5 door S16 euro spec

[RetroPug]

Bigger is not always better here.. You want to keep gas velocity high (small pipe, raspier sound) while making sure there's not too much back pressure (bigger pipe, boomy/deeper sound). Obviously the stock boxes are restrictive so a standard bore exhaust with decat and a straight through design backbox is going to be the cheapest and probably the best option..

IMHO anyway

So 2.25/2.5 would be right?

2.25 would be ideal but louder if aftermarket, 2.5 will be much, much louder, and will probably lose you low down grunt but give a slightly better top end.

Overall, the difference is predominantly noise.