I know most people say to get a full 3" turbo back exhaust, but I've noticed that the major quality brands like Apexi make 70mm (2.75) products. Most test pipes available are also not 3". For example, the N1 single is detailed as a 70-80mm exhaust. Why/where does it change diameter? Also, their down pipe is a 70mm. Does that not bottleneck the the exhaust system when the 3" is most widely recommended?

 

i think its just metric vs non metric. in the USA its not easy to find anything except 2.5" and 3" pipe, but if you go metric you get 2.75" (70mm) or 3.15"(80mm)

if we're talking about a car with a stock based turbo, i really doubt you could tell any power difference between 70mm, 3" or 80mm.

the more power you make the more it matters, but you still need to be over 400rwhp before you need something bigger than 3"