As above i know there are advantages from a bigger bore exhuast but wandering the limits on the sizes i'e I'm putting a turbo on my focus and am gunna make a custom one up but theres alot of bends on a focus and the thinner the easier it will be. I dont want to limit the engine or loose any power , i'm think i'm gunna go for 2 1/2 inch and i'm hoping for 250bhp will this be fine and will a 3 inch system see the power easier.

Thanks.