Select the following spring pressures based on engine vacuum reading:
-10 to -13 in/HG use 7 PSI Spring
-14 to -17 in/HG use 9 PSI Spring
-18 to -21 in/HG use 11 PSI Spring
-22 to -23 in/HG use 12 PSI Spring
If your going single and running a lot of boost get the big spring, and if not get the smaller spring. BUT the smaller spring is going to blow off like an SRT4 every time your even thinking about getting close to boost. So thats the difference.