Originally Posted by MrFatbooty
I see, and do you have some kind of dyno results to back up your claims? I've noticed a fad lately with people replacing their stock cats as another bolt-on and I can't say they're having very impressive results. All of the information I've seen points to the fact that the stock cat when mated with a header and catback designed to bolt up to it imposes no large restriction in flow.
There are two situations which merit the stock cat being replaced:
- When larger piping is used, it does indeed become a restriction but the smaller-diameter pipes do not flow well enough such that the stock cat would be a bottleneck.
- Deposits of smog particles and other grime get cooked on to the matrix and become physically bonded to the surfaces. These deposits grow over time and eventually limit the ability of the cat to flow. Cats are considered a wear item because of this.
Unless one of these two situations occurs, there's really no reason to replace the stock cat.
1) Catco (metal core) 2.25" 223.8 cfm
2) Catco (metal core) 2.5" 271.9 cfm
3) Catco (std. core) 2.25" 338.2 cfm
4) Catco (std. core) 2.5" 388.0 cfm
5) Random Tech 2.25" 297.1 cfm
6) Car Sound 2.25" 342.7 cfm
7) Test Pipe 2.25" 407.1 cfm
8) Stock NSX 242.1 cfm
9) Stock Type R 223.6 cfm
10) Stock Integra 218.3 cfm
These are older flow figures for a variety of cats. A number of things are clear from examining these. First, an Integra cat flows roughly half as well a test pipe. Secondly, a 2.25" Carsound (an older model) flows over 50% more than the stock cat it's replacing. Third, different brands and designs of aftermarket cats can vary widely in their flow numbers.
I'm not certain why you favor stock cats so much (or why you choose to take such a defensive stance), but it's difficult to argue that a component that reduces relative flow by 1/2 over the rest of the exhaust piping is not a major bottleneck. You also mention a 2.25" inlet/exit, of which the stock cat has neither (it actually even necks down to ~1 7/8" at certain points).
No, I don't have a direct dyno comparison to post, but anytime that exhaust flow can be increased so significantly, gains are sure to result. I believe that this is a major reason why so many people realize gains when switching to a "JDM-style" 2.5" collector header. Yes, the larger collector improves flow, but these headers are almost always installed with a Carsound or a test-pipe. A not-so-recent header test revealed that, when each was paired with a test-pipe, the power difference between even higher-end JDM headers and a 2.00" collector "USDM" header was not significant.
The "fad", IMO, is simply that people are begining to realize that it doesn't make sense to spend money to upgrade a header and cat-back with high performance pieces, only to have a small and poor-flowing bottleneck remaining between them. If people aren't seeing gains from replacing their cats, perhaps they are using one of the lesser-flowing brands. However, compared with the cost/benefit ratio of a cat-back or a header (or both), ~$175 for even 3 WHP is not a bad deal at all.