I just read the linked article. A lot of good info there for sure. Twice the exhaust flow in the center ports would need a bigger primary pipe for sure. Based on that article and what has been posted here go maybe 1 inch on the single cylinder ports after being carefully necked down from a port match at the head. Same thing for the shared ports only down to 1 5/8 as best I can see on the chart. Not to be too far off topic, what about the people who either run a single manifold or a split manifold set up like Fenton’s, Langtons, or even the factory split set up. I would assume that the primary length could at most be considered the length of the farthest port from the manifold outlet which would be the “collector”. Would there be any way to use pipe size and length comming off a manifold to help with exhaust scavenging? Or are all the gains using split manifolds over stock just a reduction in back pressure? Jay
