Differences between oe rear sway bars?

I was just casually reading through a Porsche service tech bulletin for the 987.2 platform, and was looking at some of the differences between the “basic” (what I have) and the S. In particular, the suspension components. Spring rates and front sway bar are all identical between the two models, from what they published, but clearly a lot of tuning the feel was done with the rear sway bar: So given I have the 2.9, with pdk, and no PASM, I get the softest rear anti roll bar. Are there any thoughts as to why? I’m strongly considering picking up a used S rear bar (19mm dia) because they can be found dirt cheap and seemingly work with all the other components on my car that are shared with the S level. The bigger question is - any thoughts on why there are so many different rear sway bars, and why differences between manual/pdk, and PASM/“basis”? My assumption for PASM vs non is that the electronic dampers are expected to do the heavy lifting for anti-roll in sporty situations, and a smaller bar is specced for the comfort mode, but that is just a guess. Manual vs PDK is more confusing to me, though, as the pdk weighs more, so you’d think you’d want stiffer spring to support that? Alternative confusion is that more weight in the rear wheels would drive more understeer (?), so you’d think a stiffer bar would even that out? Anyway, just an interesting table I’m curious to poll the crowd wisdom on.

My completely uneducated musing on this after messing aroudn with cars at A/x and on track for a few decades. Sway bars are all relative to front and rear stiffness. Think of it as a balanced system. If you increase the stiffness of a bar in one end that end will grip less (to a point...sometimes like a front wheel drive car the bigger front bar will increase traction for other reasons).....so a big bar in the rear encourages more oversteer. Which makes sense with the LSD as the LSD will "tighten" the rear end since both wheels would be hooked up better. I wouldn't put a bigger rear bar on an non LSD car since it would likely unload the inside wheel and allow it to slip and then spin more easily. I have no idea why the heavier PDK needs a lighter bar. My base car has had R springs and Koni sports installed. I left the bars alone though. I belive the R has a 19.6 rear bar and LSD so that makes sense. ��

So the difference between the base and the S is just a feel thing and wanting the car to rotate easier vs more casual driving, I suppose? Neither have LSD but have the biggest rear bar size difference out of any change on that matrix. I can’t imagine it is considered a safety issue by Porsche ? I’ve put a bigger rear bar on a couple previous dailies ( Audi A3, fwd, which was understeer city, and a Honda Element, 4WD, which was just kind of soft), and noticed a general sharpening of steering on both those with no real detriment I could tell. However, I really like the way my cayman handles, so that could be reason enough not to change anything. I do notice a bit of body roll and understeer, but those are quite possibly appropriate for my level and ability of driving…

I swapped the 19mm bar from my 06 Boxster S (manual) onto my 07 Cayman base (manual) which from memory measured around 1mm smaller and I noticed a difference on the offramp I take every morning.

A +1 on what Ric said. A stiffer rear sway bar will lead to a sharpening of turn in and increase the oversteer in a car. That’s generally desired by enthusiasts since most cars are set from the factory to have an understeer bias for safety reasons. some of the listed sway bar differences don’t make complete sense to me. For instance, if the standard and S models weigh the same, I would think the sway bars should be the same for a given transmission configuration. While a bit more power might make some sort of difference once power is applied, the sway bars play a significant factor in a corner regardless of power level and even when power is not applied. The following chart is helpful in seeing all the things related to the understeer/oversteer balance of a car.

Any negatives you can see?

There is no doubt that Porsche spends more time on developing the driving characteristics of their cars than most manufacturers. I'm sure they have very good computer modeling. I also know from reading all their press releases, etc. that they spend a ton of time with actual driving during the chassis development phase for new and updated models. I guess the question in my mind is whether they have different performance goals for the S versus the Base. You would guess from the above discussion the answer is yes. Just be careful drawing conclusions, as there are obviously a variety of physical features of the cars that can be tweaked to obtain their performance goals. BTW, mathmatically, the torsional stiffness of a round bar is a function of the 4th power of the diameter. Wall thickness has much less of an impact. So ballpark, the torsional stiffness ratio of a 19 mm bar versus a 17.2 mm bar is 1.5!!! To me, that make understanding the reasons behind the differences even more perplexing.

Just doing a little research this morning to correlate part numbers to sizes. I only have the rear ones figured out for now: 19.6x2.6 is part number: 987-333-701-04 19x2.7 is part number: 987-333-701-05 18.5x2.5 is part number: 987-333-701-03 17.2x2.5 is part number: 987-333-701-06

Just curious. Have you looked at rear springs, as in are there differences with those also?

Looks like cayman rear spring rates are the same, but boxters are all over the place. I imagine this has something to do with chassis stiffness differences between the two platforms.

I pulled the trigger on a rear sway bar, and got it installed (within hours of package receipt ). It, however, cast doubt on my part numbers listed above because I purchased the -04 variant on the assurance from the seller it was from a basic no PASM , no locking dif 987.2 Cayman S . I measured it at 19.3mm which tracks to a 19mm bar + paint thickness. Anyway, only a brief test drive on it so far, but definitely has crisper turn in and a “sportier” feel. I did a calculation a bit ago and think it is roughly 46% torsionally stiffer than what came off my car. That was a very crude calculation, but it lines up with experience. On the test drive, I did take a roundabout 180 at a little bit of speed to feel over/under steer, and it still pushes, but less dramatically. Overall, I’m pleased with the change, but do see why Porsche decided on a softer rear bar for presumably a more relaxed driving experience for the “basic” level car.

Have we verified that the front bar is the same? I like the idea of a tighter set up (Already changed my Base to "R" springs and Koni Red shocks) But also know the risk of "too stiff" unbalanced bars.

The only primary source from Porsche I have about this is specifically for the 987.2 models, published in 2009, and that document says they are. No mention of R variants, though. I find it confusing to parse out the options listed when searching by part number, though (eg: “with active suspension, no damper control”) On a previous car, I had a setup that was damped acceptably in heave, but the rear sway bar was too stiff for those dampers and it definitely made for an odd feel in some situations, prompting a return to the softer stock bar on that vehicle.