Type R Club of America

Hi guys:

I have been helping my son with a mild build on his 2001 Integra GSR. It currently has a USDM ITR header/cat and Mugen twinloop exhaust. I just bought a JDM 4-1 header for him but am now reading (duh) that the collector on the cat may be an issue.

So, one option is to keep the stock cat and have a 2.5" flange welded on it, but that makes it a real headache to put the stock exhaust manifold back on if needed for CARB, etc. However, the JDM 4-1 does have all heat shields so maybe it can squeak by an emissions inspection anyways. Another option is to get a 2.5" replacement cat. I know some have used SMSP in the past but I am not sure if they still do this (or contact info). But, then we do have to worry about swapping the cat and header every time there is an emissions check and that is its own hassle.

I mention the year of his car because it is OBD-2 and I believe the exhaust dimensions are the same as the USDM ITR.

Any ideas that won't break the bank? I appreciate it!

typerx wrote: ↑Tue Jan 03, 2023 9:17 pm Any ideas that won't break the bank? I appreciate it!

Obvious question but have to ask since I don't have direct experience - Do the flange sizes differ between JDM and USDM? I know there's a quarter inch difference in diameter of pipe, but are the flange bolts a different spacing or something? Could you use a donut between flanges and just bolt them together?

I don't see any company called SMSP every being CARB-legal in the CARB database. These were probably 48-state when offered?

I don't think you will find a CARB-legal setup in 2.5", simply because it's specific to model, and the model certified for was 2.25".

If in budget, I would get an aftermarket CARB-legal cat and put the proper size flanges on either side to meet your application. $4-500 + an hour or so to a muffler shop if you're not doing the work yourself. See my guide in the Tech Tips forum if you need help selecting a CARB-legal cat. It's good for GS-R too, just make sure the EO pdf contains your exact year and model in its chart.

So I had something similar made, just mated it up to my USDM ITR catback + JDM 4-1 header + GESI cat on my 2000 LS.

https://gsportbygesi.com/high-performan ... -products/

What I did was buy an adjustable 2.5in test pipe and used the ends of those and mated it up with a GESI cat.A shop that I know that has a buddy with a mobile welder and they were able to weld the pieces together with minimal cutting.

The front flange on the USDM header/cat is going to be smaller than the JDM flange so a stock US cat won't fit with a jdm header. But you shouldnt have a problem with a rear 2.5in flange bolted up to your exhaust.

Something like this + a GESI cat would work with the JDM header https://ajpdistributors.com/products/ac ... pipe-black

For some reason I don't have many pictures of my cat...

aw614 wrote: ↑Thu Jan 05, 2023 12:04 am So I had something similar made, just mated it up to my USDM ITR catback + JDM 4-1 header + GESI cat on my 2000 LS.

https://gsportbygesi.com/high-performan ... -products/

What I did was buy an adjustable 2.5in test pipe and used the ends of those and mated it up with a GESI cat.A shop that I know that has a buddy with a mobile welder and they were able to weld the pieces together with minimal cutting.

Keep in mind this particular cat won't be legal for emissions testing in CA or NY, but it's all good info in that post.

How about welding on a smaller flange to the JDM headers? It looks like there's enough room there between the bung and the flange for a skilled fabricator to make that work. That would bring the nonstandard into spec with the standard and allow it to be a bolt-on piece for anyone you were to sell it to as well.
Seems like it would be a cheaper option too since you could use your existing cat.

Arguably you might be reducing the performance gained from using a JDM header in downsizing it to 2.25" anywhere in the line though, which I now realize might be why you were looking for a 2.5" cat, but that I do not believe you will find CARB legal so it seems to me to be a "pick two" situation of - convenience/legality, cost, or performance.

Methinks it's get a new cat as you originally proposed, or modify your headers. Either way, hopefully they'll pass a visual check.

I believe the DC Sports 4-1 "JDM" header did exactly that, but I don't recall if that one was CARB legal. I just remember someone I knew who had that header in his garage, and how it had a stock sized collector

Swapping headers every two years doesn't seem too bad, but the JDM 4-1 header is kind of a pain to remove if you have a full size Koyo rad on it...

Thanks for the replies guys - for some reason I wasn't getting notifications. In any case, I think the best route will be to modify the stock cat to accept the 2.5" header. I am nervous that getting an aftermarket cat and then adding flanges will cause problems at SMOG (even if the cat is CARB-legal) because the JDM 4-1 header technically isn't. Ugh...didn't realize this was going to be a problem or figured it was very easily solved as the JDM 4-1 header is popular. I guess most just go test pipe...

typerx wrote: ↑Thu Jan 12, 2023 8:18 pm Thanks for the replies guys - for some reason I wasn't getting notifications. In any case, I think the best route will be to modify the stock cat to accept the 2.5" header. I am nervous that getting an aftermarket cat and then adding flanges will cause problems at SMOG (even if the cat is CARB-legal) because the JDM 4-1 header technically isn't. Ugh...didn't realize this was going to be a problem or figured it was very easily solved as the JDM 4-1 header is popular. I guess most just go test pipe...

Yeah... notification don't work here. I had a spat of them for the first time last week come in to my email, but then they stopped again.

I don't think flanges will cause you problems. Some CARB-legal cats are sold as universal fit and it's expected you will add flanges anyways. On my NSX I had nasty popcorn welds from the previous owner adding cats in and no one has ever blinked an eye. As long as it doesn't leak.
If you're worried about it drawing attention, just leave the stock heat shield on. I seriously doubt anyone will even notice or care unless they happened to be well-versed in B-series, and if they were they'd probably think it's cool... They're not checking part numbers or anything on the manifold, and I can bet there's no documentation saying they should be looking for 4-1 vs. 4-2-1.

I tried to pass ca smog recently with a jdm 4-1 with both shields. The guy kept looking at it with his flash light then told me it was aftermarket. Lol.

Got to love California. Where the police know who stole what and don’t arrest anyone. Then you got the tax paying citizens getting cars black listed. The carb is just another California tax/ rip off. The part itself is identical. You just get to pay extra for bureaucrats fleecing you for more cash.