Does anyone know how close the fitment on the JDM ITR header is in relation to the cat-converter on the 99 Si. If I buy a 2.5 to 2.25 donunt gasket can I make it bolt up to the stock cat or will I need some costum piping or a new converter?
Thanks

 

You could modify the stock cat to bolt up to the JDM header, but being as small and restrictive as it is, running the stock cat would basically defeat the purpose of buying the JDM header. If you'd prefer to retain the stock cat, I would suggest simply purchasing a quality "USDM-style" header instead.

 

what is needed to install a jdm itr header on a 99-00 Si? can it bolted up with a jdm cat using a Si exhaust, or does a custom cat/test pipe have to be made?

 

You'll need a custom cat. Check out SMSP's offerings.

 

your going to need a new cat or atleast a test pipe of some short so it can bolt up correct