I ran into Doug at several Norton rallies over the years. Last time I saw him was at the rally in Elma. He'd set up a little work station where he was porting 2 into 1 manifolds for Mikuni carbs. He said some manufacturers used a thicker wall thickness, which gave him more material to make them breathe with the right amount of porting. I think he also recommended starting with a port size larger than the carb. Said done right, he could get pretty close to twin Amal performance. He held the manifold up to the light, and said the more light you see around the corners, the better it will breathe. He knew a lot, and was generous with his time and information.