Well, it all started with an article in Hot Rod, which was "10 Steps to Power", and they suggested that on a daily-driven street car, moving down a step in primary header tube size would yield more real-world power on the torque side of things, since focus would not be on top-end on city streets. They suggested on a typical SBC, to move down to 1-1/2" primaries to get that torque at a lower portion of the RPM range, as you mentioned. I agree with this, especially on a 2-barrel powered 273.
I'm not engineer, but wouldn't increasing the primary tube size to something which is larger than the exhaust port equivalent be a detriment to the motor unless, you are going for high rpm?
Matthew
