I am in the market for a high flow cat, for my '98 GSR I understand carsound makes one of the better ones and I was just wondering what inlet/outlet diameter I would need with a Comptech 4-2-1 header and Comptech Catback exhaust. Do I need to have my O2 sensor welded to this cat? Also, my current oem cat has only about 45,000 miles on it, is it worth replacing with a high flow, and am I going to notice any remarkable increase in hp or torque,(I also have an aem cai) and does a high flow cat also increase the sound db's with my current setup.
Thanks for your input.

 

Any combo of header and exhaust that is designed to bolt up to the stock cat is going to maintain the same 2.25" inlet and outlet diameters. The stock cat doesn't really become much of a flow restriction unless you go to 2.5" piping or it gets really old. Since you have done neither you might as well just keep the stock cat.

As a side note, Carsound only sells universal-fit cats. You need to have flanges and oxygen sensor bungs welded on after you purchase one. Some places sell them pre-made with all of the welding necessary for a particular application. Still, on a '98 with that kind of mileage and 2.25" exhaust piping the stock cat is not a problem.

 

Thanks for the input,

 

Since we are on the subject of high flow cats.

I have a 95 gsr with about 62,000 miles, stock except for exedy clutch comptech sri and ice box. I just ordered a dc sports 4-2-1 header. I was wondering if i should go ahead and buy a high flow cat or not. I want a quiet exhaust like a rsr exmag. Would it be to my advantage to get a high flow cat or just save up for the exhaust. Thanks.

 

I have to completely disagree with you on this. The stock cat is terribly restrictive and is one of the worst "bottlenecks" in the entire exhaust system. If given the choice, I would even opt to run the stock exhaust manifold with a high-flow Carsound over running an upgraded header mated with the stock cat. In the case of a a DC or Comptech "USDM-style" header (both with 2.00" OD collectors), I would suggest a 2.25" Carsound. My shop of choice for Carsounds is SMS Products (SMSProducts@adelphia.net - 540-287-6385). Dave (the owner) will be able to provide you with a complete bolt-on cat for your application.

 

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.

 

You're getting to the point where your stock cat may indeed be clogged up. There are really only two ways to tell: put the car on a dyno and if it makes a bunch less power than it's supposed to, or take it off the car and rip it apart (it can't be put together again). With a car of that age, it's not a bad idea to replace the cat anyway.

 

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.

 

What you fail to realize is that the cat is only part of the exhaust system. Look at the flow difference between the stock cat and a test pipe. Yet when people running a test pipe along with an intake, header and exhaust put their car on a dyno they get very similar results to what they would get with the stock cat. I'm not saying that the stock cat cannot be out-flowed, the point is that given what the rest of the exhaust system is able to flow even with bolt-on mods that the cat doesn't slow anything down. Only when the rest of the exhaust system's flow exceeds the capacity of a non-clogged stock cat (i.e. 2.5" collector, etc) does it become a bottleneck. Also, the "not-so-recent header test" you're referring to was on an ITR which is already putting out 25 hp more than a GSR and benefits more from increasing the exhaust flow.

So yes, if the exhaust system is upgraded to a 2.5" collector, the engine is upgraded to at least ITR specs, or the stock cat gets clogged then by all means a replacement cat is a good idea. But on a stock LS or GSR motor with a bolt-on header and exhaust and relatively new stock cat, replacing the stock cat serves not much purpose.

 

I'm well aware that the exhaust funtions as a "system".

I assume that since you request dyno proof of my argument that you have charts to support this (though I suppose "similar results" is rather subjective and could be argued)?

If an upgraded cat-back is able to flow more than ~220 CFM (which a well-designed one should, considering that a length of 2.25" piping can flow 400+ CFM and that there are systems which have shown only a 2 WHP loss over open-header), then the cat is a restriction. From your statements, are you arguing that the rest of a modified system can't flow more than the OEM cat, even considering its absurdly small inlet and outlet?

Yes, the test was on a basically stock ITR, although the C1 and C5 are not so different that they will not react at least similarly to exhaust modifications (as header builders can attest to).

Again, I contend that it is at least as cost-effective as replacing the header and cat-back.

Also, I do hope you realize that our discussion shouldn't be perceived as anything more than a friendly, intelligent debate I think it benefits the entire forum when members are able to read and/or particpate in these discussions, and hopefully come away with a bit more knowledge. :beer: