We drafted this for the other big Miata sites, but figured it was worth a share here too.

Supermiata Xidas for the NA/NB are divided into two families based on damper length:

Xida Race
Xida Race are the original short body. Designed/optimized for pinch weld heights of 4~5.25" with ~23" tires.
One ("singles”) or three ("triples") damping adjustments.

Xida XL family: XL, GS and ACE
Xida XL/GS dampers share the same internal/external construction and valving with the Xida Race, they are just longer. Pinch weld heights of 5.25~6.5" with ~23" tires.
One ("singles") or two ("doubles") damping adjustments.
GS and XL variants use the exact same damper, just different springs, mounts, helpers.
ACE are the same length as GS/XL but with electronic semi-active dynamically controlled damping adjustments.

Why XL family:
The Xida XL will allow taller ride heights that raise the roll centers closer to OEM. This beneficial change increases natural roll resistance allowing the use of softer springs and smaller sway bars for a given roll stiffness. Big sway bars and stiff springs are the enemy of ride quality on bad roads. The taller ride heights also increase bottoming resistance. These features together allow even better ride quality on the street than Xida Race.
-You wish to install them on an Exocet or Catfish kit car that uses Miata underpinnings. The suspension geometry of these cars is slightly different than the stock Miata so they benefit from the longer length Xida
-You require speed bump or steep driveway clearance, or simply don't want/need the car that low.
-You have drop spindles on your NA/NB Miata.

Supermiata Xida Race
The original Xida Race was designed and engineered from scratch to win championships in road course, autocross, sprint and endurance racing. More track records, race wins, and national championships have been won on Xida Race than any other Miata coilover.
Xida Race have more bump travel than OEM, sometimes requiring fender or liner modification with big race tires. This critical extra bump travel is made possible by using shorter than OEM shock bodies. This results in reduced droop travel compared to our Xida XL/GS/ACE. The use of low friction internal coatings, precision assembly and pro motorsports level technology result in the supple ride quality that have made Xida’s the gold standard for NA/NB coilovers. Xida Race optimal pinch weld heights are between 4” and 5.25”.

Xida Race user:
-Xida Race users want the maximum amount of grip/performance/handling and are willing or able to use the lower ride heights.
-Xida Race’s adjustment range tops out around 5” on heavy cars (MSM, or any Miata with full options + roll bar and both tops, ~2500lbs with driver etc.)

Supermiata Xida GS
Over the years the most common Xida request has been for a daily driven street car that sees some occasional autocross or HPDE. This is what we carefully tailored the Xida GS variant for. By producing a large quantity of only one configuration, we were able to reduce the price by about $300 compared to an equivalent XL kit.

· Swaps the XL’s customizable rate, motorsport grade Hyperco springs for lower cost, standard grade 550/350lb springs.
· Swaps the XL’s high rate helper springs for low rate tender springs.
· Deletes the XL’s Torrington bearings.

Xida GS user:
-Owns a helmet and wants to run an occasional autocross or HPDE.

Supermiata Xida XL
Xida XL use the same Hyperco springs, high rate helper springs, Torrington bearings and the optional Billet Coaxial Mounts as the Xida Race.

Xida XL user:
-Requires spring rate or mount options not offered on the Xida GS.

Supermiata Xida Ace
Xida ACE are electronic dynamically self-adjusting dampers using Tractive’s DDA piston. This system controls both rebound and compression in near infinite steps over a wider range than is possible with traditional manual "clickers". The ACE system is like today’s high-end OEM systems in that it continuously adjusts the shocks at 100hz (10ms) over the full range in force increments as small as 2lbs. That's very fine tuning of damping force based on the included accelerometer and user settings from the two-button wired fob. Only a few international pro level racing classes allow this technology. When they do, the winning teams employ it.

What you will experience with ACE is an almost magical combination of eerily plush ride quality yet with instantaneous responsiveness. How plush, how instantaneous is up to you, courtesy of the control fob. Nothing rides better than ACE. They are banned in 48 states. They most interesting man in the world has Xida ACE.

Xida ACE user:
- You are not bound by competition rules restricting the use of state of the art semi-active suspension.
- You wish to achieve the absolute best ride quality and not want to stop the car, open the hood/trunk and adjust the shocks when you want to go play. Just hit a button and let the system do it for you.