I have a 2007 dodge ram 1500 SLT 4x4 5.7L hemi
Towards the end of the month i will be going to get my JBA long tube headers installed, model number 6962SJT titanium ceramic coated, competition headers. the header outlet is a 3 " pipe outside diameter/ 2 7/8" inside pipe diameter then the adapting flange goes from a 3' inlet down to a 2 1/2" pipe, my stock cats are 2.5" piping but then after the cats i would be going right back up to 3" piping running true dual exhaust with an x pipe, then super 40 series flowmasters into a 3 " inlet 5" hexagon tips. SO im guessing since im getting the whole exhaust done i might as well have 3" cats put on right? cause i feel like if i don't its going to bug me. Also if it makes any difference on the cat, i will be installing cold air intake, tune, and a 6.1 or 6.4L air intake manifold on it, then later down the road i plan on forging internals and putting boost on engine. and upgrading fuel injectors etc... so any suggestions on selecting the right cat would be great. and if i were to keep stock cats how much back pressure would i have?

 

I love posts like this lol
Let's start with why would you purposely neck 3" down to 2.5" just to bring it back up to 3"?

It doesn't really. What I don't see is how you're looking forward to rearranging your accessory drive and/or relocating your battery (depending on manifold selection).

And what I don't see here is what your plan for the inevitable destruction of your transmission is lol.

 

Okay lets start with, im trying to find the right 3" cat for my truck because i dont want to neck down... 2nd i dont plan on boosting or forging anything for a while it will take prolly around 1-2 years before i start going crazy on the mods. but ones i will be doing very soon is cold air intake, tune, and maybe by end of the year ill put a 6.1L or 6.4L intake manifold on the truck. i mainly just want to make sure i buy the correct cats since i am going to make the shop take the flange adapter piece and cut right at the welds to keep it a 3" piping from the long tubes.

I just want to make sure i get the right cat for my vehicles weight class. and a good flowing cat.

 

Any universal 3" cat will do, not really rocket science. Most normal vehicles do not have an exhaust pipe that big (yours originally wasn't) so it will handle it just fine.

Which brings me back to my original question on this:
You keep saying it like this an old school engine and you can just swap a manifold ***** nilly; it doesn't work that way anymore, not for a long time. Because of the way the stock accessory drive is designed, the stock manifold is the only one you can use unless you intend to or are willing to move the alternator and AC compressor out of the way as well as convert to an electric fan. 6.1 manifold? TB comes right out the front, as well as requiring adapter plates to mitigate the gross port mismatch. 6.4 car manifold is active (has switchable runners) and dumps straight at your battery. Truck 6.4 manifold is also active but would at least dump in the correct direction. Both 6.4 manifolds would require spacer adapters not just to mitigate gross port mismatch but to also clear the temp sensor on a pre-VVT block. Both would also require rearrangement of the accessory drive and conversion to an electric fan. None of these options is worth the effort (and more importantly, expense) on a completely stock engine. A cam swap at minimum is needed because the stock combo is pretty well matched with all its parts. Adding a later intake that flows better gives you the ability to move more air in but not use any of it. It doesn't seem like much but it adds up in expense rather quickly:
- Alt/AC relocation kit (if you can't do it yourself properly) - $1000 (only one company doing this for 3rd gens anymore), plus core charge for your front timing cover unless you send yours in for modification
- Manifold cost - varies, but 6.1's average $500-1000, 392's $350-900 & 6.4 truck's are rare to find used so usually between $600-1000 or new
- Conversion to electric fan - aftermarket kit requiring fan, shroud, temp probe and wiring, varies by manufacturer but count on $150-200 at least
- Adapter plates - $280 (same place as the accessory kit)
- Custom made intake ducting from air filter to TB - $$ varies by material

Most people don't know what they're getting into with these modern vehicles (even ones that are coming up on 20yrs old that are still different from the years before). Like I said, this isn't like the old days of carbs where you just bolt on a better manifold and gain power.

 

I would be buying an adapter kit from MMX, and once i get to it money would not be the problem with it. either i will stick to N/A or i'll end up putting a turbo on it, remachining the block and forging pistons/rods etc. I will end up doing it right and once i actually get closer to starting/doing the project i will order and look into it more completely. I dont mind rearranging components. i ended up getting Kooks high flow 3" cats. i'd rather build this truck up and rebuild over time than just buying a new truck from dealer.