I understand what you are trying to achieve Rod, but I don't understand everyone's (and your) obssession in trying to make it overly complicated with valves and actuators etc? Why not just get a properly designed 4" down pipe which splits into two 3" pipes (a la standard system) and has a couple of silencer boxes each side under the car with a side exit exhaust either side - should be able to keep the noise down that way at ALL times and you won't ever be restricting the exhaust gasses?