Im still unsure about where the backpressure sensor is on the s4 6pi system.
I plan on finishing up my exhaust tomorrow but I dont want to screw up too bad. Will removing the big main cat cause my aux ports not to open? if so, How difficult and expensive is it to make an rpm switch like the s5

 

There is no backpressure sensor. The actuators are connected directly to the exhaust. Follow the lines from the actuators.

 

You can buy a rpm switch for about fifty bucks or buy a kit from JAYCAR. Combine that with a common solenoid like the ones on the vacuum rack (EGR, Relied etc) plus two pieces of vacuum hose.

One vacuum line tee'd to the airpumps output hose to the solenoid. The other vacuum hose from the solenoid to the aux port feedline. Not expensive or difficult to do.

 

If the ACV has been removed you also need an adjustable bleed valve tee'd into the line. I wouldn't put full air pump pressure into the actuators...