In my mind's eye, the bore changes we're discussing here, let's just give an example.....say going from a 4.250 bore of a 383 to a 4.342 of the 400 isn't enough of a difference to "undo" the added power of the larger bore, if that makes sense. While the pressure "may" decrease from the smaller bore to the larger, (and I'm not certain it does), it's not a large enough amount so as the larger bore of the 400 loses power. To me, that's a great example too, since the 383 and 400 share everything else in common BUT the bore size. Lets just say their compression is the same at 9.5 and the rest of their builds are also the same. The 400 wins every time and the only difference IS the bore size.