"HEY GRUMPY WHY DON,T THEY MAKE SQUARE TUBE HEADERS?"
you might be thinking about something like this
but you would more likely see guys fabricate something like this
http://en.wikipedia.org/wiki/Hydraulic_diameter
http://www.lmnoeng.com/darcy.htm
http://en.wikipedia.org/wiki/Reynolds_Number
http://www.eng-tips.com/faqs.cfm?fid=1236
SHORT ANSWER..
SQUARE PIPE FLOWS LESS FOR A GIVEN CROSS SECTIONAL AREA, it would weigh more and loose heat faster
but the real problems are these,
(1) all inside corners would flow at significantly lower velocity than the center of the tube
(2) a round header has approximately 80% of the surface area of a square tube having the same outside dimension, this means square tube weights more and looses heat faster due to its 20% plus increased surface area MUCH faster, and thus the scavenging inertia of the exhaust gases are reduced and more heats dissipated in the engine compartment.
(3)gasses flowing over a surface that changes direction would cause turbulence at any change in direction over about 15 degrees, and a 90 degree bend in square tube flows significantly lower that a radias round tube making the same change in direction
(4) square tube tends to move and change dimension when rapidly heated and cooled making stress failures more common.
you might be thinking about something like this
but you would more likely see guys fabricate something like this
http://en.wikipedia.org/wiki/Hydraulic_diameter
http://www.lmnoeng.com/darcy.htm
http://en.wikipedia.org/wiki/Reynolds_Number
http://www.eng-tips.com/faqs.cfm?fid=1236
SHORT ANSWER..
SQUARE PIPE FLOWS LESS FOR A GIVEN CROSS SECTIONAL AREA, it would weigh more and loose heat faster
but the real problems are these,
(1) all inside corners would flow at significantly lower velocity than the center of the tube
(2) a round header has approximately 80% of the surface area of a square tube having the same outside dimension, this means square tube weights more and looses heat faster due to its 20% plus increased surface area MUCH faster, and thus the scavenging inertia of the exhaust gases are reduced and more heats dissipated in the engine compartment.
(3)gasses flowing over a surface that changes direction would cause turbulence at any change in direction over about 15 degrees, and a 90 degree bend in square tube flows significantly lower that a radias round tube making the same change in direction
(4) square tube tends to move and change dimension when rapidly heated and cooled making stress failures more common.