Thanks, guys. I've followed all the tech info - great stuff.
(I wasn't belittling any of the input, BTW, I don't know how that got construed J/. Clearly the input/info was from someone way more knowledgeable/experienced that I.)
OK, so there are no shims that are different than the clutch discs, it's just clutch discs one can add...
I had assumed they were something different, as I'd read you can add shims and not need machining, but adding clutches involved machining the housing (?). As I'm not looking to do any machining - either to change the ramp angles (which seems to be the best approach if one was looking to really get in to tailor/tune the engagement/disengagement performance of the diff), or to add additional clutches - maybe I should just leave my LSD stock. It seems to work fine - I was just looking to get a little additional perfromance if it was an easy mod while 'in there'. I thought it was just a parts addition one could do in the rebuild - and if was as simple as adding a few shims, I'd go for it. (Also wasn't looking to fab my own parts, but to buy 'em).
The info I had gathered prior to my post told me I could add shims to increase/alter preload (and that too much preload would increase understeer). It said the shims come in pairs - one being a Belleville spring 'as big as the discs', and are approx 2mm thick and the other being the dog-eared plates to shim up/increase the static locking. I saw that BMW/ZF uses the same angle for the coast and power ramps to drive the pinion shaft (arrow in pic), and shallower ramps and/or altered ramp geometry would increase the mechanical advantage increasing the lockup %. (and that increasing lockup this way, vs. just altering the static preload is really the better way to go.)
As for the differences I'd get between 25% and 40% lockup, I looked into the engineering to try to figure out what it might mean as far as driving the car. (I grok the ZF way of citing the LSDs perfromance - 25% LSD meaning 25% of the torque applied to the faster wheel is applied to the slower wheel). With the 25% lockup LSD the high torque side provides as much as 62.5% of total, while low side could be as little as 37.5%. This yields a bias ratio of 1.67 (1.67 times more torque is applied on the high side). A 40% LSD would have a 2.33 bias ratio. It would increase rear tire wear on the inside wheel during cornering. It would increase understeer in tight corners in the dry, and increase oversteer in slippery conditions. It would get me less inside wheelspin accelerating out of corners and more power being translated to the ground in that situation. This seemed like a reasonable tradeoff - and if I could get it 'for free' (cost = an additional set of "shims"), it looked like something to go for.
If I can't increase the % lockup without machining (i.e. if adding any additional discs beyond the default included in the stock 25% LSD config would necessiate machining the housing to fit them) I guess the 25% limited slip will do as is.
I appreciate the info - I learn from the knowldege of others here, and try to share things I know too. It's great to know if I wanted to add discs, I could get them via Maxmillian or Aardvarc. Also good to learn that if I really was interested in altering/tailoring my diffs performance, machining and the use of an expert diff builder is required.
Tom (aka visionaut)