the 2 3/4" was my personal preference as i tried a 2 1/2" system and a 2 3/4" system with the same backbox and design and front flange and the 2 3/4" was noticebly better. this was with a front mount intercooled et engine with a standard turbo at 1 bar boost. I wouldnt normally expect it to be needed on 215bhpish flywheel figure but that was the performance results i felt
.But this is just one engine. i have not tried the same test out on other s12's/engines
I need to make myself a new system at some point this year. i think a 2 3/4" will still be sufficient at around 270bhp. mines just a little bit patchwork quilt from having so many different turbos,backboxes and manifolds.i will definatly make this one myself