These are Racecomp Engineering’s Bilstein direct replacement struts. These have been valved specifically to work with Racecomp Engineering’s popular yellow springs. Although you can use your factory springs, as well as other compatible lowering springs. Bilstein is a trusted household name and pioneers of suspension components utilized by factory car manufactures and race teams worldwide. Teaming together with Racecomp Engineering makes for a serious performance replacement strut for those of you who don’t want the inherent harshness of some coilover sytems.