Posted in  Powertrain, Tech Stories

Get Locked Up


Torque converters for circle track

By Brandon Flannery

Automatic transmissions have become a staple of circle track racing. Time and technology have brought those once-sloppy “slush boxes” from scorn to superiority. Unless the rules call for a transmission that matches what originally came in the car, the preferred automatic is the PowerGlide. Though long out of production, they enjoyed a revival in the 1980s that never stopped as racers and manufacturers learned how to tune them for the track. Their lower weight alone increases speed and reduces wear and tear on the valvetrain and drivetrain. A properly dialed-in Powerglide can use as little as 18 horsepower.

Transferring engine power to the transmission is done through a torque converter. They work much like a clutch, but use fluid pressure instead of a mechanical coupling. Engine speed and an internal pump cause the fluid to turn finned components that route the fluid against itself to drive the transmission.

742018_2Torque converters have a pre-set “stall speed,” which is the rpm level at which the converter becomes “locked up,” or transmits full power to the transmission. Below this, there is no grip and the transmission is disengaged from the engine. Unlike their mechanical counterparts, “fully locked” fluid couplings rarely achieve 100 percent efficiency. Average slip-page in a stock torque converter is 8 to 10 percent and can be as much as 12. Unwanted byproducts of this slippage include excess heat and power loss. At 10 percent, an engine turning 5,000 rpm can lose 500 rpm in the process. In a racing scenario, even 6 percent is considered high.

So how does a racer achieve a better coupling for circle track racing? We turned to Scott Miller of TCI for answers.


“Racers have to work within their track and class rules,” Miller says. “Many of them require a converter that looks like a factory stock piece. Finding a stock converter that has a low enough stall speed can be done, but it will slip and waste power.”

The solution? An aftermarket converter inside a stock housing that is purpose-built. TCI converters reduce slippage down to 2 to 3 percent. That translates to reduced heat and power when you need it.

The ideal converter should stall just enough to get the car in gear without killing the engine, generally between 1,000 and 1,600 rpm. That converter should be locked up above the stall speed anywhere on the track, including yellow flag restarts.

“On the track, you want the converter to act as much like a direct-drive unit as possible,” says Miller. “This will let you pick up the throttle on your turn exit and use all of the engine’s torque to power out of the turn. If your stall is higher than that point, you are giving up power.”

This differs from a drag racing converter that relies on a higher stall. Those engines stay in the power band from start to finish, with an average 1,000- to 1,500-drop in rpm between shifts. Circle track racers are on and off the gas, and even if peak torque is around 5,000 rpm, it would be wasteful to have the converter stall up to that point.

Additionally, a high stall can cause wheel spin as it “grabs” at a high engine rpm, and cause problems in the corners.

The logical answer would be to use a converter with the lowest stall possible. Unfortunately, torque converters with lower stalls are generally much larger, and a larger converter means more weight. Excess weight, especially when stretched out at the back of the crankshaft’s centerline, hurts acceleration. Smaller converters weigh much less, but often come with a higher stall. It becomes down to balancing stall speed and rpm.

Obviously, if the rules allow a smaller converter, one should. A 12-inch converter can weigh around 32 to 33 pounds with fluid, while a 10-inch unit checks in 6 to 10 pounds lighter. In terms of rotating weight, that is significant, particularly among classes with lower horsepower engines.

Thankfully the crew at TCI can build low-stall torque converters in 12-, 11- and 10-inch sizes to fit any rules. Their units come with precision-certified stall speeds to your specs and are strengthened on the inside to withstand the extreme pressure changes circle track racing creates.

“We add a heavy-duty stator and replace all thrust washers with bearings where possible,” says Miller. “We also create a stronger turbine and impeller pump by furnace brazing the fins, and for the circle track converters, we go back in and hand-tack them for added reinforcement. Stock fins are usually stamped into place and the ends folded over. Under racing stress, this is a very weak design, and the resulting flexing will cause unwanted slippage. Obviously, the stronger we can make those components, the more efficient our fluid coupling will be.”

Since torque converters generate a lot of heat, and heat is bad for the rest of the transmission, Miller feels an external cooler should be used whenever possible. Though some feel having the trans fluid ran through the bottom of the radiator helps bring the transmission up to operating temperature in the pits, Miller feels the trade-off of engine heat back into the system under racing isn’t worth it.

“Plus, in the event of a damaged radiator, water can get into the transmission’s fluid system and cause all kinds of damage,” he says. “It’s cheap insurance to separate the two systems.”

As always, the number of variables when selecting the proper torque converter are numerous and best results are going to be achieved by skipping the guessing step and calling somebody like the TCI TRANSHELP hotline to speak with one of their trained professionals. They’ll get you sorted out and on your way to victory lane in no time.



Cheating is a part of racing, especially when it falls into “creatively interpreting the rules.” There are some tracks that require a “stock appearing converter” and say nothing more. Racers there can use a “gutted” converter that is there to meet the rules, and pair it with a clutchless PowerGlide. This has the effect of working as a direct drive, and the significant reduction in weight helps to make power.

CPG_1693For those tracks that require a “functional, stock-appearing” converter, TCI does make a non-functioning, direct-drive “dummy converter” in 10- and 11-inch diameters that can be tailored a number of ways. Some tracks check for dummy converters by using a heat gun, and some have minimum amounts of fluid that need to be drained out for it to pass. For these, TCI adds a drain plug, allowing racers to fill them with fluid. Some racers under the heat gun rules have been known to add a little sand to the converter instead of fluid to create enough friction to fall within the heat specs. Generally, the amount of sand weighs less than the converter filled with fluid, and this saves weight, which makes power.

“We have even built a converter that pumped hot fluid from the transmission into the converter,” says Scott. “But by the time you do all that, as efficient as our race converters are, you’re not really gaining enough advantage to outweigh the risk.”

The question comes down to: What is it worth to you? Like everything in life, there are options. Even in circle track torque converters.



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