maybe true for the UK, but the USDM ITR (0V13 casting number) has a cast 4 to2 with a unique part number that is different the GSR's cast manifold. The collector is 2" though. I've been told that the ITR downpipe does bolt upto the GSR's cast manifold. AFAIK all USDM Integras have the same 2" cat size for and aft, the ITR USDM's B pipe flares back out to 2 and 1/4 after the three bolt flange.
The 96 and 97 JDM ITR casting is larger and has a 8k23 casting number. The downpipe looks the same as the USDM one, the JDM stays at 2.36mm OD after the rear O2 fitting while the USDM goes down to something like 2.0 OD.
The JDM 4 to 1 header puts out the same ponies but much more torque vs the JDM cast 4 to 2 to 1 manifold and downpipe. Test in the USDM ITR show that going to a decat or high flowing cat makes about 4 HP, I would imagine cutting off the neckdown on at the very end of my ITR down pipe just after the 02 is worth a few HP too. So that puts peak HP of a slightly mod USDM ITR within a few HP of a JDM 4to1 with JDM 2.5" cat, but the JDM has more torque where it is needed. Note the USDM ITR's cast manifold is also smaller than the JDM ITR's cast manifold, so even with a fixed downpipe and 2.5 cat the USDM manifold may be down a pony or two.
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