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Converter Confusion – Part One
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Why is stall speed so important?
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By Wayne Scraba Photos by J.W. Performance Transmissions |
Wander through the pits of any drag race, large or small, and count the numbers of stick-shift transmissions. Not many. The overwhelming majority of racecars are now automatic-equipped. There's a reason or two for this: Automatics are generally consistent. Automatics are almost always easy on parts. Automatics are more cost-efficient than sticks. Automatics generally require far less maintenance.
So far, so good, but one place inside an automatic combination that can create considerable confusion is the torque converter. If you chose the wrong converter, you'll end up with a slug at best and a non-raceable car at the worst. We've all seen the poor soul who can't even get his or her car to stage, let alone make a lap down the 1320. What follows is a detailed look at choosing the right converter for your racecar. Increasing the Stall Speed Of The Converter… Let's start at the beginning: What's the purpose of increasing the stall speed of the converter? Increasing the stall speed allows the car to leave the starting line at a higher rpm (usually at a point where a modified engine is producing more torque). That translates into a quicker ET slip and increased speed numbers. Because of this, the most critical component in the performance mix of an automatic-equipped racecar is the torque converter. The real secrets though, are how converters work, how they're rated, and how these ratings have an effect upon the performance of your particular combination. The combination is the key. The engine, engine tune-up, torque converter, transmission, rear-axle ratio, slick size and a host of other variables make up this “combination”. But we'll get back to the big picture later. When considering torque converter stall speed, there are two different terms (and, consequently, two entirely different numbers) used: "Rated Stall" (it's often referred to as "foot-brake stall") as well as "Flash Speed." What's the difference? 
Foot-brake stall is just that: You load the converter by stomping one foot on the brake pedal 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 very different. Flash occurs the instant you release the foot-brake and the rotating inertia (which is "stored" in the engine-flexplate-converter) is released. This flash speed can be anywhere from 500-2500 RPM higher than the foot-brake-stall speed. In a racecar, the typical “flash –the-converter process” usually works like this: Stage at idle, hold the brakes with one foot and "flash" the converter (flooring the gas pedal with the other foot, and simultaneously releasing the foot-brake) the instant the last yellow on the Christmas Tree comes on. The sensation is a form of “slingshot” effect as the car leaves. Some cars work better by flashing the converter. Some combinations work better by outright stalling the converter. Some work better with trans brakes. Others work better off the foot-brake.
Flash stall can change just by reworking the load that the converter sees. For instance, if the car in question has a 4.88:1 rear-axle ratio and the flash stall achieved is 2,800 RPM on a launch, a change to a 4.10:1 gear might see the flash speed increase to 3,000 RPM. Why? 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 5.13:1 ratio, the flash speed will have a tendency to go down. The reason for this is, the converter has begun to couple up progressively as the load it sees gets lower. Thus, our theoretical combination with a 5.13:1 gear would make the car easier to move when compared to the 4.88:1 or 4.10:1 combinations, but the car would experience less flash speed.
So far so good, but there is something muddled here: 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 that contributes to converter stall speed. More on these variables next issue. Watch for it. Special thanks to J.W. Performance Transmissions for supplying critical information provided in this article: J.W. Performance Transmissions 1826 Baldwin Street Rockledge , FL 32955 PH: 321-632-6205
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