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There’s an old saying in drag racing that we definitely didn’t just make up: “Anybody can be fast once.” Regardless of whether that’s something racers actually say or not, it’s still true. The difference between one-shot wonders and drivers who win races and championships is that the latter aren’t just fast, but that they are fast all the time, regardless of circumstances. On a sunny day with good air when the track is hooking, it’s not too hard to look like a hero even with your tune set to “kill,” but when it’s late at night, the heat has gone out of the asphalt, and you’re stuck in the wrong lane in the finals, the make-or-break challenge is often just getting a clean pass.
The ability to take some of the uncertainty out of that equation, coupled with affordable and flexible engine control and datalogging hardware and software, has led to a surge in the popularity of drag race traction control. With that surge has come a certain amount of pushback as well, with some racers deriding traction control as a crutch or a shortcut to the winner’s circle.
The truth is that it’s much like any other tool in the box for a successful racer. It’s not a substitute for hard work and doing laps, and it’s within the reach of just about anyone going quick enough to benefit from it in terms of learning the basics, implementing a system, and fine-tuning it to deliver consistent results. To get a good grounding in what traction control can and can’t do, we’ve enlisted the expertise of Holley’s Ryan Witte, who walked us through the important things to know about how it’s implemented in Version 6 of the Holley EFI system.
At its core, what we refer to as traction control is really just keeping wheel slip within the desired range. Unless you’re bracket racing rental cars (and there is nothing wrong with that) your race car will have more than enough power on tap to overwhelm the available grip from a dead stop. A traction control system automates the process of applying that power without exceeding the desired amount of wheelspin, and compensates for less-than-ideal track conditions in the process.
“There are multiple forms of traction control - there’s what’s called ‘perfect pass’ traction control or driveshaft traction control, which is just where you plot a driveshaft (speed) curve over time,” Witte explains. “It’s just a graph of where the driveshaft speed should be at or below at any given moment.” When driveshaft speed is below the line, the tire is hooked, but if it exceeds the reference RPM for that moment in the run, the tire has excessive slip and power needs to be pulled back to get wheel speed back into the desired range.
In addition to this primary driveshaft RPM versus time strategy, it’s also possible to work with different data inputs. Per Witte, “There are other ways to do it. There’s front versus rear wheel speed - comparative wheel speed or wheel slip are other terms for it. That’s not something that is preset in our software; it’s something you have to do using our advanced table features, which is kind of a ‘build your own adventure’ section of the software where you can create special functions like that to your heart’s content.”
The advantage of using comparative wheel speed instead of driveshaft RPM versus time is that it reflects what is actually occurring in terms of wheel slip instead of relying on the ‘dead reckoning’ navigation of the perfect pass approach. The disadvantage, as you might imagine, is that getting useful front wheel speed data requires the wheel to actually be consistently in contact with the ground.
Front vs. rear wheel speed readouts only work properly when the front wheels are still touching the ground.
Witte makes the obvious clear, saying, “Front versus rear wheel speed only works when you have the front and rear wheels on the ground. So what I tell people is that if you’re in a drag racing situation and you know your front wheels are going to be off the ground occasionally, use that “perfect pass’ time-based traction control for the section where you don’t know if your wheels will be on the ground, and then put the front vs rear wheel speed on top of it at whatever point in the run you are confident your wheels will be on the ground.”
He also points out that it’s possible to use the same set of features to limit how much your car carries the front wheels, but it’s not necessarily the best use of the technology. “Some people use that as a wheelie control as well, by saying, well, if my front wheels are off the ground I want them on the ground, so they use the timing control that would normally be used for traction control for wheelie control,” Witte admits. “In my opinion that’s not the best way to do that, but people do use it that way, and if it’s working for them, who am I to argue? I haven’t seen their data.”
Regardless of which strategy is used to provide real-time data to the ECU, there is still work to be done by the tuner. Witte says, “People expect it to ‘just work,’ that you can call someone and get the numbers to plug in. The biggest misconception is that it works for everything straight out of the box. What works on a single-turbo, thousand-horsepower, LS is not the same as what you are going to put in for a big block, methanol, twin-turbo car.”
For the driveshaft RPM versus time strategy, the first step is getting accurate data as a starting point, and that means testing. “You have to have a good pass. You actually have to have done it,” Witte explains. “You end up taking the data log of the pass you want to copy, and we have a feature in the software that can literally import that curve of driveshaft speed starting from transbrake release as your starting point.” Once that ‘perfect pass’ is mapped out, the system is ready to tweak to properly control just how much slip is allowed.
In this screenshot, you can see data from a log file represented as yellow dots on the driveshaft RPM versus time plot. The Retard A (blue) and B (green) limits currently selected are also shown.
“It can be as simple as opening the log, zeroing it, and pushing the import button. Then you can start dragging dots where you want them,” he explains. “Once I get them there, most racers know enough about the traction control to just run with it. They’ve often used some other form of traction control in the past, and we designed ours to be intuitive and similar to what they may have used before, so they can easily transition into ours. Some concepts like advanced traction control are complex by nature, so there’s only so much you can do, but it’s not rocket surgery at the end of the day. It’s just timing. You pull some out until the tire is under control, then you put it back in as fast as the car will let you.”
