I copied this from the Maximum Motorsports site. I hope this helps...
Q. Why do you have two different coil-over kits?
A. The dimensions and construction features of the Bilstein strut housing are different from other struts. This requires subtle changes to many of the kit's components.
Q. What are the benefits of switching to a coil-over front suspension?
A. Better handling and better ride quality! When the spring is in the stock location on the control arm, the minimum spring rate for performance handling is 700 lbs/in. Better handling can be had with higher spring rates, but ride quality begins to suffer with rates over 850 lbs/in. A coil over kit allows the use of wheel rates that are much higher than those obtained with an 850lbs/in spring in the stock location. This will dramatically reduce body roll and brake dive. Other benefits include easily adjustable ride height, the ability to do corner weighting, ease of measuring bumpsteer, less weight, and a wide selection of available spring rates.
Q. How do I compare a coil-over spring rate to my conventional spring in the stock location?
A. The spring rates must be converted into wheel rates. The wheel rate is the spring rate measured at the wheel. The wheel rate (for the front) of a Mustang is ¼ of the spring rate of a spring in the stock location. For example, An 800 lb/in stock location spring has a wheel rate of 200 lb/in. For a coil-over suspension, the wheel rate is 9/10th of the coil-over spring's rate. A typical coil-over spring rate for street performance handling would be 350 lb/in, which provides a wheel rate of 315 lbs/in.
Q. How does a coil-over spring with a higher wheel rate ride better than a conventional spring with a lower wheel rate?
A. Although the coil-over spring in the above example increases the wheel rate by over 60%, the ride quality will actually improve. A conventional spring located on the control arm contributes to ride harshness because of friction in the control arm bushings and ball joints. A coil-over kit eliminates this friction by acting directly on the spindle and upper strut mount. Coil-over springs are also much lighter than a conventional spring. The resulting reduction in unsprung weight allows the suspension to more easily follow bumps in the road.
Q. What springs are available from MM?
A. Standard 2.5" inside diameter coil-over springs are available in 25 lb/in increments between 175lb/in and 600lb/in, and in several standard free lengths. The wide range of springs available means that coil-over suspension systems are easily tuned by swapping springs to balance your car's handling.
Q. What are my options on setting ride height?
A. Ride height can be easily adjusted by changing the position of the lower spring perch (The spring perch is raised and lowered by rotating it on the threaded sleeve). The total range of adjustment is more than adequate when the correct spring is chosen for your application.
Q. Are there any disadvantages of running a coil-over suspension?
A1. There may be tire and wheel clearance issues, depending on the wheel size, back spacing, and tire size. For example, our coil-over kit will not interfere with a 275/40/17 tire on a 17X9 Cobra wheel on an '87 Mustang with '95 spindles. The Konig Villain wheels will require a ¼" wheel spacer for clearance.
A2. Because the upper spring perch lowers the point where the bumpstop contacts the chassis, all non-Bilstein coil-over conversions will reduce the amount of available bump travel (no matter what others tell you). We have seen other kits that reduced the available bump travel by over one inch. To maximize bump travel, we carefully designed the upper spring perch assembly-- it provides over ½" more travel than most other kits. Also, our exclusive bumpstop helps regain some of the lost travel because it is shorter and softer than conventional Mustang bumpstops. Don't be tempted to regain bump travel by not using bumpstops. Damage to your car, CC plate or strut will occur when the suspension bottoms.
Q. I've been told that If I use a stiff enough spring for racing, my suspension will never bottom and I don't need bumpstops.
A. If you are running a spring that stiff, you will be hurting your car's ability to absorb bumps (large or small), which reduces your car's overall cornering grip. To quote Carroll Smith in Engineer to Win, "If you are not USING the bump stops, you are running stiffer springs than you need and are therefore giving away some cornering power."