Here's a thought of mine...
I used to have a Stillen header, which is a 4-1 design, and I always had a problem with the design of it. It had a very nice header piece, with equal length primaries, and it had a V8 style 2.5" triangle collector where it meets the b-pipe. But, where I think they messed up, is that the secondary pipe is only a 2". So it always troubled me that the gases come flying down the primaries, merges, and then literally hits a wall, as it suddenly gets crammed into a 2" pipe without a smooth transition. I wish I still had that header, so I could fab up a 2.5" downpipe and see if it would've performed better. I also wonder if that's the reason that the average whp gain with the 4-1 Stillen was about 7-10, whereas the 4-2-1 Hotshots and SSACs tend to make more high rpm power, at about 9-14 whp. Hmmm...
Here's a pic of my old header that should help my story...
