Yeah, I'm familiar with X-design (it's existence and general purpose, not the S/W itself which I believe is proprietary to Burns). But X-design requires, as any header designer logically would, a bunch of inputs specific to the particular engine the header is designed for. Displacement, cam timing, desired power band, port lengths and cross sections, compression ratio, etc. Change these parameters, and the answers (pipe diameters and lengths, collector lengths, etc) will also logically change, no? So I'm not so convinced that one set of primary diameters is correct for all LSx motors from mild 346s to wild 427s or bigger ...

On Dynomation, when two pipe diameters are close in power, you can observe that the "best" diameter will change when you alter the exhaust valve open point by just a few degrees. In the real world, we just don't have the luxury of experimenting with multiple combinations of header pipe diameters and lengths, so we go with a more generic design which gets us close. Some of those generic designs, like the LG Motorsports long tube merge collector 4-1 headers, seem to consistently produce more area under the torque curve than other designs. I would hope, and even expect, that the ATS headers would yield similar results. But to date, there is a dearth of empirical data to support that expectation one way or the other.

And as I said, I really like those headers and expect to use them on my project. And my comment about "obsession" was directed to those who automatically assume bigger is better when it comes to pipe diameter.

We make 2 sizes of header primary tubes, but for the big cubic inch displacement (400 cu in and higher) the 1-7/8" is the best we have found.

Tyler