Though not an expert on intake design at all, my understanding is that the stacked design allowed each runner to be the same length, feeding each cylinder the same A/F charge. A small aside regarding this question is the design of the bottom vs. top outlet ports of the later model intakes. I've noticed that one has slightly rounded corners compared to the other being more squared. It is subtle though definite and I've noticed the difference on more than one iintake. Without a manifold in front of me forget which is which. I'd like to know what the deal is there?