Toranamat69, thanks for posting the link although I would discourage anyone from using those parts. The primary diameter is too large and the collector area is too large. Both of these things contribute to lower velocity and increased reversion. The net result would be a loss of low and mid RPM power/torque and if there is a gain in top RPM power it isnt likely to be much.
Look at the headers that Peewee had made. These improved power at all ranges. 1.5" OD primary runners is the correct choice of available tubing sizes. The smaller you can make the collector volume the better. Tuned length is nice if you can do it but it is more important to build a low restriction manifold that does NOT easily reverse flow and if possible having primaries that exert some "pull" or scavenge on the adjacent primaries.
