PAC(Petersen American Corporation) is one of the largest spring manufacturers in the world, if not the biggest. They are OE manufacturers for most any company you can think of that use springs, from Catepillar to GM and Ford. They manufacture everything from ball point pen springs to springs used in trucking and shipping industry. Their racing arm is PAC, they do make springs for many aftermarket companies IMO they are the best spring out there. They also make custom springs for many of the top professional teams in racing. From NHRA to Nascar, F1, Rally and Sprint cars. One thing they are not is inexpensive. But then again what top shelf parts in racing are?