A reasonably good explanation of the impacts of runner length on manifolds can be found here;
enginebasics.com/Advanced%20Engine%20Tuning/Short%20Vs%20Long%20Tube%20Headers.html
Although it should be noted that not all of what is discussed here is relevant to the J-pipe vs turbo manifold discussion as the J-pipe converges into one pipe (or two) very early on. But still, the idea that there is a trade-off between
- lag induced by the extra volume pre turbo post exhaust port
- better flow and lower cylinder temps due to the extra volume allowing the exhaust pulse to leave the cylinder more easily.
is pretty relevant to us. Also, greater top end could be found with longer manifold and greater low end torque could be found with a shorter log style manifold.
As for the different J-pipe designs I would imagine this to be pretty crucial - if you introduce a restriction like the merge collector on the N/A manifold and then open it out with larger J-pipes like in the photo that Mike posted you may find that you speed up the exhaust flow only to reduce its velocity in the J-pipe and then have it meet another restriction at the turbine - effectively creating a resonance chamber, fundamental to the design of many mufflers (made to take the energy out of exhaust gasses!).
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