Exhaust Manifold

As I have read it form different books (e.g A. Bell's "Four-Stroke Performance Tuning") and sites it is quite difficult to plan the best exhaust manifold. To get the peak power from exhaust, it takes lots of experimenting. For a starting point there are some formulas of the basic sizes of the manifolds pipes. I have found one at http://www.mez.co.uk/mezporting/exhaust_length.html. My current values shown on picture on right. I have also a drawing of a flange for c20xe exhaust manifold in dwg-format (see attatchments). Once I get it done, I'll post a picture here as well.

Why 6000rpm? As I'm going for MegaSquirt and I've been told that 20XEs 60-2 trigger wheel is good for up to 6500rpms, then I want to get my peak before I'm at my limit. Yes, I know that original EMS is 'good' for 7200rpm's, but that's not the case here.