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There are many spring options available for the 1984 - 1989 300ZX. In addition to the normal aftermarket springs available (Eibach, Tokico, Suspension Techniques, Intrax, etc) you can also use springs from other Z31s. All Z31 springs interchange with no fitment issues. 87-89 turbos have fairly stiff springs and the springs from an 88SE (SS if you prefer) are about the hardest springs you can get without special ordering a set. Some common aftermarket spring rates are shown below. Note the Eibachs, they are variable rate springs. Variable rate springs are often touted as "wuss springs" because of their softness when uncompressed. Note however that Eibach's progressive rate springs are actually stiffer than the stock Z31 springs for most of their travel. Linear rate springs are generally better for performance but much worse in the ride department. All the non-Nissan springs below will lower your car approximately ¾ to 1-½ inch from stock depending on the condition of your current springs. 

SS springs are a rare commodity now, as the Nissan supply of them was depleted in March 2000. A trickle of SS springs are available at various large dealerships as they pop up in Nissan's inventory in Japan. Check Courtesy Nissan if you insist on buying them new ($112 EACH, $85 EACH from courtesy. You need 4.) Since they were so popular, they should start popping up in junkyards. All stock springs are color coded for their application. SS springs have white and purple marks on the rear springs while the fronts have orange and pink marks. If you get harder springs you also need firmer shocks to hold the springs. 

While the Z31 springs will fit in any Z31, there will be a slight shift in ride height depending on what model you have and what springs you end up using. So, if you use the 84 2 seater GL spring in an 88 2+2 turbo, you might find your car sitting slightly lower than before. 

^  Aftermarket Springs Available  ^^^^
^ ^  Front\\ (in lb/in)  ^  Rear\\ (in lb/in)  ^  Lowers\\ approx.\\ f/r  ^
|  AVO / Chassis Dynamics  |  ???  |  ???  |  35mm  |
|  B+G  |  138-147  |  175  |  1.6"  |
|  Eibach  |  140-240  |  183-254  |  0.75"  |
|  Intrax  |  107-423  |  119  |  0"/1.5"  |
|  Jamex  |  ???  |  ???  |  35mm  |
|  Kingsprings  |  190  |  300  |  25-30mm  |
|  Motorsport Auto  |  152  |  168  |  0.75"  |
|  Suspension Techniques  |  200  |  226  |  1.0"  |
|  The Specialty Shop  |  300  |  324  |  ½"/0"  |
|  Tokico  |  150  |  165  |  2"/0"  |
|  Whiteline  |  180  |  240  |  30-40mm  |
|  Z31parts.com - Type 1  |  300  |  325  |  1"/½"  |
|  Z31parts.com - Type 2  |  300  |  325  |  1½"/1"  |
|  Nissan - 88 SS  |  207  |  224  |  0*  |
|  Nissan - 84AE + 86T  |  146  |  188.2  |  0*  |
|  Nissan - 87-89 turbo  |  196  |  188.2  |  0*  |
|  Nissan - All others  |  136.1  |  140  |  0*  |
* see charts [[information:stock_springs|here]] to determine affected ride height. The most you should see would be about 1/2" difference.