Right bit confused really, read on here for big power you need 3" and spoke to blue flame and they say you need 3". But I phoned the place that does the exhaust systems next door to red dot and they say 2.5 is good for 400 and that I would loose lower end power. Don't know what to do know r who to beleive. What are you lot running.
Also would I be able to get just backbox changed?