Originally Posted by KARgt13
The front bumpers are the same, the splitters are different but not much in thickness so I'd say half an inch with the GT being taller
You were right!
So here's the duffmod for springs...Boss front springs and top coil removed from rear springs on Brembo GT. Best compomise and cost.
I swapped the front springs on my '12 brembo gt with those from the regular boss (not LS!) and it lowered the front exactly 1/2" and increased neg camber slightly (along with about +12% rate). For the rear, I calculated the spring rate of the brembo gt spring if one coil were removed and if you assume the top coil is of the average mean diameter (which it is not), the rate comes out the same (within 4lbs/in) as the regular boss rear spring (not LS!) . However, the cut brembo spring actually ends up a little softer in rate than the boss rear spring (because the top coil has different mean diameter)and also lowers the rear a little more than the boss springs would but still keeps a very slight forward rake on the car. These are probably the stiffest springs you could get away with without replacing the stock dampers. Also, I did not need an alignment correction or adj phb with my mod.
The comments in other threads about staggered tire size and balance are more applicable to cars lowered 1" or more due to change in front roll center height and camber (gain front traction) as well as change in antisquat of the rear (lose rear traction). And syiffer frony rayes work better when defeating the stability control. Oh, remember that the boss mustangs increased rear stabar size with the increase in rear tire width (I retained my rear stabar). In this case, with just 1/2" drop in front and 3/4" rear, the main reason for not using the boss rear springs was just slightly better match to the dampers. Btw if/when I eventually add anti-squat brackets I will probably put the boss rear springs in.
I would recommend my mod to anyone who doesn't want to lower their car much or spend s lot of money to do it. This based on street evaluation since I haven't taken it on the roadcourse or autox in this form as of yet. The FRPP and Steeda springs are great but require new dampers unless you have a boss to start with...and likely an adj phb and alignment. So do my mod for now and then save up for new dampers if you still want to go further with suspension mods.
Just wanted to share my experience and opinion regarding this, even though it does not consider the koni sports. Hope it helps someone.
Boss Springs and Koni Sports - The Mustang Source - Ford Mustang Forums