Once again, having that knowledge base of how your car reacts to timing changes is key to an effective traction control strategy. He continues, adding, “The only thing you need is to actually spend enough time with your car to know what it’s going to do. You have to have had a combination together long enough to know what changing the timing curve is going to do. If you know pulling 2 degrees is a big change on your car, that’s all you really need. Because with traction control you need to decide, 'am I pulling 2 degrees or am I pulling 20?' If you don’t know what doing that does to the car in the first place, then you are just guessing. That’s OK, but it puts you at a disadvantage when it comes to figuring out your power management. Anyone who has done this for a while runs a launch retard; they know the difference between a good track and a bad track and how they set it up for that. And if you know that, you have what you need to run traction control.”
Here, we see the ignition tables for the A and B retard. At the point where the pink crosshairs meet, the A table is off-setting timing by the amount shown in the very top row of data.
Per Witte, “The customer decides how aggressive to make it. We just give you the tool, and the tool is that we can pull timing, and we can shut cylinders off. It’s up to the tuner to decide when and how to remove that power. The general rule of thumb is to do it in stages. The way we typically set things up is to start by pulling a small amount of timing out - the tire is starting to get away, let’s see if taking the timing back is enough. If that isn’t enough, you ramp into taking more and more timing out, and if that isn’t enough, then you progress into starting to drop cylinders. One cylinder off, two, three, four. At that point, that’s as far as we typically go. If you need to shut four cylinders off, you really messed up… We could do more, sure, but at some point you have to take responsibility as the tuner and get a little closer on power.”
When asked what he sees giving new users the most trouble, Witte had this to say: “The biggest mistake is giving it too much leeway - they let the tire run away too far before they try to catch it. Our traction control works best when it catches it right when it starts. I’ll see people try to give 200 RPM on a driveshaft curve before they start to correct, and that’s usually way too much. You want to start at 25 or 50 RPM.
One of the marvels of our time is just how powerful and fast microprocessors have become, and Holley’s EFI system is no exception. That’s what makes it possible for the traction control features to work seamlessly and invisibly when set up properly. “The cycle is on the order of ten thousandths of a second,” says Witte. “It’s faster than a cylinder firing event, so you can always remove power before the next cylinder fires, which is the most important thing. As long as you’re faster than that, which is RPM dependent of course, you can have control.”
When using the differential wheel speed method, the tuner creates a map that shows desired timing offset for any given amount of slip. The interface should be very familiar to anyone who has worked with general EFI timing or AFR tuning, and offers a smoothing function to interpolate between user-entered values.
“The traction control, when set up correctly, is faster than the driver [in making changes]. It will have corrected the slip going down the track before the driver can even react. You have to unlearn that if you have had to pedal the car a lot. But if you’ve learned that habit, there are underlying issues with the car that you should work on first. Traction control won’t take a car that spins every other pass and turn it into a bracket car. That’s not how it works,” he explains. “The people that I’ve helped typically have a scenario they are tuning to. They’re just tuning for slightly different conditions, and with front versus rear wheel slip, you hardly have to touch it. With a perfect pass, you end up dialing a number, basically. You have to know what you want to do. So if you go to a really good track, and all of a sudden you need to go two tenths faster, you either need to turn off traction control, or you need to make a guess about what your driveshaft speed curve has to be to go two tenths faster. Time-based, it’s accurate data in, accurate data out. If you have an accurate driveshaft RPM curve you are trying to hit, and you know your car can run it or close to it, it works amazingly well.”
With that understanding, you might wonder if traction control is a useful tool for a racer who maximizes their competition possibilities by running the same car under different rule sets, specifically going from one limited-tire class to another. Granted, compared to the vast majority of people, this is an edge case, but Witte laughs and says, “I live in the edge case.”
“The hard part is slick versus radial, because the amount of slip you’ll want between them, or even between one kind of slick and another, is completely different,” he explains. “Radial, you’ll want your front versus rear wheel speed offset to be somewhere between zero and three percent before you take action. With a slick-tire car, I’ve run them as high as 20 percent slip before I really try to pull it back in. And it takes a gutsy driver to do that. The number is the number, but you have to take it with a grain of salt with front versus rear wheel speed because a slick grows, it wads up, it changes diameter dynamically throughout a run. So technically speaking, you never have a truly accurate slip percent, but if you have a number that is consistent pass to pass, you just tune to it. The 20 percent you see may actually be ten or fifteen percent, but it’s still substantially higher than the radial.”
As with any other use of drag racing traction control, it comes down to knowing your car, running it enough in testing to have a picture of what the run you want to make looks like, and then letting the system duplicate that picture consistently. That’s the true value of traction control - eliminating as many variables as possible to get cleanly to the stripe time after time. So when is it not appropriate? Witte concludes, saying, “If you have a brand new car, you’re not going to run traction control the first outing. Now if it runs really well, you may the second time. It’s designed to be a band-aid. A very effective band-aid, but the point of it isn’t to make you faster. It’s to give you that coverage in case you misjudge the track or conditions change slightly, and you’re off a bit. Use it for last chance qualifying and going rounds. If you’re trying to set a record or run a big number, turn it off.”