My original headers and sidepipes had the same flanges which I labeled "Flange 1". Mine also cracked and it was not related to engine tune. There are large gaps where there isn't any flange, allowing exhaust gas to leak to adjacent pipes, which effectively puts the edge of the pipe in the exhaust stream. The edges of the pipes overheated and cracked in those areas. I built sidepipes and headers using "Flange 2", which contains the exhaust to its respective pipe. I have not had any failure in that area since.
