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Porsche Stability Management System

A racerís perspective

By Jack Miller

Thank you to Jeff Southall of Porsche Cars of North America for the technical information in this article.

April 29, 2001 marked the official return to Formula One of electronic driver aids, including traction control. In racing as elsewhere, technology that enhances (or interferes with, depending on your perspective) human performance is controversial. Potential Porsche buyers face a similar controversy in deciding whether or not to purchase Porsche Stability Management System (PSM) in the new Carrera 2, Boxster, or Boxster S. PSM is standard in the Carrera 4 and Turbo and unavailable in the new GT2.

If you never intend to race your new Porsche, the decision to purchase PSM is simple. If you can afford it, buy it. It provides a level of safety impossible to achieve by driver skill alone. Hereís why. PSM monitors the ABS sensors (which measure the speed of each wheel), engine speed (RPM), throttle position (via E-Gas), gear selection, lateral acceleration (side to side), yaw (the car spinning in a circle), and steering wheel position. This enables the PSM to detect oversteer and understeer. It basically determines the slip angle of the front and rear tires, or more simply, when the car is not going where the steering wheel is pointed. Oversteer is minimized by automatically applying the brake on the outer front wheel in a bend, slowing the rotation of the car; understeer is minimized by applying the brake on the inner rear wheel, speeding the carís rotation. No driver will be able to do that until Porsche develops a car with four brake pedals. However, PSM is not only a braking system. If you lift off the throttle in a low traction situation (wet, snow, etc.) and the back of the car gets loose, PSM will increase the engine speed (blip the throttle) to keep the car in line. Also, if traction is low, PSM can use engine braking (EDC Ė engine drag torque control) to slow the car. PSM can calculate the amount of available traction by comparing wheel speeds at all four corners of the car.

Recognizing that even street drivers expect excitement from their Porsches, PSM allows approximately seven percent slip angle before intervening. Five to seven percent is generally agreed to be the limit for modern, high performance tires. The biggest difference between PSM and the other systems on the market today (Mercedes Benz, BMW, Jaguar, etc.) is that PSM is programmed to allow a good deal of slip, as you can see. All of these other systems clamp down the moment any slip (i.e., fun driving) is detected.

However, if you require more fun, you can turn the PSM off. When you "turn it off," you are taking only the outputs offline. The PSM system is still collecting data from the ABS system, the yaw sensor, the lateral acceleration sensors and the steering wheel position sensor. If you have PSM off, and the levels of slip are exceeded, and you do not touch the brakes, the car will continue to slide. If you have not exceeded the levels of slip allowed, and apply the brakes (no matter how hard), PSM will not active its outputs. However, if you have exceeded the levels, AND apply the brakes (no matter how hard), PSM will activate until the car has regained control or you get off the brakes, at which point PSM stops outputting. PSM assumes that since you hit the brakes that you are not comfortable with the level of sliding and that you want it to help. This answers the question, posed by Mike Furnish on the PCASD forum, that inspired this article, "what happens in a spin when you put both feet in?" Presuming that you put in the correct two pedals, PSM will activate.

So what about PSM and racing? At this point in my career, PSM is an asset to my racing. It has allowed me to more confidently explore the limits of traction on the first few laps at a new track, particularly in scarier corners, e.g., Turn 8 at Willow Springs. I was very happy to have it at Phoenix International Raceway, a track with concrete barriers everywhere. When PSM activates you can feel it, much like you can feel ABS. It will show you where you are losing traction while keeping you on the track if the loss was unintentional. When it engages, it may slow you down where you might not want it to later, i.e., where you really do want more oversteer, but on those first few practice laps, who cares? You can actually throttle steer the car quite well with PSM on as long as you are smooth, the yaw is not excessive, and the corner is fast enough to allow smooth inputs. This in itself is a good training tool. So PSM is good for practice, but what about when it matters, during timed laps?

In a time trial situation, it would depend on the course whether it would matter if PSM were on or off. On a tight road course, you would most likely want it off. On an autocross track, you want it off for sure. If you had sufficient presence of mind on a road course you could turn it on and off depending on the corner. You could make sure itís off for Turn 2 and 4 at Willow Springs, turns where throttle steering comes into play. You could turn it on for Turn 8, the last place on earth you want to see your tail catching up with you. I've never done this, but it illustrates the point.

So far, so good. Since you can turn PSM off, why wouldnít you want to buy it, even for a car you intend to race? It seems like the best of both worlds. However, remember above where I said that when PSM is off, it is still collecting data and if you hit the brakes when the levels of slip are exceeded, it will intervene. That could be a negative in one racing technique, trail braking, where you are obviously on the brakes and turning. There are two reasons to trail brake, one in which PSM is neutral or even a positive, and one in which it can interfere with the driverís intention. The first is when you are trail braking to lengthen the straight or to maintain a higher speed through the first part of a turn. In this case, you want the car to stay on its directed path. If things are going as intended, PSM is very unlikely to engage even though you are on the brakes. If it does, it is probably because you lost rear traction in a pretty big way. By engaging it didnít cost you time since your intention was to slow down anyway and it may have saved you from spinning. The second use of trail braking serves a different purpose. If you are trail braking to induce some oversteer intentionally to tighten the corner, PSM could interfere in the same way as when it is on and you lift to oversteer. While I have a lot of experience throttle steering the car, with PSM on and off, I donít brake to loosen the rear of my 996 C2. Lifting is normally sufficient. However, I have seen this technique, in the form of left-foot braking, used in a friendís 993 C4 in Turn 4 at Willow and Turn 5b at Spring Mountain and presume it would be useful in the newer 996 C4. Since the 993 does not have PSM, I cannot tell you to what extent it would have interfered. If you are smooth, probably very little, if at all. But, this is one possible negative to weigh against the aforementioned positives. I think itís worth it, but let me give the last word to Porsche.

"We wanted the car to perform like a Porsche not a family saloon, so the system has been designed for minimal intrusion," explained Thomas Herold, the Carrera 4 Project Manager. "Its limits are really high and you can reach the same lateral g-force number with the system in or out on a steady state cornering circle. Thus, if you are a good driver, you can keep the power on in a drift and even adjust the carís attitude on power in a corner without interference. But if you lift off suddenly or brake, and the car is in danger of destabilizing, the system will reach out and save you."

"The difference is small around the Nurburgring for a skilled test driver," he explained. "Within one second a lap in fact. This is the way the car is made. If you are smooth, there is no interference from the system. But if you are ragged, the system will be cutting in all the time to stabilize the car, so an aggressive driver will be slower with the system on."1

1. The web link for this quote's source citation is no longer available on the Internet.

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