just a simple question! the toda racing header 4-2-1 has diametre of 45-50-60(mm) it doesn't say if it's od or id! and the mugen catback exhuast i just ordered has a diametre of 60.5mm and doesn't specify if it's od or id. they both just have the diametre symbol. so if i get a hi-flow cat of 2.5 inches would this be a perfect match?
if it's not and the header end is smaller than the catback then really is there a power lost cause theorectically from the headers end the whole exhaust system is 60mm where it's od or id. am i totally wrong?

any comments welcomed

 

In general headers and catbacks are measured OD, where cats are measured ID.

 

As far as the Toda diagram goes, those measurements I'm pretty sure are ID since the collector end is 65mm OD. The Mugen exhaust is 60.5mm OD.

 

j00 answers lies in da saerch... if you get lucky matching up keywords...

https://honda-tech.com/zerothread?id=274617

cat is 2.5" id, and exhaust 60.5mm od