The most flow is out the top part of the port----reversion effect takes place letting the exhaust flow backwards into the port and then cylinder-----The header should be bigger than the port size----like the distance around the hole should equal the circumferance of the header tube. offset the flange upwards so that there is a dam to stop the reversion flow at the TOP where the flow would be more---reversion on valve overlap will dilute the mixture in the cylinder and even contaminate the mixture in the intake, possibly even to the point of igniting it occasionally(backfire)