it does depend, a head could only flow 150 cfm but make 400 BHP relatively easy with 2 bar of boost (like a totally standard cossie on a T34, greys for example) or it could flow well over 200 cfm and make the same power with 21 psi (these are examples, i only really know about aspirated shit)

Normally aspirated, you'd need either a lot of ccs, a lot of revs, and a head / heads and induction system that flow at least 250 cfm to make 400 bhp afaik

My numbers are gonna be miles out!!