Exhaust sizing questions with JDM ITR header and USDM exhaust

[aw614]

I am trying to figure out what to do for my new catalytic converter setup when I replace me old one and could use some assistance. I know the JDM ITR header has a 2.5in collector and that the JDM exhaust goes down to a 2.25in from I believe the catalytic converter.

But I also noticed a 2.5in 3 bolt flange will bolt up to the stock ITR exhaust's flange that is 2.25in.

My question, is there anything wrong just making the setup 2.5in all around without running a reducer pipe of 2.5in to 2.25in at the catalytic converter? I know I won't get the best power with this setup, but I also value the quiet exhaust and the stock looking exhaust tip.

This was what I was looking to do in the pictures. My current exhaust had piping reduced from 2.5 to 2.25 in, but I used a 2.5in flange which welded up without an issue, but I did not realize this at the time. But I feel if I do get rid of my stock ITR catback, having the 2.5in already will make it better for potential future upgrades.

Pictures of what I was mocking up along with my old cat. Test pipes are good for exhaust piping

[jayhaire]

It's been over a month without a reply.

I want to help. I've had a few setups over the years:
- JDM ITR 4-1 header, carsound cat, JDM mugen catback
- JDM ITR 4-1 header, JDM ITR cat, JDM ITR catback...no reducer at the end of the cat nor the beginning of the b-pipe.

[Stin1]

I have a jdm itr header and a Oem ek9 exhaust. The ek9 and dc2 catalytic converter are the same thing. From my experience a Usdm Oem catalytic converter would not bolt to the jdm b pipe. The spacing on your pic appears to be able to accommodate both the Usdm and jdm spacing. The reducer isn’t necessary imo for fitment . Performance maybe . I don’t think decibels are going to vary much if you are using quality tubing. That thin stuff will be louder.

[coolhandluke]

I have a jdm itr header and a Oem ek9 exhaust. The ek9 and dc2 catalytic converter are the same thing. From my experience a Usdm Oem catalytic converter would not bolt to the jdm b pipe. The spacing on your pic appears to be able to accommodate both the Usdm and jdm spacing. The reducer isn’t necessary imo for fitment . Performance maybe . I don’t think decibels are going to vary much if you are using quality tubing. That thin stuff will be louder.

From memory:
JDM ITR header has a 2.5'' collector / USDM 2.25''
JDM ITR cat has a 2.5'' inlet and 2.36'' outlet / USDM 2.25''
JDM ITR exhaust is 2.36'' / USDM 2.25''
*All sizes interior diameter

[aw614]

Thanks everyone for the size and listed combos.

I have a jdm itr header and a Oem ek9 exhaust. The ek9 and dc2 catalytic converter are the same thing. From my experience a Usdm Oem catalytic converter would not bolt to the jdm b pipe. The spacing on your pic appears to be able to accommodate both the Usdm and jdm spacing. The reducer isn’t necessary imo for fitment . Performance maybe . I don’t think decibels are going to vary much if you are using quality tubing. That thin stuff will be louder.

I noticed this as I looked at my exhaust. The flange I ran with the reducer on my current setup was sold to me as 2.25in and then welded to the reducer by a friend. Didn't realize the flange was actually 2.5in when I compared it to the various test pipes and flanges I bought. Looks like I shouldn't have an issue running the stock exhaust again.

Go figure, the 2.25in vibrant flange that was also supposed to be 2.25in was also 2.5in lol.

When I see @coolhandluke on the sizes, I was basically trying to mimic the JDM ITR cat. But since the 2.5in flange bolts up to the stock exhaust. I am just going forward with that. Not looking for absolute power and I like the stock exhaust quietness