Originally Posted by Green Dragon

High flow cats are just less restrictive (and in our case with the M54's manifold, much less restrictive) and clean up your emissions enough to pass. You have options. You can run 100 cells per square inch, 200,300,400 cpsi, etc and also an option of substrate, ceramic or a metal foil, both of which are treated to cataylize and clean up some pollutants. There are a few factors that influence which one to go with and alot depends on how clean your emissions need to be. Check with your local code first. If they sniff test, you need to be fairly clean. If they are doing a visual with a computer check looking for "ready status" with the ecu, not so much.

Our stock exhaust manifold is designed to be very clean, but is compromise on performance and efficiency. (about 15-20 hp loss)That's okay for most. They want the cats as close to the heat source to get them hot and expedite the cleaning of the emissions at start up. Aftermarket HF cats are going to flow much better, and not really losing hardly any gains from the installed headers. Its honestly the way to go. Its just a question of whats best for you and your area.

As far as software goes, its not needed, unless your working around the CEL, trying to get it off and you live in an area that doesn't require emissions, etc.

The shark Injector just compliments the performance upgrade, but does not turn off the post cat o2 senors,etc, so you will have a CEL if your not running cats and post o2 sensors.

Active Autowerk has a flash that turns off the post o2 sensors to eliminate the CEL, and give a performance boost as well.

You can flash your ECU to a Euro flash(EU2)(free) as well to accomplish similar gains of reported better drivability , responce, and a few extra ponies.