Torque Converter Fluid Coupling: Tell Me Something I Don’t Know!

When most racers discuss the torque converter application for performance and drag racing, the first words that come to mind are “stall speed.” Of course, when you look in a general racing catalog, the most standard of converter selections simply list generalities such as transmission application, converter diameter, and stall speed.

Though stall speed plays an integral role in operating your racing engine in its “sweet spot,” it is a total misconception that a converter’s stall speed is constant. The truth is that stall speed is a variable as a function of the engine’s constantly changing torque output as a race car travels down the track — that is where the science of torque converter “coupling” comes into play.

Ronnie Combs, Mechanical Design Engineer at Circle D Specialties, combines the company’s converter experience with new computer technology that aids with its designs.

Our engineering department is utilizing Computational Fluid Dynamics (CFD) software. This software allows you to work out a great deal of experimentation while avoiding the trial and error method of swapping the converter in and out with different internals. – Ronnie Combs

For a standard racing converter, the rule of thumb is to achieve a general 5- to 8-percent lockup-slippage at the finish line – depending on what type of racing you are competing in. This fluid coupling definition is the difference between engine RPM entering and exiting the converter. With a typical 1:1 ratio of a transmission in high gear, the slippage ratio is easily calculated (see figure 1.)

You can calculate the converter slippage ratio by deriving MPH to tire RPM — then calculating through the rear gear ratio. Our simplistic bracket car uses a data recorder tachometer with a driveshaft sensor that easily graphs the two RPM differences onto our laptop computer.

When you grab the typical off-the-shelf torque converter with an advertised 4,000 RPM stall speed, is that designed to reach the specific stall using a 600-horsepower small-block with mild performance torque? Or will it offer the advertised stall and proper lockup characteristics only for a power-added, 1,000-horsepower big-block with a monster amount of torque?

Any involved torque converter manufacturer is going to design and build your converter based around multiple questions, usually on an order form or a website page. The questions are myriad, such as application (street, street/strip, race only,) total vehicle weight, drive tire size, engine horsepower, and torque specs (many and very specific questions also based on cam, heads, and other components.

In total, a typical questionnaire, such as this excerpt from the Circle D Specialties can ask over 30-variables in which to factor a torque converter design to exactly match your car.

If a torque converter is “too loose” for your engine’s power output versus multifold variables (above) many racers refer to this problem as “driving through” the converter on the top end of the track by overpowering its slippage design and finish line RPM is too high.

On the flip side, an incorrect torque converter on the “tight” side that is made for more power than a racer’s combination actually has, will have racers chasing what they think are engine problems when the car does not perform because it is pulling down or struggling during a pass.

Computational Fluid Dynamics Software

Experimentation, software, and experience are a key combination that is paying off for Circle D Specialties.

“With a great amount of testing and logging of real-world changes over the years combined with this computer technology, we trust our work with the software pretty well,” Combs explains. “Though we can never replace the real-world input from our customers, the CFD software has well-overtaken any trial and error experimentation in the racecar.”

The one-way bladed disc between the pump and the turbine assemblies is the stator. The stator plays an integral part in a performance or racing converter that causes torque multiplication. Thus, the stator is highly modified in different manners to change converter characteristics. This on-screen experimentation is a key point where CFD can engineer pressures and fluid flow characteristics for racing.

Major auto manufacturers have been utilizing CFD software for years, and now Circle D is well into its fourth year of embracing the software technology.

“Auto manufacturers design new converters based around new engine specifications for fuel economy. But instead of fuel economy, we’re engineering better high-performance converters,” Combs says.

“We can control the torque multiplication a little more with the CFD software,” he adds. “Especially with the stator, we can relocate and alter shapes of the stator blades, and it will show us via the K-factor readouts what will happen.”

K-factor For Converter Engineering

The mathematical science resulting in the torque converter’s overall RPM stall is called the K-factor. This K-factor is understood from the math formula K-factor = RPM ÷ sqrt(Torque).

To define this general equation, let’s say your engine creates 500 ft-lb of torque at your converter stall of 4,000 RPM. The square foot of that is 22.36. If your converter stalls at 4,000 RPM, you divide 4,000 by 22.36, and you get a K-Factor of 178.89. Let’s say you increase your torque to 550 ft-lb with an engine upgrade. By reverse calculating the equation related to the same torque converter and K-factor, you can predict your stall to be 4,195 RPM.

K-factor numbers are often used in the world of OEM converters to refer to their overall efficiency in an effort for fuel economy. Those general calculations are used in the performance and racing world, but racing engineers like those at Circle D also specifically compute fluid dynamics and K-factor numbers between the converter’s individual components.

The CFD software allows the engineers at Circle D to apply a new shape and/or angle to a stator blade and analyze the fluid dynamics by the directional flow. It also allows for the relationship between pump, stator, and impeller to be reviewed for varying fluid pressures.

“Everything is based on the speed ratio of the converter,” Combs says. “So, if you are in a stall condition, you have a speed ratio of zero. If you were to have a perfect lockup with no difference between a torque converter’s input and output, the number would be 1. The differences between those two extremes are speed ratios our CFD software provides, and we work with.”

Remember, all components inside a converter are coupled with the transmission fluid flowing between each section. That is why these fluid dynamics are so critical. Even in a lockup converter, the fluid coupling effect needs to be spot-on up to the point of actuating the lockup in order for that overall system to be as efficient as possible.

Lock Up Transmission and Converters

Circle D not only hosts an impressive clientele list with non-lockup converters, but it also works with a huge list of later model transmission customers related to the lockup converter. Surprisingly, Ronnie Combs explains that the two technologies are less different than you might think.

Combs explains Tahakum Autosports’ recent accomplishments. “They have a very tricky 2,000-plus horsepower powerplant in front of their Powerglide transmission/converter that we developed. There is no room inside that converter for a traditional flat blade stator. The CFD software helped us narrow our development down to just two effective stator options allowing these guys to set the stock LS block record in their Mustang at 6.696 at 331 kph (205 mph) in the 1/4-mile.

“There are not many big differences between a lockup and non-lockup application,” Combs adds. “When we develop these stators, we’re not really concerned if you have lockup or not. Our lockup converters for performance offer great technology, but when it comes to stators, we want all the stall and fluid coupling to be equally efficient; it’s just one version that ultimately has the lockup passages and clutch assembly.”

In either lockup or non-lockup case, the converter design needs to reach the proper stall and then fluid coupling either at the finish line (non-lockup) or at the point of transmission lockup.

Educated Decisions

Choosing a torque converter based on just an advertised stall speed is like trying to hit a baseball blindfolded. You can swing the bat and possibly come in contact with the pitch, but the odds are equally as remote. Achieving an overall torque converter design without an experienced shop factoring in your complete racecar combination is as futile as just blindly ordering a described “3,000-stall converter.”

Add the option of computer technology eliminating so many other variables within the stator, pump, and impeller blades, and you are well on your way to hitting the ball out of the park when sliding in your next torque converter.

Article Sources

About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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