[SD455DJ]

I'm doing essentially the same thing as you are on my 350HO Custom S. I will be replacing the kink-bent 2 1/4" headpipes (aka downpipes) with the Pypes 2 1/2" mandrel downpipes (DGA10S 2.5" mandrel bent downpipes) with a reducer to the 2 1/4" pipes to the mufflers. I am going to replace the standard style regular reverse flow mufflers (restrictive) with 2 1/4" DynoMax UltraFlo's or the MagnaFlow equivalent. This will allow the regular performance 400, like yours, to breath better. If it was making 375 hp (flywheel) or more, I would definitely go 2 1/2" all the way back. I'm not sure if you 310 hp/400 tq was measured on a chassis dyno, if it was, your proposed system is better than what you currently have and will help preserve more power than your stock system. We have found the most hp is preserved with 2 1/2" mandrel downpipes for at least the first 3 ft. and substituting hi-flow mufflers, even with sections of 2 1/4" pipe here and there further back in the system. As mentioned before the exhaust gases are cooling as they move down the pipe and are spreading out as well, so slightly smaller pipe isn't as big a restriction at lower rpm's and lower hp engines.

Dennis