[Frank A]

The rate at which the flap opens can be regulated by the diameter of the vacuum line that feeds the flap bellows, larger diameter more volume transfer the faster the valve opens. The vacuum canister should have more than enough capacity to handle this. The plastic lines used currently have a 1mm I.D the combination of the small diameter and the volume air needed to actuate these devices does two things, regulates the time between normal and activated state, and lessens the affect of pressure pulses creating pulsed transition from normal to to actuated.
Besides I have always been a fan of a 2 stroke like power curve, just as long as it is consistent. And I'm the only driver on this buss!