Torque Converter Selection - Transparent Performance

How many of you have an automatic transmission in your hot rod? Whether you love them or hate them, automatic-equipped machines are the most common, and one area that's sure to create confusion is the torque converter selection process. Choose the wrong converter and you'll end up with a dud at best and an undriveable car at worst. Following is a look at selecting the right converter for your car.

Stall Speed
Why increase stall speed? For high-performance applications, a higher stall speed allows the car to leave at a higher rpm (usually at a point where the engine is producing more torque). This translates into better performance, quicker elapsed times, and increased speeds. Because of this, the most critical component is the torque converter. That's no secret; the real mystery is how converters are rated, and how those ratings have an effect on your car's performance.

When discussing torque converter stall speed, there are two different terms: "Rated Stall" (it's common for enthusiasts to call this "foot brake stall") and "Flash Speed." What's the difference? Foot brake stall is just that. You load the converter by stomping one foot on the brakes and the other on the gas. Watch the tach. The rpm at which the converter overpowers the brakes is simply the "foot brake stall."

"Flash speed," on the other hand, is quite different. Flash occurs the instant you release the foot brake and the rotating inertia (which is "stored" in the engine flexplate) is released. In many cases, this flash speed can be anywhere from 500-2,500 rpm higher than the foot brake stall speed. In the case of a race car, a drag racer will stage at idle, hold the brakes, and "flash" the converter (flooring the gas pedal) the instant the last yellow on the Tree comes on. The same technique can be used with a street car to increase the engine rpm level when the car leaves a stop light.

According to B&M, flash stall can change just by reworking the load that the converter sees. For instance, if the car in question has a 4.10:1 rear axle ratio and the flash stall achieved is 2,800 rpm on a launch, a change to a 3.90:1 gear might see the flash speed increase to 3,000 rpm. Why? B&M claims the effective load on the converter went up. It also works the other way.

If the same car had the rear gear swapped for a 4.33:1 ratio, this would cause the flash stall speed to go down. B&M explains the reason for this is because the converter has begun to couple up progressively as the load it sees gets lower. Thus, our theoretical combination with a 4.30:1 gear would make the car easier to move when compared to the 4.10:1 or 3.90:1 combinations.

So far so good, but herein lies the grief: Some converter manufacturers advertise the stall speed of their converters by the "foot brake" method, while others prefer to use the "flash speed" figure. As you can imagine, this causes no end to confusion-and it's compounded even more by the complex array of variables, which contribute to converter stall speed. More on these variables later.

Converter Size
Generally speaking, the larger the torque converter, the more torque (and horsepower) it absorbs. Because of this, a larger converter will usually have less stall speed than a smaller-diameter converter. The Catch-22 is the fin angle inside the converter. Because of fin angle changes, it is possible (both in theory and in practice) to produce a converter in something like an 11-inch size that actually has more stall speed than a 10-inch converter. The stall speed changes gained by reworking fin angles obviously have some limits. You can only increase the stall speed so far before you're forced to go with a smaller-diameter converter.