K.J. Jones
December 11, 2007
MSD's Programmable Digital-7 Plus ignition (PN 7531; $900) was the best-kept secret-until now-for dialing in traction consistency on a drag-radial Mustang.

Horse Sense: NMRA racers should consider themselves lucky the sanction even permits the use of MSD's 7531 ignition box in some of the heads-up classes. In a classic, racing-rules catch-22 (where a kick-butt part is outlawed because it works too well), the National Hot Rod Association-the "almighty" when it comes to organized, sanctioned drag racing-banned 7531 from use in any of its categories, after testing it and discovering the power it possesses.

The word bite, when used as a noun in drag racing, is one of several terms that refer to traction-the coming together of tire and racing surface. Traction makes all the difference between whether your 'Stang leaves the line like a bullet or roasts the tires the moment you let go of the transbrake button or drop the clutch. Yes, drag racers definitely know the importance of good traction, and they work hard tuning their cars to get the best possible bite each and every lap.

While getting a 'Stang to hook on slicks is tough, figuring out how to make it work on today's popular drag-radial tires can be a bigger nightmare. There's a fine line of variables that influence the success potential of 1,000 or so horsepower and 11-inch-wide (tread width) sticky radials. Chassis dynamics are important and, of course, a driver's ability is critical. But effectively managing (manipulating) the horsepower probably plays the biggest role in a drag-radial racer's setup.

Phil Clemmons plots a timing curve. He uses several timing-retard features of the MSD Digital-7 Plus to help plant the BFG 315/50-15s on his drag-radial 'Stang.

Boost (supercharger/turbo applications) and progressive nitrous controllers are two of the better-known devices radial-tire racers use to help manage how their cars leave the starting line. With either of these control methods, a racer basically eases in boost or nitrous flow in gradual increments. Calculations with these controllers are critical because track conditions, weather, and other variables must be taken into account when the settings are made. Ultimately, they impact how well the entire package performs-from the Tree to the stripe.

When things are good, some Mustangs are capable of leaving with the rear end perfectly squatted and the front end stretching upward, seemingly on the verge of liftoff and proceeding down the track with the rear tires sticking like glue. When the stars for good traction are properly aligned, some radial Mustangs even hook so hard they literally take flight in monster wheelstands, rear bumper scraping the track and rear wheels off the ground.

Five years ago, Autotronic Controls, maker of the MSD Ignition systems and components, introduced its popular Programmable Digital-7 ignition box (PN 7530), a landmark product that introduced a previously unfathomable feature that allowed racers to control timing in each cylinder of an engine. For those using power adders, this was a major development because it opened the door to more precise tuning, better performance, and a way to alleviate tire spin by retarding the timing. It also helped reduce some of the carnage that was common-especially among the nitrous ranks-thanks to timing settings that were slightly off the mark.

A closer look at a Digital-7 Plus data screen. Note the small spike midway through the curve (at the tip of the pen), which indicates the tires briefly lost traction roughly two seconds into the run.

These days, the Programmable Digital-7 Plus (PN 7531; $900) has everyone's attention-especially in the drag-radial world-because of its many different ways to control timing or rpm at the launch. "It gives you infinite tunability of the ignition in numerous ways," according to '04 NMRA Drag Radial champion Phil Clemmons. The 7531's most-notable and controversial features include the linear launch-retard curve, the programmable, time-based rev limiter, and the slew-rate rev limiter. The ignition can also retard timing by using intake-manifold boost or vacuum pressure on supercharged and turbocharged engines.

For racers trying to maximize traction with timing adjustments, the Digital-7 Plus features a time-based (0-2.5 seconds) launch-retard that can be programmed to pull out between 0 and 15 degrees from the total timing value (BTDC), in tenth-of-a-degree increments. The retard function is activated upon release of the transbrake or clutch and the introduction of 12 volts. When power is taken away, the retarded timing is ramped back to the initial, total-timing value within the programmed time margin (the box allows a maximum of 25 degrees of total timing that can be retarded). This function can be helpful for dialing in solid 60-foot times on the drag-radial tire.

The rev-limit curve allows a drag-radial racer to map an rpm-suppressing curve based on time. The map can be set up to increase in 100-rpm increments, across a time window of 0.01-12.5 seconds.

The slew rate, or "slew," is the feature of MSD's 7531 ignition that's commonly argued to be traction control. Phil says, "It really isn't traction control at all." Slew is an rpm limit-to-time (seconds) ratio that controls an engine's rate of acceleration in each gear. For example, if 5,000 rpm is the selected slew rate for an engine, the engine will be limited to 5,000 rpm per second from First through Third gear with a C4 automatic transmission. It represents a maximum, per-gear rpm value (acceleration rate) you don't want the engine to exceed.

Slew is a great rev-limiting feature. According to a few drag-radial racers we spoke with, however, slew is not the attribute of 7531 they use when trying to improve their cars' bite potential. Complexity of the tire and the erratic (non-linear) gear-to-gear power curves common to most drag-radial cars are among the reasons racers choose to use other features of the box to help them hook. The drag-radial racers we spoke with agreed it's just too difficult to establish an ideal rate of acceleration in each gear on a drag-radial car.

Any time an engine gets into the rev limiter, it affects performance. So, as with most rev limiters, slew can slow you down if it isn't adjusted properly. Theoretically, with slew, if the tires break loose during a pass or something else causes an unexpected spike in the rate of acceleration, the engine will run against the rev limiter until it's once again accelerating below the slew-rate rev limit. So, in a way, slew can be considered a safety feature of sorts.

The options and versatility of the MSD Programmable Digital-7 Plus ignition are designed to help make drag cars-on radials or on slicks-more consistent. The features definitely can help drag-radial racers achieve better traction. But, like many other ways of achieving killer hook, the ignition and its many settings require a lot of trial and error to get things right, simply because the timing and rev-limit adjustments are based on acquired run data (the 7531's data acquisition functionality and additional dataloggers). A unanimous comment from the users we interviewed was, "If you don't go out and test with it, you'll never get it right." These are wise words to live by if you're trying to race on drag radials and plan to use the MSD Programmable Digital-7 Plus.

The 7531's ability to capture run data that's used for creating timing and rev-limit "maps" is an attribute that drag-radial racers agree is a major ally in the war against tire spin. Here's a sample of just the rpm trace for a turbo Mustang's dragstrip pass (top) and all the acquired data of the 'Stang's run (bottom). The custom timing and rev-limit curves this box enables racers to create could have considerable influence on getting the tricky tires to stick.