So I have a strightpiped '88 (cut off before the 3rd cat) I just read another thread saying back pressure is needed (2psi) to open aux ports or your idle may be shabby and you'll loose torque. I was wondering if me cutting the exaust will effect this and also where is the sensor that checks the back pressure because there is just a black bar underneath my car that's not connected to anything.

(EDIT: IM A DUMBASS. so I just reliazed that the black bar does go to the cat, so new set of questions, will this cause the car to die completely at idle and could I circumvent this by putting the bar into a different pipe?)

 

It is unclear (downright confusing, actually) what you mean by "bar".

In any case, there is no sensor that checks back pressure. This pressure is a power source that operates the 5th & 6th ports and they are controlled by a solenoid in you rat's nest.
In your S4 this "backpressure" is essentially a vacuum source that sucks the solenoids open.
On the S5 the system uses positive pressure from the air pump to push the valves open.
Same end result, different approach.

 

Very sorry about using the word bar for lack of a better term ik it is confusing. I should have said pipe, it is under the right side of the car and has a little oval on the end that connects directly to the 3rd cat. So if I connect that to the 3rd cat it will solve that issue?? Or will I need to reattach my full exuast??

 

Not sure but that "little oval" thing might be a one way check valve. Blow through it in both directions to confirm.
Then sure, connect it to your cat and see what happens.
Worst case is the aux ports don't function, best case is that they do.
In either case you've neither lost nor harmed anything.

 

If you got a backpressure pipe going from the catalytic converter back up to the engine it's for the exhaust backpressure. If you got straight pipes there probably nowhere for it to connect to, remove it and plug the larger rubber hose it connects to.

If you've still got your air pump and ACV, there's a dump hose on the ACV that blows off air at 3800 RPM. I plug the hose just before it reaches the plastic silencer which continues down beneath the fan. If you tap a tee in there you can steal enough pressure to activate the aux ports on time.