Pics & Review of My Bilstein PSS10 Lowered Red Turbo by cannga

8 minute(s) of a 612 minute read

2-1-2009

cannga:

For those following this thread and new to suspension mods, let me summarize what I've learned in case you are interested. There are 4 common mods that people have added to the Turbo, with number 4 probably the least frequent: 

1. Front camber adjustment to reduce push/understeer. Stock is -.4 or so, a common target is about -1.3. 

2. Lowering and stiffening the car. 2 ways to do this: With coilover such as Bilstein, or lowering springs...

3. Stiffer Anti-Roll Bar (the drop links that connect the anti-roll bar to the suspension are sometimes also replaced). 

And the adventure continues . Up next: the effect of front negative camber and front toe-out. (Please note: I am NOT recommending this to anyone, merely describing my experiment/experience.)

When I posted the above 7 months ago, someone asked the very valid question that, which of the top 2 changes, Bilstein versus alignment changes, is responsible for the dramatic improvement in the Turbo’s behavior. At the time, I said Bilstein, which now seems to me like the wrong answer, or at least the wrong way to approach that very good question. 

In retrospect,the simplistic answer I gave, that Bilstein was the "star" of the show, was partially wrong for the following reason: you can’t really compare the 2 modifications because they affect 2 different aspects of a car’s handling, which are, a. overall body/chassis control (Bilstein), and b. cornering and turn-in behavior (alignment changes). As always, I don’t claim to be a professional, just sharing amateur-level experience here:

Bilstein: When you accelerate, the stock Turbo’s squats; when you brake, it dives; in corners, it leans and its rear end has vertical & lateral motions. The Bilstein helps to control these motions, which to my thinking are related to the car’s body controls, in corners obviously, but majority of the times, it involves “straight line” behavior.

Alignment Changes: To me, affect mainly the car’s behavior in corners. How the front of the car turns in corner (understeer/oversteer), how the front responds to steering input (turn-in response), and how the rear end rotates.

If you are new to this and have some trouble understand what I am talking about, I encourage you to take test drives in the C2, or any RWD Porsche. (I love comparison drives; they have been critical to my learning.) I think your experience would match my observations below:

1. The Turbo, as set up from the factory, understeers. It is beyond the scope of this post to explain in nit picking details, but basically if you approach a corner at speed, the nose wants to go straight. To me, the Turbo does this more than both the C2S and the GT2 that I’ve driven.

2. Turn-in behavior: The steering of the Turbo is stubbornly straight. The good thing: Great & solid center feel. The bad thing: If you move from center, it wants to go back straight. This is different, in a bad way IMHO, from both the C2S and the GT2 that I’ve driven.

3. Rear end rotation: It is extremely hard, actually impossible, to get the rear end to rotate in stock form. The car just wants to go straight. It is safe behavior and I understand why Porsche set up the car this way, but personally I would prefer more rear end rotation.

Subjectively, the result of 1-3 above is that the car feels un-natural and a little "dead" (as in not "alive" ) when it enters a curve. It doesn't follow the curve, instead wanting to go straight, and perhaps as result, it leans a lot. In addition, when you apply throttle coming out of a curve, the rear end doesn't rotate so it's so much harder to perform a power slide (am sort of kidding with this last one).

The 2 alignment changes that I have made to my car are: increasing negative front camber to minus 1.2 degree, and change the front toe setting from toe-in to slight toe-out at minus 0.02 degree. The first change (front negative camber) is common among enthusiasts, but the second (front toe out) I’ve only seen in racing/GT3 forums, and is used for selected drivers by (at least) some very well known tuners here in Southern California, including mine. My tuner (a great and very careful one) was hesitant to do the toe-out for me, probably because I don't track and I am only a beginner level driver, but I "pushed" him towards it because I was curious and wanted to learn, with the idea that I will set the toe back to factory level eventually. As it turns out, there are pluses and minuses, but I am now having second thoughts about going back to the factory toe level (toe in) at all and I do "get" what the tuners and track guys are trying to do with this setting.

The car now feels "alive" , when you move the steering wheel, the car responds and turns quickly, and has more willingness to turn, and the car's body & rear end rotates nicely to follow the curve. (BTW, another very noticeable effect is the steering wheel effort now feels much lighter.) IMHO the alignment change makes the car much more similar to the GT2 when it comes to turn-in response (very quick and responsive in the GT2) and corner behavior. It is now so much more fun on twisty roads and feels very fast and responsive. True also, as it feels more sporty, it also has become more edgy: There are 2 curves, a 43 mph and a 71 mph one, that I've done hundreds of times as they are on my way to work; I am now perhaps faster in the 43 curve because the car appears to lean less but slower in the 71 just because, like the GT2, it feels like you have to pay attention at the edge or the thing is going to give, and I get scared easily. 

Some reading material if you are interested:

http://www.ozebiz.com.au/racetech/theory/align.html

....If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it's clear that toe-out encourages the initiation of a turn, while toe-in discourages it.

....The toe setting on a particular car becomes a tradeoff between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-the never-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straightaway for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.

http://www.mgf.ultimatemg.com/group2/suspension/toe.htm

...If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it's clear that toe-out encourages the initiation of a turn, while toe-in discourages it.