Quite likely, E36.

As for the bump issue - you're looking at what is related to bump sensitivity. Think about what the normal force on the specific tyre is doing as you go over a bump (compression followed by dropoff) - so naturally the wheel slip is going to fluctuate as the normal force fluctuates. It's really no surprise that you'd enter ABS at this point - the hard part is getting out of it. Yet another reason to stay away from the bumps or keep the ABS off on-track...

Well-tuned ABS should not induce porpoising. However, when you've changed much of the suspension tuning, it's no surprise that things may no longer be in synch.

 

Another reason the 964 cup cars had ABS shut off switches is that when the car is going backwards ... ie SPIN, the ABS does not work the same way as it does when you are going forward. You do not have the same ability to stop when in a spin as you do with the ABS off and can have full lock up. Ask John C. He has first hand experience with his 964 cup ... he spun going up the esses at the Glen ... and luckily was able to avoid any barriers due to the ABS shut off switch which he was able to turn on (turning off ABS) during the spin.

AA

 

That's what you call serious presence of mind !! That section of track has been known to claim some victims and it's also exactly what I was thinking about when I mentioned that downside of ABS.

 

Hmm, it appears the new ABS works fine in reverse. My spin at TMS was a double and it was working like a champ after I ran through the oil spill from Cherry's nuked 964.

 

The Porsche racing ABS is radically different from the street. I have the racing ABS on my '02 cup car, the biggest difference is the actuator speed - the racing ABS fires at a much faster rate than the street version, so it keeps the tire right at the edge of locking up. It also takes a hell of an effort to trigger it, the only time that I've engaged it was in full avoidance of a slower car that chopped me going into a corner. I also know, from personal experience, that the wheels can be locked in a spin, but everytime I've done this (well, only twice) the car was sideways when I locked up, so the wheels weren't turning at any appreciable rate. I do know other non-p cars that have not locked up in a spin, causing unfortunate events. I hope to not find out what happens if I'm going backwards and try this, but if I do, I'll let you know.
The team I bought the car from commented that they hit the ABS all the time and really counted on it. They are also a lot faster than me.

 

924RACR already touched on a lot of good points in this discussion. As he said the ABS on street cars are tuned to how the factory built the car. Most cars going on the track have already deviated pretty far from this specification.

Another point to realize is that production street car ABS is achieving two things at once. First it is preventing the wheels from locking up in order to optimize deceleration. But what it is also doing is trying to keep the wheels at an average level of slip that is slightly less than "optimal for max deceleration" so that you also have some traction leftover for lateral needs (turning!).

As ABS systems have improved mechanically and electronically over the years, this "slip left over for turning" is reduced in a straight line as the systems not only are more accurately keeping the wheels at the targeted slip level, but they can also better determine when the driver wants to turn or not, and therefore optimize deceleration better in a straight line.

Additionally, there is another major difference between street and race car ABS. For a street car, the ABS must always assume that "the worst' is coming in the next few milliseconds So even if the ABS sees that it is on a certain surface, it also is always making sure that it is ready when the surface changes. A street car might go from glare ice to dry asphalt, and then back - however a race car normally would be operating in a much narrow band of surfaces. Because of this race car abs can be a lot more aggressive with the level of slip and deceleration, and with the accuracy it maintains the optimum level of slip (since it is not required to react as well to such radical changes in surface traction).

So..........

If you are running a car on the track that has been heavily modified (suspension brakes race-tires, etc) with an older technology ABS, you might be better off disabling the ABS in the dry. My answer on this would depend heavily on each individual car / driver situation.

In the wet, however, I would have a greater tendency to leave the ABS on, as the benefits of finer modulation and more rapid response are more crucial in the wet. Additionally in the wet there the relationship between traction and the amount of slip changes: in the wet the relative amount of traction lost when you drive the wheels past their optimum slip level is much higher.

The question of "when have we ever seen proof" is a good one, and it is impossible to have one set of data that will apply to every circumstance because every car, ABS, tire, driver, etc is different.

I took an SAE applied vehicle dynamics course a couple months ago and was fortunate to have the class instructor also be my in-car instructor. We had a Escort GT2 g-meter in the car and were running several simple exercises of stopping with ABS on and off, both in a straight line and in a turn. All 4 drivers in the car (including instructor) were brake (ABS) development engineers with at least 5 years each in the field, and additionally the instructor was a active SCCA racer.

The result? NONE of us could beat the ABS in a straight line or in a turn. In the turning situations the benefit was very drastic, and even in the straight line, none of us got within 5% of the ABS's deceleration capability (as measured by the G-meter). And this was on a mildly modified street car. (suspension, tires, brakepads, wheels changed from stock).

A couple of posts including FixedWing's mentioned the track driving technique of braking up until you feel ABS but not much more. I also subscribe to this technique most of the time for the following reasons:

1) it makes the braking event more smooth (better balanced weight transfer and therefore more predictable turn-in characteristics)
2) Most cars I have driven on the track fall somewhere into the continuum of older and/or majorly modified (including a 95 968CS, sometimes with R-Tires)
3) It makes it so that I don't have to radically change my driving behavior if I get into a car that doesn't have ABS.
4) I know I'm leaving a little bit more of a margin of safety for an unforseen circumstance or a small driver mistake. (this would differ between a track day or a real race, the latter which I have not started yet, SpecMiata in process )

However when in a car with a more modern ABS system and no modifications, such as my (previous) S2000 and Opel Speedster, I would tend to use the ABS more often as they were very well optimized (except for rough surfaces).

That said - if I'm in a panic stop in a ABS equipped car (street or track), my foot is to the floor as hard as I can push, absolutely no questions asked.

Cheers,
Keith