Suspecting partially clogged cats as a reason for poor fuel economy. I have a OBX exhaust stored away. What I want to do is to remove the stock manifold, gut the pre-cat, and then splice a 3" Magnaflow SS cat into the OBX header-back. (I have a much more...elegant plan for when the budget allows).

What I initially wanted to do was to run two of these cats in series. But then I realized that the cat I'm looking at looks like it's a dual monolithic-core, so maybe I only need one? If you're wondering why I don't just gut the cats and be done with it...I need some kind of emissions on my car to ease my conscience, even if my state requires it not.

 

Should have thought about this before buying a rotary

Just teasing

 

Yeah, it's more of a minimizing-damage thing. When I get the money (or a welder) I'make my own exhaust; a RB true-dual header followed by a H-pipe and parallel cats, leading down to side-exit mufflers.

But until then I want to gut the stock pre-cat and just install the OBX exhaust with a cat spliced in. If it ends-up making a few HP no biggie.

 

Your Idea should be just fine.(for your budget too.)
you can think about the HI Flo cats and maybe a Race cat later on down the line.
Remember though,you have to keep your air pump on,as you have an S5.

 

Until I do the Renesis swap, that is.

 

Why?
.it'll cost ya a Boat load of cash..I mean really,if you are just building it for Power,then go TII and get a standalone,big turbo,etc.
The Renesis is gonna create headaches fitment wise,where the TII slaps right in.
I'd do some Research on your "Goal" before you find out your wallet is gonna have a big hole in it.

 

Honestly I don't know what I want in her, at least on a consistent basis. One day I like the idea of the Renesis, the next a 13B-RE. I have done research and do know it will require a standalone and custom engine mounts. But it's a newer engine with better replacement parts availability, better internal technology, better fuel economy, potentially simpler (wires v. vacuum, coil-on-plug, less complex manifold structure) to maintain.