Hey, I have some questions for anyone who has done this modification. I am considering installing a Gen 3 subframe into my 91. I am looking for any guidance or advice you can give me on this.
My subframe isn't in the car yet so filter what I say appropriately. The car's coming apart for the swap now.
You have significant work to do on the subframe no matter whether you start with a SHO or SLO/Sable subframe; my opinion is that the SLO subframe is a lot easier to get and no harder to work with. I've heard that there was a design change on the Gen 3 subframes in '98 so some techniques may differ, mine is a late '98 Sable subframe.
1) You've got to cut off the Gen 3 trans mount and weld on a mount cut from a Gen 1/2 subframe, and you need to make a jig to get it properly located. It overhangs the rail so you need to do something to support it on the outside of the subframe rail.
2) You need to make a pocket in the subframe rail below the mount for MTX clearance; I've seen one that just had a hole cut in the rail but that strikes me as weakening the rail too much. I made up a pocket from a section of big rectangular tubing (.090 wall? .125 wall? Don't remember) and welded it into the subframe.
3) You need to fab up a front mount. This proved easier than expected, the subframe's got funny angles top/bottom here but you just drill a 1/4in hole through both layers of the crossmember where the mount stud needs to go, then put a 6in long piece of 1/4in drill rod in a holesaw arbor with a 1.75in holesaw on it and run the rod (and holesaw) up through the crossmember guided by the small hole, weld a piece of 1.75 x .125 wall tubing (old rollbar tubing in my case) up through the now-big hole to form the mount tower, and weld a plate to the top extending out to the edges of the crossmember for the mount to sit on. Nut goes right up through the tube from the bottom. Took me 2 hrs to think of it and an hour to actually do it.
4) From my measurements the rear Duratec SLO mount tower is close enough to be usable if you slot the hole a bit. I think there's folks who disagree with that conclusion. I'll know if my answer's right in a couple weeks.
The Gen 3 steering rack is a high-zoot ZF Servotronic rack typical of those found in BMWs, Audis, Jaguars, the Lincoln LS, a few Cadillacs, etc. The Gen 1 rack, by comparison, is scrap metal. I'm not aware of anyone who's yet actually hooked up the SARC box from the Gen 3 to implement the variable-assist feature, but it looks trivial to do (the vehicle-speed pulse-count input to the box is the same as the Gen 1/2 VSS output) and I plan to do it.
I'm not really doing this for improved handling or anything like that, because I don't think there's much to be gained in raw grip. I'm doing it for ride quality and the Servotronic steering.
FWIW I had the Servotronic rack on a Gen 1 subframe and everything fit fine, so I expect that if you didn't care about the better-isolated Gen 3 lower control arm setup you could probably just bolt up the Gen 3 rack to the stock subframe, work up the plumbing and call it a day.
