I had a 2.5inch system on my first n/a, people say that's too much, however.. We all know starletsportif has the 90bhp n/a, well mine was only running the catback and filter (standard mani, his has a rolla mani) and his was only fractionally quicker. It's all chineese wispers with the optimal size for the exhaust system on n/a's. Obviously going excessively large is silly but you get my drift.