pretty sloppy converter that will never reach maximum efficiency.
Torque converter specialists have unique machines for determining stall. However, the best way to test
the true stall speed of your converter is to do a flash stall test. This involves rolling down the road at about 15-20km/h and then standing on the throttle. The rpm to which the tacho immediately jumps to when the car locks up and takes off is your flash stall speed. Ideally, your converter should be set up so that the stall speed is pretty close or just below the rpm where your engine makes maximum torque. This will give you the greatest initial acceleration, and is why drag cars use high stall speed rated converters.
Lock-Up Converters
The turbine always turns slower than the impeller, which is why automatic transmission equiped cars, most notably older ones, have higher fuel consumption. Maximum efficiency is around 92-96 per cent and being a fluid coupling, you will never acheive 100 per cent efficiency. Lock-up converters use a hydraulically operated piston to press a friction/clutch plate (that’s coupled to the turbine) into contact with the inside of the front cover of the converter housing. This locks the two halves together, but only occurs as the speed of the turbine and the impeller converge.
A lock-up converter should not be confused with a trans brake. Trans brakes are used exclusively in drag racing to launch the car hard with good repeatability. They work by engaging first and reverse gear clutch packs simultaneously via a solenoid valve (coupled to a cabin mounted release switch). This locks the transmission in neutral, prohibiting drive to the back wheels. Now that you don’t have to rely on the foot brake to hold the car, you can get the revs up closer to stall speed. When the driver’s ready to go, simply release the button which disengages the gearbox out of reverse, leaving it hooked up in first and you launch so hard that it pins you back in your seat.
So, that’s about as simple as we can explain, what is a very complicated topic and hopefully it’s cleared up some of the mystery surrounding torque converters.
