when you had it set for 9 psi. With one turbo on runaway boost got about (19 or 18 psi + 9 psi) / 2 gets you 13 or 14 psi. The balance tube evened out the manifold pressure as it should have. The reason for the detonation was because of instant boost spike . You said it was slight detonation. If you didn't have the balance tube, it would not have been slight and could have blown the motor from harder detonation. The Nissan engineers knew what they were doing when they put that there. I added an additonal balance hose before the TB joining my BOV for my application.