I am currently running a Spoon 4-2-1 header w/carsound cat and stock catback on my ITR. Will I need a new cat or will i have to modify anything if i plan on putting the Mugen Twinloop catback on? I would think so, because i was told with JDM exhausts u need a shorter cat. any info appreciated. thx.

 

You will be fine.

Spoon 4-2-1 + USDM lenghted Cat + "USDM" Mugen exhaust = OK.
Just get the "USDM" Mugen exhaust w/ USDM lengthed b-pipe. You're good to go.

However, there are "JDM" Mugen exhaust which has long b-pipe which requires shorter cat.

 

Are there any difference in terms of power between the "USDM" mugen twinloop and "JDM" one? Or is the bpipe of the "JDM" one just slightly longer?

 

just length.

 

If you have a JDM length one - just run a JDM sized test pipe (equivalent size to JDM cat) or have the b-pipe cut back to accomodate a USDM sized cat.