When I do a search I am getting different answers from different people. I am going to be getting the Downey headers for my 3.0, unless there are better ones that someone knows off. Their exhaust system with free flow cat is a 2.5" system. Anyhow I see that people run these, but then other people say that the 2.5 does not have enough back pressure and that you should run a 2.25" exhaust system. I would really love if anybody has any comparisions or info about what is better. I will in the near future be rebuilding my engine and getting OS valves, bored out, and other machine work done. My sig has my truck info if you need more.
Thanks

 

The only thing about the downey headers that isnt really adding up to me is the head pipe dia. The stock xover pipe is somthing like 1.5" but the downey pipe running from each header to the cat is 2". I havent got around to it yet but when I do build my own xover for my headers I think I will go with 1.875" or 1.75" head pipes back into the merger before the cat. I was just crunching through surface area numbers and it looks to me like a pair of 2" head pipes are too much and may give the exhaust gas too much time to cool down compared to 1.75" pipe. The area after the header is the second most critical area for scavanging and keeping exhaust velocity up second only to the individual cylinder head pipes. By the time you get to the tail pipe the exhaust gasses have cooled so much that velocity probably isnt an issue nor should it be if the 1st two stages of the exhaust are doing thier job correctly.

Just my .02

 

I always point people here:
http://www.torquetechexh.com/whichsize.htm

Outside of the babble on the page, there's a nice chart at the bottom that relates engine size to exhaust size.

 

Here is another great article on exhaust theory