I call my car stock. To be honest, its not totally. The time I ran in my sig was with the following:
No strut brace (not all came with 'em anyway)
K&N, no snorkel
Steeda Tri-ax (should have been stock)
No spare/jack
Ford 9mm wires (stockers had an open #8)
Champion coppers
Tires at 25 psi
Now I don't know about where everyone else lives, but if I called my car
modified I'd get laughed at. Sure when the h-pipe, gears and timing adjuster go on, it won't be stock anymore. But right now there is nothing that would raise an eyebrow under a warranty.
I guess that would be how I'd define
stock. If you do it to an 03, and it doesn't even cross your mind that it would effect the warranty, then you're still stock. Unless of course another Mustang guy(girl) is poking around and wants to know.
