There are two different sets of Bilstein shocks available for the B-cars, including the Impala. Bilstein lists the shocks as different versions for the '77-'90 cars and the '91-96 cars, although these are completely interchangeable under most circumstances.
Here are the intended applications and Bilstein part numbers:

The shocks they specify for the '91-'96 models were specifically developed to Chevy SEO 9C1 (police) platform engineering parameters, and are actually the same (except for cosmetics) as those sold through GM under the SEO 8X3 option on the 9C1 Caprice. They are all painted yellow, but the factory 8X3 versions have black boots and Delco-Bilstein stickers and a broadcast code sticker while the OEM Bilstein ones have blue boots and regular OEM Bilstein stickers. The OEM versions are much cheaper than the 8X3 versions purchased through GM!

The 1516/1517 shocks feature slightly firmer compression but also much looser rebound settings than the 1104/0929 shocks, which Chevy believes to help control the car better when it is going over large bumps, uneven roads, or when it is coming down from being airborne. The 1516/1517 shocks are also progressively damped, which means they have a variable rate that offers less resistance to light inputs, and much greater resistance to large inputs. This gives a more comfortable ride, especially over rough
roads, and still offers greater control than the original factory shocks when the situation demands. Unfortunately the variable rate also makes them feel floaty during normal driving, especially when compared to the DeCarbon shocks that are standard on the Impala. The 1104/0929 shocks are a linear damped shock, and do indeed "feel" much firmer and offer more control, especially with lighter inputs.

Another issue is the 1517 rear shock was designed for 1" longer travel compared to the 0929 shock. This was also specifically at GM's request, primarily to facilitate tire changing on the '91-'92 Caprice 9C1 with the skirted rear wheel openings. Obviously on the '93 and up Caprice or the Impala which have radiused rear wheel openings, this is NOT an issue.

Here are the specifications on the Bilstein B-car shocks:

Bilstein B-car Front shocks:

Bilstein B-car Rear shocks:

Bilstein commented that the 1516/1517 shocks are one of the only ones they have done where the compression rate is higher than the rebound. These were done specifically for Chevy at the SEO 9C1 platform engineer's  request. This does give them more of a "built-in float" than the 1104/0929 shocks. It helps cushion an impact (high compression), but the light rebound makes the car feel somewhat floaty. These specs are in Newtons per mm.

I have previously installed the SEO 8X3 (1516/1517 shocks) in my Impala, also with stiffer rear springs and the Herb Adams swaybars. I also had the 1104/0929 shocks on my '88 9C1 and am installing them on my '95 9C1 as well. From my direct experience the 1104/0929 shocks definitely feel much stiffer overall and a much less floaty than the 1516/1517s. Based on how much I liked the 1104/0929s, I will be upgrading the 8X3 shocks on my Impala to custom valved units that are much stiffer overall. I like the
stiffer 1104/0929 shocks much more than the softer 1516/1517s, but I must say that drinking a cup of coffee without chipping my teeth in my 9C1 has been a bit more difficult than in the Impala! <g>

The bottom line is if you want a more comfortable ride with less jounce, then go for the '91-'96 (1516/1517, also SEO 8X3) shocks. I would call these the "Grandpa" versions of the Bilsteins, they will far outlast the stock DeCarbon shocks on the Impala, but are a little too soft feeling for my taste. If you like a shock a little firmer than the OEM DeCarbons, then go for the '77-'90 (1104/0929) Bilsteins. These are firmer without being too firm, and they work extremely well with upgraded swaybars and springs.

Bilstein has also indicated to me that they could do any custom valving you want for $55 per shock. That way one could have any rate of compression or rebound damping they wanted. That is what I am considering for my Impala, I plan on selecting something about 25% stiffer than the 1104/0929 shocks in that case.

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