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I see a number of folks are running "P" springs now. Was the recommendation/decision to use "P" springs based on performance difference when compared to the "K" spring, or appearance, or something else?
I believe they are identical other than the K springs lowering the car 1" more than the P. While I'm no expert on this when you lower the car more you introduce more handling issues that need to be compensated for. I have the P springs and if you're looking to slam your car the P springs are not it. My understanding is:
Here is a good review of the Ford Racing M-FR3-MGTAA supsension pack:
"The standard lowering springs provided in the kit drop the car by one inch. If you prefer to go lower, and we did, you can use the optional M-5300-K springs for a one and a half inch drop instead. When ordering a kit for yourself, be sure to specify this up front. According to Kershaw, the additional half inch of drop is for looks only. “With the 1″ drop it maintains excellent road manners. We offer a 1.5″ drop also but this offers virtually no improvement in handling but does decrease the ride quality substantially,” he explained."
I think one of those is a progressive rate spring the K if I remember right. That is not good for track use, I don't like them on the street but that's up to you. You want a spring that has a straight rate the you can feel gradually building pressure on that corner. Not a spring that is soft to start moves a bunch then stops abruptly causing spikes in compliance.