Type R Club of America

This is an OD measurement of my JDM 4-1 '98 spec head collector. It's a little blurry but you can see it's about 2.5" +/-. My battery was dead so don't have screen readout.
These are two donut gaskets, the big one is for JDM size collectors (e.g. the '98 spec above). The small one is for USDM spec.
This is measuring the USDM spec donut gasket, which is approx. 2".
The JDM sized gasket is about 2.5".
This is my '96 spec JDM ITR head collector, which looks to be the same as JDM '98 spec, at approx. 2.5" +/-. Test fitting the donut gaskets, as you would guess only the larger 2.5" gasket fits. And the JDM oem cat fits right on as well. My conclusion is that the USDM ITR head collectors are about 2", and have the small 2" gaskets, that allow them to fit in the USDM cats.

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Thanks man, I'll try to measure more accurately including outer diameter. What we really need is for someone to measure their USDM ITR header that they are positive is USDM lol. Otherwise all these threads are kind of speculation. The JDM '96 spec header is confusing.
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Here you go...

DC231 wrote: ↑Mon Jul 03, 2023 9:25 pm
IMG_20230703_221512676_HDR.jpg

Thanks man!! I think the evidence is now conclusive. The ITR USDM header has a collector of about 2" (outer diameter). Now we can sleep peacefully at night.

I can't believe it has 2" piping. Totally makes sense as to why they wake up so easily w/ a header and exhaust upgrade.

JDMJNKY wrote: ↑Thu Jul 06, 2023 11:42 am I can't believe it has 2" piping. Totally makes sense as to why they wake up so easily w/ a header and exhaust upgrade.

Good point sir. Now we know!

[quote=JDMJNKY post_id=22063 time=1688661761 user_id=230]
I can't believe it has 2" piping. Totally makes sense as to why they wake up so easily w/ a header and exhaust upgrade.
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It is surprising how small it is. The inner diameter of the inside pipe is actually much smaller at 1.615 inch. The 2 inch is the outside diameter of the outer pipe. Huge bottleneck for sure.

Dave7CDMTYPER wrote: ↑Thu Jul 06, 2023 5:17 pm Good point sir. Now we know!

haha agreed.

DC231 wrote: ↑Thu Jul 06, 2023 9:36 pm It is surprising how small it is. The inner diameter of the inside pipe is actually much smaller at 1.615 inch. The 2 inch is the outside diameter of the outer pipe. Huge bottleneck for sure.

Extremely. I don't think I've ever kept a car stock long enough to take data. This is insane a 1.8L w/ 1.6" piping.


random facts (googled - 1800cc street bike exhaust piping size):

OTS & Customs: 1 3/8", 1 1/2", 1 5/8" which is ITR piping size , and 1 3/4"

"The 1 3/8" pipes gave good mid-range power, but not as good top end as the 1 5/8" pipes."

"Honda model offered in the mid to late sixties. had a small diameter pipe inside a larger tube. Honda knew the smaller diameter offered better performance, but the American market looked askance on weeny pipes. The pipe in a pipe satisfied both performance and the market idiosyncrasies."

http://www.fonema.se/qpipe/qpipe.htm