I did the Koni inserts with my WRX. As long as you get the bumpstop lengths right (you may need to trim a stocker) you should be good to go with minimal impact to ride comfort.
They don't like too much spring rate, but I'm assuming from the sound of it that the Swifts are one of those largely cosmetic spring setups that kinda sorta have a little extra stiffness.
In regards to the spring rates from what I can find:
| Front Lbs/in | Rear Lbs/in |
FXT OEM | 118 | 157 |
FXT Swift | 179 | 185 |
FXT King | 148 | 196 |
However I'm reasonably sure the swifts are progressive vs the OEM being linear, so I'm not sure if the listed numbers for the swifts are peak or average or what. I'd say I need to dig more, but swift is pretty much the only manufacturer left for fozzy springs besides king, so my options are limited unless I trackdown old stock of whiteline springs or similar.
The king rates on the table are the guesstimate of what they should be off their generic claim of around 25% stiffer than stock from what I can find, but is in no way a guaranteed figure.
I'd give it a shot with stock springs first to see if you like the damping, and then tinker with aftermarket springs rather than change two things at once.
One of the main reasons I am doing this is for the drop for autox regs. Without the springs I don't think I will be near my goal. Added to it the additional costs for alignments and paying for the installation and so fourth twice. I'm reasonably confident I can modify and assemble the struts/inserts, but I have no where to install them on the fozzy itself.
My main concern right now is trying to decipher and understand is the differences between the OEM, swift, and impreza springs/struts, and how that relates to how to best install the inserts and what sort of actual drop it yields. Basically from what I'm seeing the distance from the bottom of the strut to the spring perch itself on the fozzy vs impreza is the same. The difference is the length of the continuous OD of the housing above the spring perch and the length of the actual dampener shaft itself.
Pic taken from here by Peaty:
http://www.subaruforester.org/vbulletin/f70/impreza-struts-vs-forester-struts-difference-7607/So theoretically if I cut the fozzy's housing down to the point where the insert seats on the bottom of the housing properly (without a spacer), there is no tangible difference between using FXT donor struts or WRX donor struts as the housing above the spring perch would be shortened to WRX length and the insert's shaft would make it the same overall length as the WRX strut as thats what it was designed for in the first place.
Where it gets interesting is if I use the fozzy's housing I have room to set the height of the dampener itself, as I have the option to not cut the top of the housing down all the way, and instead raise the insert higher in the housing using a spacer.
Where to cut it would be a simple choice if I were using WRX springs, as I wouldn't want it mounted any higher as the stroke of the insert was designed for the wrx so I'd lop it off and no spacer. However I am planning on using fozzy lowering springs, which are equivilent to WRX lift springs according to King springs catalog. These springs are designed for a different stroke than the WRX springs, especially being progressive.
Which gives me 4 options:
1.) Make it match forester strut length at the expense of travel, however the strut would have room to decompress when the full weight of the vehicle is not on it.
2.) Make it match WRX strut length which would theoretically give me more travel, but would increase the preload on the spring and limit the suspensions ability to decompress when the full weight is off of it.
3.) Split the difference between forester and WRX strut length by measuring the distance between the insert and the bottom of the fozzy housing, splitting the distance, and installing a spacer.
4.) Figure out how much the Swift springs actually compress when the full weight of the vehicle is on them on forester length struts, split the distance it compresses, and make the spacer based off of that.
In the forum link in the previous post they used a 3/4" spacer, I have to do more digging to comprehend how they came up with that spacer size to ensure it meets my needs so the struts have room to decompress while having enough travel so they don't bottom out constantly.
So much mix and matching, and jumping back and fourth from tab to tab on different threads, different specs for fozzys and imprezzas, its making my head spin.
I have my takeoff forester struts you can hack apart if you wish.
I might take you up on that offer once I get a more comprehensive game plan together.