2mm will make the different already. More air to go in to the air chamber and combusion room. That's why some bore it until 64 or 67mm... After market TB like Jun is 70mm at inner diameter. But bigger doesn't mean better, big enough for ur engine OK lar. Don't tell me a stock b16a is going to put a 67mm or 70mm TB.
Most of our cars are Dump Out .. and i know about Dump In from someone. any sifu has any comment on it ? which one is better ? any benefit for Dump In ? my car only running 7psi btw