I am looking to modify the standard STi 5 backbox to make it flow a little better. Is this possible and has anyone tried it? How restrictive is it in standard form on a 280bhp Sti, surely it would be less restrictive on a Uk Turbo with PPP at 237bhp, am I right in thinking this?

I know it will probably make it a little louder but only marginally.

 

dunno the exact standard figure but sti 5 typeR with just backbox swap made 305 bhp