As a general rule youll want to keep the header primary tube size inside diameter similar in size to the engines exhaust port exit size.
larger sizes tend to reduce exhaust gas velocity thus reducing the cylinder scavenging efficiency,
smaller sizes tend to block the exhaust port causing a restriction to flow and turbulence.
the problem youll find is that about 90% of the commercial headers either don,t line up 100% correctly with the engines port exits , or the shape of the exhaust port and header entrance don,t match or both.
ports like these where the exhaust port is roughly centered but slightly smaller , where theres a step the exhaust gases flow over are far better to have that the opposite condition where the header partially blocks the exhaust port, as the minor step tends to reduce reversion and in most cases has a slightly helpful effect on scavenging efficiency.
keep in mind the ideal condition will be where the inertia of the fast exiting exhaust gases help to drag in the following intake runner charge of fuel/air mix and flush out the expended previously burnt gases.
remember exhaust gases are physically forced out, of the exhaust port by a fast rising piston, while the intake charge is drawn in, by the dual effect of higher outside air pressure pushing it in and inertia of the mass of air in the intake runner trying to follow the previously burnt and exiting exhaust gasses
RELATED INFO
https://www.youtube.com/watch?v=rXzRwi2C2xM
http://www.stahlheaders.com/ports.htm
viewtopic.php?f=56&t=185
viewtopic.php?f=56&t=495
viewtopic.php?f=52&t=4081&p=35069&hilit=tumble+swirl#p35069
viewtopic.php?f=56&t=789&p=40231&hilit=reversion#p40231
larger sizes tend to reduce exhaust gas velocity thus reducing the cylinder scavenging efficiency,
smaller sizes tend to block the exhaust port causing a restriction to flow and turbulence.
the problem youll find is that about 90% of the commercial headers either don,t line up 100% correctly with the engines port exits , or the shape of the exhaust port and header entrance don,t match or both.
ports like these where the exhaust port is roughly centered but slightly smaller , where theres a step the exhaust gases flow over are far better to have that the opposite condition where the header partially blocks the exhaust port, as the minor step tends to reduce reversion and in most cases has a slightly helpful effect on scavenging efficiency.
keep in mind the ideal condition will be where the inertia of the fast exiting exhaust gases help to drag in the following intake runner charge of fuel/air mix and flush out the expended previously burnt gases.
remember exhaust gases are physically forced out, of the exhaust port by a fast rising piston, while the intake charge is drawn in, by the dual effect of higher outside air pressure pushing it in and inertia of the mass of air in the intake runner trying to follow the previously burnt and exiting exhaust gasses
RELATED INFO
https://www.youtube.com/watch?v=rXzRwi2C2xM
http://www.stahlheaders.com/ports.htm
viewtopic.php?f=56&t=185
viewtopic.php?f=56&t=495
viewtopic.php?f=52&t=4081&p=35069&hilit=tumble+swirl#p35069
viewtopic.php?f=56&t=789&p=40231&hilit=reversion#p40231
