With your 103" & 255 Cams you might want to try 22 Discs & Closed End Cap to get the Max out of your tune,, Look at it this way your Fatshotz came with 10 Discs and a Open End Cap (open end cap = 10 discs) so you have the flow of 20 Discs from the pipes when they were new (THESE PIPES WERE MADE FOR 96" MOTORS WITH A CAT IN THE HEAD PIPE)So with 15 discs and closed end cap you have less flow than before,, add the fact that you have a 103" & 255's,,, seems to me you will get a better tune with 22 discs instead of 15