To broaden the powerband,the head pipe can go as much as 50% larger than the exhaust port area.The header pipe should be six to eight times its diameter for maximum power.For a broader powerband, it can be as large as 11 times the header diameter.A slight increase in head pipe diameter in relation to the diffuser con can prove to be the ideal.Remember that the head pipe is measuerd from th exhaust port window,& includes the exhaust passage of the cylinder & the header manifold.