A coil-over shock design is desirable primarily for adjusting ride height, using inexpensive standardized springs available in various rates, and having appropriate shock valving. As you may know, there are no direct-fit coilover kits currently available in the USA for the Z31 300ZX, apparently, because there simply is not enough perceived demand. JIC did make front coilovers, but they were hideously expensive and I have not found them available for some time. Over the last couple years, while scouting Japanese online auctions, I found a few sets of coilovers for the Z31, all of which appeared to be either custom jobs or off-the-shelf adaptations.
Pictures of some Z31 coilover setups.
Not that the 88SS springs and Tokico 5-way adjustable shocks I had in the car were that bad at all, in fact they were very good. Unfortunately, OEM-type high performance replacements can not provide the same performance as a much more sport-oriented and well designed coil-over setup, with emphasis being on well-designed. simply don't buy the crap and you won't get the crap. When looking for performance oriented suspension components, consider Ground Control, Tein, Tanabe, JIC and other popular name-brands for quality components. Remember "garbage in garbage out" ... "quality is as quality does".
When I started my research into coilover setup, I figured I would end up doing something like Brian's idea (link). Also, I would recommend reading his page, as it covers basic coilover information that will not be covered here.
Cost being the primary factor involved, things really changed when Kris-86T let people on Z31.com know that S13 coilovers could be adapted to the Z31. Keep in mind there are 295 kinds of coilovers for S13 240SX's some have the same dimensions as mine, some don't. I can't vouch for the fact that most of the setups are like the Tanabe's I bought, so do bear this in mind if you do decide to go this route for coilovers.
You can compare and contrast the S13 and Z31 all day and all night. What it boils down to is the fact that S13's and Z31's are "close enough" overall to use units designed for the S13 on a Z31.
I found a really inexpensive group-buy on zilvia.net for the Tanabe Sustec Pro S-OC Coilovers for the S13. and decided to go for it. This being a cheaper set, it did not include upper mounts. I scouted around and found some cheaper S13 camber plates on ebay, which ended up being TEIN pilo units. Now came the quest to actually figure out what needed to be done to install everything.
Rear Coilover Conversion: I used the lower sleeves from a set of old Z31 OEM shocks (I believe all are the same), and stock Z31 NA rear upper mounts. The stock jounce "pad" is cut off allowing the remaining plate area for the upper spring seat to rest on and enough of a lip to keep the spring seat in place:
A few washers were used to shim the damper rod "down" and get slightly more travel and ride height. Assembled picture:
The bushings and sleeves removed from the bottom of the old shocks are bonded together. That is, the rubber bushing has to be cut away from the outside of the sleeve. I cut the majority of the rubber off using a hack saw, and then a bench grinder to smooth it out leaving just enough rubber to provide a snug fit into the inside of the lower bushing on the S13 coilover. With the rears installed and adjusted all the way up, the car sits roughly an inch lower than stock:
Some people have said that this rear location was not designed to support the weight of the car. I initially agreed, but upon closer inspection determined a few things:
Front Coilover Conversion: I used 87-89 strut tubes after determining that S13/S14 front spindles would fit the lower ball Joint and tie rod, but would alter suspension geometry and wheel placement if used on a Z31. In addition, 5-lug front hubs are expensive for S13 and 4-lug S14 cars. This route seemed entirely too complicated and expensive, so I stuck with something reasonable. In a sense, the new dampers were installed "into" stock strut housings.
The housings or "tubes" as they are usually called, were cut off about 2 1/8" just above where they enter the spindle assembly:
The lower part of the "tube" is expanded, but the pipe expander used could not go more than 2" or so into the pipe. Most explanders are like this, because they are only designed to flare a pipe to fit over another pipe. I did not want to weld the pipe to the actual outer tube of the damper, so another section of the strut tube was cut off. and expanded, welded onto the first part and then the coilover shock was inserted, lined up with the lower part of the bracket and slowly welded to the strut tube. That's the reason you see two welds here. Had the expander been able to reach farther into the pipe, it would not have been necessary. The strut tubes are THICK! At least 1/4" steel; tough stuff:
Completed setup, quickly painted with Rust-Olium for that long lasting shine. For comparison, on the right is a stock strut with ST spring and Tokico Illumina damper:
One of the upper mount holes on each side had to be slotted for S13 camber plate fitment. Otherwise, they fit and function perfectly:
With the spring perch adjusted all the way up and the damper positioned as low as it can go in the expanded strut tube, the ride height is under 1/2" below stock. With my wheels, which are pretty close to the stock strut tubes to begin with, I can't drop the car more than 1.5" below stock without very slight rubbing between the tire and the lower spring seat. I think a small (3mm) wheel spacer will fix the problem. Keep in mind these are 87-89 strut tubes, so the problem could be worse with 84-86 units. I'm sure stock wheels or something with a slightly larger backspacing than mine (lower offset) would have no clearance issues. I don't think I have too much of a reason to lower the car more than 1.5" besides occasional autocross and track events.
Completed installation pictures:
A couple side shots:
This project turned out to be a complete success for me. The car handles better than it ever has, and ride quality is hardly (if any) worse than the former configuration. Ride height and camber are now adjustable, and now there is acceptable spring rate and damping force for track use. Front camber is adjustable from slightly positive to negative ~3 degrees (eye balling it). I set it at about -1 degree for daily driving. There is very slightly more vibration noise in the car from the front suspension, which is to be expected with pillow-ball upper mounts. When you consider the price of a set of ST or 88SS springs ($200) and good shocks ($400+), anyone can see why this is such a good deal! I can only kick myself for having spent the money on other kinds of springs and shocks before finally deciding to go this route.
Tanabe Sustec Pro S-OC Coilover kit: $650 shipped, new.
TEIN camber plates: $160 shipped, new.
Welding: $50 cash.
88T struts: Free from parts car, used.
Z31 NA rear upper mounts: Free from parts car, used.
Total Cost: $860