Torque Converters - Converter Chronicles

You can tell quite a bit about a converter just by looking at it. Note the angle of the multiple tabs on the larger converter on the left versus the tabs on the smaller converter on the right. Note how the tabs on the small converter are angled in the opposite direction compared to the larger converter. These tabs position the fins in the converter and have been moved to increase the stall speed of this mainly because it was originally used on a low-torque four-cylinder engine.

You can tell quite a bit about a converter just by looking at it. Note the angle of the mu

The reason for increasing stall speed is directly related to the engine's torque curve. With a manual transmission, you are allowed the freedom to pick the starting-line rpm you want to apply when you dump the clutch. In a car with a good suspension and sticky tires, you can use extra rpm to put the engine closer to its torque peak, launching the car even more quickly. Adding stall speed to the torque converter increases the power available to accelerate the car more quickly.

Unfortunately, additional stall speed also increases the slippage in the converter. For a drag-race car, the cost of this additional slippage is worth the price, but for a street car it creates other problems. A high-stall converter like a 4,000-rpm unit is not only lazy from a part-throttle-efficiency standpoint, but it also generates much more heat. Heat is the enemy of any automatic transmission and will radically reduce its life span if not mediated by an efficient trans cooler.

The issues of slippage and heat are the main reasons that street converters should be approached with a serious amount of compromise when it comes to stall speed. A converter that might have a stall speed of 5,000 rpm for a pure drag car would be completely miserable on the street. Because of the lack of traction and limited tires, it's not critical that a street engine launch at peak-torque rpm. For most hot street engines, converters in the 2,200 to 2,600-rpm range are considered streetable but will still require a very efficient trans cooler to keep them out of the heat zone.

Another important part of the torque converter and stall-speed equation is converter diameter. Larger-diameter converters are usually more efficient and deliver less slippage, while smaller-diameter converters offer more stall speed and more slippage.

Vehicle weight is another major factor that should be considered when selecting a converter. Big cars weighing over 3,300 pounds generally need a converter with more torque multiplication. A flyweight 2,800-pound car like an early Mustang or Chevy II could use less multiplication to successfully launch on their relatively smaller rear tires.

With the advent of overdrive automatic transmissions like the TH200-4R, the TH700-R4/4L60E, the massive 4L80E, the Ford AOD, and all the rest, the new-car companies were striving for improved fuel economy using overdrive. Overdrive ratios drive the torque converter at speeds generally below normal coupling speed, creating increased slippage and heat, which is unacceptable. This created the demand for a lockup converter employing an internal clutch inside the converter to directly connect the turbine with the input shaft of the transmission just like a manual-transmission clutch.

The arrow points to the stator, the small propeller-shaped component that redirects fluid flow into the impeller at maximum stall speed to multiply torque. The stator is fitted with an overrunning clutch that allows it to freewheel once the turbine and impeller approach lockup.

The arrow points to the stator, the small propeller-shaped component that redirects fluid

Many aftermarket companies offer performance lockup torque converters that can provide a higher stall speed without sacrificing the advantages of the lockup converter for highway use. The downside to these converters is that they are heavier and more expensive than non-lockup converters.

It's also important not to undersell converter weight as a contributing factor to maximizing acceleration. The combination of a smaller-diameter and lighter converter means that less engine torque is required for acceleration. This is not a huge factor in the overall lower-e.t. scheme, but it does contribute.

This has been a quick jaunt through world of torque converters, but it should at least inspire some intelligent questions to ask your converter company of choice as to the best converter for your application. It's all about creating an overall tuned package that complements the rest of your car. That should make you and your car happy.