what piston compression can i use with a turbo setup running 91 octane DD, pushing 400whp? and if i was to remove the turbo setup down the line use the same block pistons etc with just a bolt on set up. (h/i/e) is it possible?

 

With the right head (cams, porting, manifold, etc), you could pull 400whp from 10:1 and still have decent power if you go NA. The problem is you're asking to do that with just 91 octane. You'd have better luck with meth injection, since you'll be walking a very fine line at 400whp on 91 octane.