If not mistaken, original is 2" from the extractor to the muffler. But don't bother to alter it into bigger size as it will decrease your power, if you like to have a louder or more aggressive sound, change out the whole system. You can try PM "Semperfi" as he have the whole new complete set of HKS muffler original from Japan for GTT. This might help you to get better HP and more aggressive sound.
Hi..? some story about me..must to say here i'm never get any education on automotive(so far and not related then what i'm study before),never know to google in net(like now) or magazine.never work in cars work...