Several years ago, the idea of going circle track racing with an automatic transmission in your car was considered lunacy. Automatic transmissions back in the day weren't called "slush boxes" without a reason--they were inefficient, killed throttle response, and were a big horsepower drain.

Today, automatic trannies in race cars have become much more common as a way to keep costs down. Since it's nearly impossible to find a Monte Carlo (or any other GM metric chassis for that matter), a Thunderbird, or a Taurus with a stick, pulling a car out of a junkyard to transform it into a race car means you are also getting an auto tranny. To save money for racers in the lower classes, the rule books simply state that you have to race what came with the car--and that means the automatic transmission is now part of the race package.

You can race a stock transmission and get by with it, but there is no way a stock piece designed primarily for a smooth ride and low production costs is going to provide the same performance of a torque converter purpose-built for racing. There are other modifications that can be made to improve the performance of an automatic transmission, but swapping torque converters is by far the easiest and packs the most bang for the buck.

As a rule, stock torque converters are too large in diameter and too heavy. Both factors are critical in terms of performance because the converter essentially hangs off the back of the crankshaft and must be spun at engine rpm. Excess weight, especially when it's located at a greater distance to the centerline of the crank, makes it difficult for the engine to accelerate. Additionally, stock torque converters simply aren't built to withstand the additional horsepower and frequent wide-open-throttle conditions common in racing.

The Competitive Advantage

A torque converter that is purpose-built for racing may look similar to the stock version on the outside (except smaller), but there are many differences that make them more efficient when it comes to transferring power from the engine, through the transmission, and to the rear wheels. Proper design is critical because a torque converter has no direct mechanical linkage between its input and output portions. Instead, it uses the resistance of moving vanes through the automatic transmission fluid to provide the power linkage between the crank and the transmission. This is known as a "viscous coupling."

"There are several things we do to improve the performance of our torque converters over a stock unit," explains Scott Miller of TCI Automotive, one of the top manufacturers of high-performance automatic transmissions and torque converters. "All of our converters that we have for circle track applications use Torrington-style bearings instead of washers like in a stock converter. So everything turns more easily, which reduces drag in the converter and allows the engine to spin up more quickly. Combine that with the lighter overall weight and the smaller diameter of our converters, and you get significantly more power coming out of the turns.

"A second thing we do is all of our performance converters are furnace-brazed. The pieces inside the converter are furnace-brazed, which makes the converter stronger and more efficient. Under heavy loads--such as a more powerful race motor in full acceleration--the fins inside a non-furnace-brazed converter will flex. When that happens, the converter loses efficiency and less power is put to the ground."

Understanding Stall

As a torque converter spins faster, it gains efficiency until it reaches its stall speed. "Stall" is the speed, in revolutions per minute, at which a converter gains maximum efficiency. In stock car racing, there is no purpose in racing with your engine rpm below the torque converter's stall speed, because that means you are failing to put the greatest amount of your engine's power to the ground. Also, even after you have reached the converter's stall speed, there is still a certain amount of slippage because of the viscous coupling. In this area, however, all converters are not created equal. Stock converters may allow as much as 10 percent slippage after stall, while TCI's performance converters use tighter clearances and more aggressive designs to reduce slippage after stall is achieved to between 2 and 3 percent. For a racer, that's easy math.

To determine your minimum stall, you must be able to predict the minimum engine speed during racing conditions. This is usually the moment you pick up the throttle on turn exit--the time you need all the engine torque available to power out of the turn. If you race a converter with a higher stall than this rpm, you are going to be giving up power on the track. The easy solution would at first seem to be to pick the lowest stall converter available, but this creates problems of its own. As the stall number drops, design requirements usually force the converter to be larger. And, as we've already discussed, a larger converter comes with a greater moment of inertia, which makes it harder for the engine to accelerate. The fight becomes a battle between lowering the stall speed and keeping off weight.

Fortunately, torque converters purpose-built for racing can be more efficient at stall and smaller than their stock brethren. Among converters with the same design, the smaller unit will always have the higher stall numbers. "Normally, the smaller the converter, the higher the stall is," Miller says, explaining how TCI can build a smaller converter with a lower stall number than most stock units. "But we can change the fin angles and modify the stator inside and actually lower the stall. That helps when comparing a stock converter that's 30-plus pounds to one of our 10-inch converters. The modifications allow us to lower the stall level, but when comparing our 10-inch converter to our 11-inch version, the bigger unit is going to have the lower stall speed."