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In the modern era of performance, racers have learned to leave no stone unturned. Gone are the days of guessing which jets to run, where to shift, or what gear to run. Thanks to today’s data-logging computers, gathering the necessary information to make educated tuning decisions is much easier than in decades gone by.
With hard data, some tuning is straightforward, while other decisions are best made with solid driver feedback and experience. It’s usually the latter combination that gets you to the winner’s circle. There are, however, areas when experience and data weigh equally. One such is the torque converter. This nifty device, which is technically a fluid coupler, has a massive effect on performance in a drag racing vehicle.
Torque converters have the ability to improve how your vehicle performs if you understand how to select the right one for your build.
What A Torque Converter Does
Unlike a clutch, a torque converter must provide slip, so the engine can remain running in gear at idle without stalling. It also provides a level of slip at part throttle, and a high level of coupling at wide open throttle. Further complicating the matter is designing the ability to hold a desired RPM on the starting line. Now consider that depending on category, there can be a wide range of launch RPM and an equally diverse range of horsepower, torque and vehicle weights, thus, making it a real challenge to find the perfect converter.
Because of this, torque converters have become highly specialized, with manufacturers developing different sizes and styles to work at peak efficiency with everything from mild performance applications to 5,000 horsepower Pro Modified race cars. The current racing climate is so competitive that racers never stop looking for better efficiency. And this can lead you down some interesting paths as you determine the “right” amount of slip and coupling power from a torque converter.
Depending on your combination and style of racing, a torque converter can be used as a tuning tool.
“Torque converter performance can be complicated to understand,” said Marty Chance of Neal Chance Racing Converters. “That’s because the converter plays a few roles. It allows the engine to remain running while the vehicle is stationary, yet in gear, it allows slip or stall for best starting line performance, and more controlled slip or even lock-up during acceleration. You could say its efficiency changes on demand,” he explained.
A racer can make adjustments to their converter at the track if needed to change how much it slips and where it brings in the power.
Because of this, the opportunity exists to alter the performance of the torque converter in separate areas and essentially tune it for the starting line, top end, or both.
“With the vehicle specifications and using engine RPM, driveshaft RPM and gear ratio spread data, we can determine the performance of the torque converter,” said Chance. “Converter specialists can reliably build converters that are dialed into a specific vehicle, or specific combination or parts. This is important, as horsepower, torque, RPM range, gearing, vehicle weight is very specific from customer to customer. Plus, there’s the potential for race-only or streetability, so a lot comes into play when designing a custom torque converter.”
Should You Measure Converter Pressure?
One hot topic as of late is converter pressure. We’ve heard the term thrown around and wanted to learn more.
Chance stated, “Racers think this is a measure of efficiency but it’s not and there’s no way to measure converter pressure. They see a drop in pressure and get worried, but the pressure in your cooler line is not a measure of converter pressure. The bottom cooler line is an exhaust flow out of the torque converter. Measuring the pressure isn’t worthless, but it’s not a measure of efficiency,” he added.
Monitoring torque converter pressure can be useful, but you have to understand what you’re looking at to make the right adjustments based on the data you see.
So where you measure converter pressure does matter and it’s going to impact the readings you get.
“We constantly get calls from customers who are concerned about converter ‘pressure drop’ on the top end [of the track]. They think they are losing efficiency and MPH from this pressure drop. They have determined that the converter pressure is dropping because they have been told by someone to put a pressure gauge on their bottom cooler line to determine converter pressure,” Chance says.
“We have to explain that cooler line pressure is NOT converter pressure. The pressure in your cooler line is exhaust flow from the torque converter. However, a torque converter has a vortex flow. Typically, there’s 28 ports, each port making a vortex and all 28 together in a sphere. As the engine RPM increases, the vortex gets stronger and as that happens it allows less pressure to get to the center of the converter—it’s much like the eye of a hurricane or tornado,” Chance explains.
The center of the converter, in between the input shaft and the stater support, is where the exhaust flow comes out of the converter to go back into the transmission. As the vortex flow gets stronger in the torque converter less fluid is able to get to the center of the converter, which becomes a low-pressure point, again, much like a tornado. Therefore, when the engine RPM goes up, the pressure in the cooler line, which is exhaust flow, goes down. When you shift gears the RPM drops, the vortex gets weaker, allowing more pressure to go to the center of the converter. This means more fluid flow out of the cooler line; hence the pressure increases in the cooler line. In conclusion, there is no way to measure converter pressure, and it is normal for the cooler line pressure to drop at higher RPM.
The torque converter needs to be matched to the combination you're running. If the converter isn't efficient in the right place for your combination, the package's overall performance will suffer.
“The converter and transmission have to work together,” Chance added. “There’s a hydraulic power transfer. The transmission controls the supply of hydraulic flow into the converter, so don’t assume it also controls it going out. “
What Should You Look At?
“Most of the higher-end transmission companies try to get the transmission and converter back to the hydraulic state of balance between what the manufacturer designed and what’s optimum for racing. It’s a delicate balance between the volume (not pressure) vs. the natural restriction of what can flow out of the converter via the vortex flow. We unbalance that by raising the transmission fluid pressure and because of this most converters are dramatically overcharged. So, what most transmission builders have done to get it back in balance is to limit the feed hole to the converter,” Chance says.
The torque converter needs to be designed to work together at all points of a run.
“Racers must understand that slippage is not your enemy. The word ‘stall’ means slippage. Do not judge your converter by looking at the engine-to-driveshaft ratio at the finish line. Judge it from the starting line. You want the rate of efficiency to match the power of the motor so you can accelerate the vehicle at the fastest possible rate,” Chance says.
A critical component when it comes to extracting more performance out of your torque converter is how you approach the load sensitivity. You need to make sure the converter is happy and able to help move the G-meter in a positive direction.
“You’re looking to make driveshaft speed and improve the G-meter. It’s about load sensitivity. With a two-speed, the gap between low and high is too much and you don’t have enough power to recover the RPM compared to a three-speed. And that can change the level of slip. So, you’re managing slip for the load sensitivity of the engine. You’re looking at the recovery after the shift. The G-meter tells the story. You want the engine speed up so it can provide the best possible acceleration and this doesn’t come from what racers relate as pressure drop in the converter,” Chance says.
As the run progresses, you want the torque converter to stay in its “happy place” and remain as efficient as possible.
A certain amount of converter slippage can be a good thing, but you need to be mindful of what can happen when you go too far.
“Guys are always talking about slippage and wanting to make it less or better. Define better by equating the percentage of slip to quicker elapsed time. If we make the converter slip less but slowed the [RPM] recovery after the shift, then we made the car slower. You always need a rate that keeps the car accelerating, and you can’t simply judge that at the finish line as a percentage of slip. When you see the slippage get better, meaning less, and the Gmeter slows down that’s an example of losing vortex flow. We assume that engine-to-driveshaft ratio is equal to power transfer, but it’s not. The biggest mistake any transmission man or racer can make is not understanding the hydraulic issues within the converter and transmission. Be it cavitation or some other issue. We’ve seen guys have problems when the car slows and say the clutches and bands look great, no problem here, but could it be aerating the fluid,” Chance explains.
As you can see, efficiency is critical to how a torque converter performs. When you improve flow and maximize the efficiency of your torque converter combination, you’ll unlock a new level of performance, while also helping your vehicle be more consistent at the track.
