Yes. That makes sense. I understand about the scavenging near the head but that's not where this started. What I said I didn't believe was that one exhaust system, past the header, caused vacuum where another did not. It's a myth. A pressure differential makes sense. There's no arguing that.

I think the part that was the least 'palpable' was the thought stated that a smaller exhaust pipe caused more vacuum. What vacuum and why more because the pipe was smaller? Doesn't make sense. It would make more sense that the larger pipe allowed more flow which should have been countered by adjustment to use the better flow.