I talked to procharger friday and was told the 12 rib option is discontinued because it has so many problems. I was intending to switch from the 350 to the 412 in the '91 Z28 and switch from the 8 rib to the 12 rib with nastier pullies to really crank on this P1SC (the 8 rib slips even on this little baby!). I also intended to get an F1 for the 415 in the '92 B4C and I dont want to use 8 rib and I hate the problematic cog on daily drivers, so what is the real deal with the 12 rib? I know there are clearance problems (there are even with the 8 rib). but what else?