Originally Posted by houston

9853 is the most open design nova pipe made (as well as the 9886) , it is geared toward low end grunt but it also can be tuned for RIPPIN top without being leaned to the moon , i run this pipe on the truck 95% of the time and the 9901ss the other 5% if i want to tame the engine down a touch and gain a little runtime (usually within 35-40 sec of each other)

9901ss is an efra legal pipe , it is made to be restrictive on purpose but out of many of the efra legal pipes it is one of the best
9853ss is not efra legal

hope this helps