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Automatic Adjustment—Controlling torque converter slip with a dump-valve improves performance off the line and down the track


Written By Jason Reiss

Photography Courtesy Of The Manufacturers

We’ve all heard the old saying, automatic transmissions win races, and perhaps that’s never been more true than it is today. But how much better would that automatic transmission be if it could act like it had a pair of torque converters inside? With one small-diameter, high-stall unit that could help the car to get up on top of the transbrake (especially in turbocharged applications) and another, larger-diameter converter that could be used downtrack to minimize slippage and drive the transmission as hard as possible to maximize elapsed time potential and minimize wasted horsepower, it would be the best of both worlds.

Two torque converters, you say? Well, not really, but by using solenoids to open and close fluid flow circuits to starve and then fill the torque converter on demand, the behavior of the converter can be altered. You may have heard of this practice as I did, with people calling it a ‘dump-valve’ setup. So let’s explore…

This universal system from M&M Transmissions can be used on your choice of transmission; all of the functions are self-contained. M&M can give you a starting point for how to operate the system, and then it’s up to you to tune it to your given combination and track conditions.

First, remember that all of the horsepower the engine creates is transferred through the transmission fluid inside the torque converter on its way to the transmission’s gearset, so the more accurate the converter’s operation can be to the specific conditions, the more horsepower the driveline can relay to the track surface. But keep in mind that how a torque converter needs to behave on the starting line to get a car moving is not necessarily the most efficient way to transfer power at the big end of the track; it is this conundrum that is solved by controlling the converter slippage through the valve system.

Here’s one of M&M’s internal dump-valve systems installed into one of the company’s Turbo 400 transmissions.

Recent discussions with transmission gurus Mark Micke of M&M Transmission and Dave Klaput of Proformance Racing Transmissions detail exactly how this can be done using today’s technology, with the savvy mechanical control of transmission fluid application control making it possible.

Before we go further, let’s let Micke give us a quick rundown on how the fluid acts inside the converter.

“There’s no mechanical connection between the turbine and the pump—it’s only fluid. So what we’ll call torque converter pressure is the result of the difference between the pump and the turbine. Let’s say you’re on the transbrake at 4,000 rpm. The pump is turning at 4,000 rpm, but the turbine is not. There’s a pressure differential between the two; as the pump drives the turbine closer to its speed, the pressure will actually drop. What we’ve learned how to do is manipulate that pressure, regardless of what the pump and turbine are doing together,” says Micke.

Micke says that there are two different solenoids used in one of M&M Transmissions’ dump-valve configurations: the positive dump solenoid, which controls the amount of fluid going into the torque converter, and then the exhaust dump, which controls how much fluid comes out of the converter.

“We manipulate both; the positive dump has a pretty dramatic effect, because you’re restricting the fluid to affect the stall speed by cutting off the fluid supply,” says Micke.

Proformance’s external-dump system mounted to the side of a Reid-case Turbo 400 with all the bells and whistles inside.

“I chose a different path,” says Klaput explains of the Proformance design, which he says was the first on the market inside the transmissions of legendary drag radial racers Kevin Fiscus, the Bruder brothers, and Mustang Mike among others. Klaput notes that it is substantially different from that used by M&M, although the ultimate goal is the same.

“I spent an exorbitant amount of time on my transmission dyno figuring out what each feed orifice in the transmission does. The only way you’re going to control fluid is to have a sized orifice and know what the pressure is in that orifice. For example, if we have a transmission that has 250 psi of line pressure, I’m going to feed that through, let’s say a .149-inch feed restrictor, I’m going to move X amount of gallons of oil per minute. Once you figure out what that does, you can get a better angle on how to manipulate each converter and what each combination needs specifically. A twin-turbo big-block isn’t going to require the same type of feed restrictor that a single-turbo, small-block in X275 is going to need,” says Klaput.

He’s developed his own proprietary feed-side dump, which is a diverter valve. He machines an area into the pump that he calls the bathtub, it features two feed orifices; one off the main regulator valve that feeds the bathtub, and another that feeds the torque converter. The bathtub has a .250-inch channel that runs around and back into the valve body area, with a mechanical dump that turns it on an off. He says the recovery time on this is instantaneous and predictable, and his turbo customers have been extremely successful with this configuration.

