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.
I got a request from one Skyline brader to have a Raya Minum2. So, I've decided to send out an SMS broadcast to those who are in my phonebook. Here are the details: