240 degrees - that's the magic number. A cam with over this amount has both valves open at the same time in the siamese ports. Less is as Panic described. On a full race engine, 1 3/4's will do, some drag cars go up a bit from there but not much. Headers work best if smooth but we use them to provide exhaust and intake emptying/filling. Laminar flow is not in the picture. From a physical standpoint it's impossible. The siamese exhaust ports are that size because they are transitioning down to a single port size. Your pipes are simply too big. Length is OK for low RPM
