Sorry, misspoke on the Z3 racks. Anyhow, dug up my old notes from when I was talking to a BMW racer who confirmed most of this with a friend at ZF (who made the racks). there's so much misinformation out there, who knows who to trust (including rack doctor and RealOEM - which has been proven to be wrong on this). I'm going with what i consider the most credible sources and my own personal measurements (which I consider very credible).
The Z3 2.8 rack is linear and 2.7 turns lock to lock. I know this because I measured it personally, and it's what's on my car right now. It is a 14.5:1 linear rate, as I recall (53mm per revolution).
The Z3 1.9 racks are a mixed bag. Seems that some of them have the same rack as the 2.8, and some have one with 3.2 turns and progressive, said to be the same as the standard e36. I've personally measured one taken directly off of a 1.9 and it was 3.2 turns and was progressive. I wish I remembered what year, because I suspect BMW did some changes over years for one reason or another. Maybe it wasn't original to the car or something, IDK. There's a lot of conflicting information otu there and RealOEM is definitely not correct on this point.
e36 regular racks are 15.4:1 VARIABLE rate (45mm per revolution) (I measured this directly off the two sets I've had)
Z3 M racks are identical to regular e36 racks (progressive) but shorter travel due to smaller wheel-wells, according to everything I've ever read (though I've never had one myself). IIRC it's a 15.4:1 VARIABLE rate. But, these are pretty much unobtanium on the open market, so who really cares......
e36 M3 racks 92-94 are a slower ratio (not sure exactly what, but ZF had it listed as 39mm per revolution, so the slowest), and are either linear or mildly variable, depending on who you trust. I think that's a ratio of something like 17:1 but don't feel like doing real math.
e36 M3 1995 rack is actually the worst of all of them all. still variable and 39mm per revolution, but a different variable ratio due to different steering geometry. Everyone agrees the '95 rack is the worst of all of them. Who cares, don't get a '95.....
e36 M3 1996+ racks are the same overall ratio (45mm per revolution) and travel as the regular e36 racks, but are linear supposedly. IDK. M3s I've driven certainly didn't feel like they had linear racks to me, but whatever. I suppose there could be a small variable rate at center, smaller than on the e36 non-M, or not. Maybe they set the lash different from the factory to make it feel that way. IDK. I mean, you have your old rack from an actual M3 sitting there. Go do some measurements..........let us know.
Oh, and just so you know...the stock e30 rack is a pathetic 20.5:1 ratio, or right around there. So don't be surprised you can out-turn the e30s that still have the OEM rack on it lol.
In any case, the Z3 2.8 (and at least some 1.9s) definitely has a quicker ratio than your stock M3 rack, but it's not a night-and-day difference, based directly on the numbers. Most of the reviews you read about how amazing they are were probably from e30 guys, lol....since it IS night and day vs. an e30 rack. IDK.
bottom line is the Z3 2.8 is the best rack for e30/e36 (IIRC the e36 rack is a similar ratio as well, but IDK). So, you have the best rack. Several of us have those racks locally and I've never heard anyone call it twitchy or nervous, though. But, that's the internet for you.
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turns lock-to-lock is irrelevant to ratio. Just like suspension bump-stops, racks can have their stops adjusted to account for tire/chassis clearance. So a rack with 2.7 turns lock to lock CAN be the same ratio as one with 3.8 turns lock to lock. But the latter may just turn a tighter angle due to large wheel wells or something. In any case, that's the fallacy with saying "oh, this rack is faster because it's less turns lock to lock" without actually measuring how far the rack itself is actually extending......