Whats the advantage over having them seperate?

 

I assume you mean, why is the stock rear not a coil over shock setup. Cheaper would likely be the primary reason, lighter, less parts, no spring compressor needed, and no real reason to go to a coil over on a stock car.

 

so your saying rear coilover is better for the reasons listed above or that the stock setup is better for the reasons above?

 

The stock setup is cheaper from the assembly and servicing points of view (you don't have to deal with the springs to install or replace the shocks). Fewer pieces, since the springs fit between fixed in place seats on the axle tubes and the chassis. For 99.9% of the driving population, that's more than good enough.

It's when you get seriously into tuning your handling for auto-x, open-tracking, or other competitive at-the-limit driving that you get involved with selecting springs of different stiffness than stock, and setting ride heights that differ from OE as well. With c/o's you have a much wider variety of spring rates and lengths than you have with OE-type springs. You can have the rate you need at the ride height you want. Plus, since you have infinite adjustability of the individual spring perch settings, you can affect your corner weights relative to each other (yet another tuneable handling variable that you pretty much have to tinker with, actually).

Norm

 

alright, I know someone sells a ride height adjustment for the rear with different spring rates, would this have the same effect as a coil over from what you described above?

 

Sounds generally similar, though to be rather picky there will be slight differences in overall effect (that you might or might not even notice in back to back testing). If it's cheaper than c/o's, you're happy with whatever shocks, and you aren't in contention for the SCCA National Solo II CP crown, there isn't much reason not to. Not to mention that the static weight of the rear of the car is still being carried where there is OE structure designed with that in mind (read: no concern about shock mounts having to carry both the spring loads AND the shock loads). FWIW, height-adjustment mechanisms for stock location springs are readily available for Mustangs and I haven't heard of any problems. AFAIK, this should be SP-legal (given that this height adjustment is/can be defined as a coil spring perch mod).

Norm

 

WITHOUT QUESTION the best thing going for an F bod is an AllStar Performance rear spring seat made to work with a 5" spring. This is a steel sleeve and aluminum nut/seat that accepts a 5" NON PIGTAILED spring. You get ride height adjustment AND have TONS of spring choices available. The springs you are going to be looking for are 5" x 11" race springs. Eibach and Hypercoil make them.
The seats are available from Lane Automotive. LANEAUTOMOTIVE.com. They are not pictured but you are looking for part #ALL56119. Springs are sold everywhere but I like to use STRANOPARTS.com. Give them a call as the site is not great. Sam can recommend spring rates.
I use this setup on my ESP car with a 125lb Eibach spring.