So I ordered two magnaflow performance catalytic converters the other day. i went to 2 local muffler shops today and the first one said that I ordered the wrong ones and the second one said that the 06 Silverado has the best performance cats on the market. Soooo I have two questions: Will putting a "pre OBDII" cat in my truck make it run bad or just run codes? and is it true that GM's cat is the most high flow cat available? Also, JEGS told me the cat would work on my truck, it didn't say that is was pre OBDII in the catalog. It says that is is OBDII complaint. The box for the cat is pre obdII. It makes me mad that now I have to pay for shipping so I can return them. Another question, do high flow cats really do that much because i was thinking about ordering another set for Dynatech but if its not gonna make my truck sound any louder or get any quicker there is no point. Thanks, Chad.