Proformance modifies the transmission pump by machining an area into the pump, which provides access to the ‘bathtub,’ which features two orifices. By altering the lubrication circuit and tailoring the orifice size to one they feel will work best with your combination, Proformance is able to provide the racer with tuning guidelines for best performance.

The dump system, regardless of manufacturer, is especially effective with small-displacement engines that have difficulty spooling larger turbochargers, where horsepower capability goes from nothing to everything in the blink of an eye. The positive dump bleeds the fluid out of the converter, which forces it to be extremely inefficient and act like a small-diameter converter. This soaks up horsepower and will allow the turbo to spool up quickly. But once the engine is under full boost, a much tighter converter is necessary. This is where the exhaust dump valve comes into play. Think of it like this: with the valve wide open, you’ve got a large hole that exhausts the fluid from the converter. But with it clamped down, you’ve now dropped that orifice size substantially and restricted the fluid from getting out, which subsequently increases the pressure inside the converter to minimize inefficiency and maximize performance downtrack.

In years past, changing track conditions—and the change of seasons—necessitated the all-too-familiar torque converter swap circus. Micke says that’s basically a thing of the past, and as long as the torque converter is spec’d correctly for the application, one converter might be able to be used in all conditions rather than a racer carrying several different units.

The inner workings of one of Proformance’s Turbo 400 transmissions. As you can see, it’s configured differently than the M&M internal system. Ultimately, the concept is the same — control the torque converter to maximize the performance on the track. Proformance’s Dave Klaput says he’s tested each converter manufacturer’s parts individually, and each one requires a different configuration when it comes to setup of the dump system.

To his benefit, Klaput has worked with all of the major converter manufacturers to run their different converters and stator designs on his transmission dyno; this allows him to be able to decide the best setup for each given combination with confidence.

Regardless of the manufacturer of the system—be it Proformance Racing Transmissions, M&M Transmission, or another—the control can be handled by the racer’s choice of standalone vehicle management systems, as the capability exists within the packages offered by FuelTech, Haltech, Holley, and many others. There are dedicated controls for the valves; some tuners like to do control it based on time, while others prefer RPM-based maps.

“There’s no one way to do it. We can provide a starting point based on your combination, and then we can change from there. You run it differently in the summer than you would in the spring and fall,” says Micke.

“I machine this area and press in a puck to make a bridge to allow access to the ‘bathtub,’” says Klaput. “The tiny hole is there from the factory. It’s there to allow line pressure to the tip of the regulator valve, so it can move smoothly to allow consistent line pressure.”

For more input on the tuning side of how this all comes together, we enlisted input from JPC Racing’s Eric Holiday, who is one of the more prolific tuners out there, especially when it comes to the small-cube turbo combinations these systems benefit most.

“If you have a really good converter and you’re on a really good track, depending on the setup of the car, you may only need the dump to spool. And in less ideal conditions, you may loosen the converter to get down the track,” says Holliday.

But it’s not all roses. Even with all of his experience, he says it still took him the better part of a year to master the use of the system and understand what to do and when to do it. Working hand-in-hand with Klaput while tuning customer cars has helped him to understand what he should be looking for and why.

“He’s specific. He tells me that we need to target X amount of cooler line pressure on the transbrake with a turbo car, we need to stay under this number in high gear, and then you have to use his tools to achieve these different pressure numbers throughout the run,” says Holliday.

Holliday says the main sections to look at when tuning and analyzing run performance are driveshaft speed, engine rpm, g-meter readings, boost, and cooler line pressure.

For example, he was tuning Tony Hobson’s car in NMRA Renegade last season. He says the difference of 15 pounds of cooler line pressure on the transbrake could be the difference between a 1.12 short time and a 1.17 short time with the exact same tune-up.

“If your cooler line pressure is too low, you’re effectively blowing through the converter—you’re not putting any power down, and it’s just slow. But if you have too much, it’s super-aggressive almost like a stick shift, where it wants to lock up right away, and it can be detrimental on the starting line as far as traction is concerned. Finding that sweet spot of what works best for your car, your converter, power, and tune-up style is pretty critical,” he says.

“It comes down to feeding the torque converter properly. There’s lots of ways to skin the cat,” sums up Klaput.

With the proper quantity of food, you can keep even the most ornery combination from getting hangry; instead, set yourself up to win races with one torque converter that acts like two when you pop the button on the ’brake.


M&M Transmissions

(573) 363- 4136


Proformance Racing Transmissions

(815)  308- 5161


JPC Racing

(410) 729-0005