Hi i need to install a wideband AFR into my car and i need to know if i need to drill a hole and weld a new fitting or if it could just replace my current O2 sensor? i wouldnt like my car to be stuck in closed loop... haha. i have an 87 S4. help would be appreciated

 

Your best bet is to weld in a new bung. The stock O2 hole is pretty close to the exhaust ports, which tends to overheat sensors. Weld in a new bung about 2 feet from the manifold/turbo and the wideband will be much happier, and you can still run the stock O2 for closed loop.

 

Also, install the O2 sensor bung at the top of the pipe between the 9:00 and 3:00 position to keep it away from condensation that collects in the bottom of the pipe. You can fabricate a heat sink to keep heat away from the sensor. Here is a diagram from http://www.innovatemotorsports.com

 

Im guessing it would have to be placed before the cat? And would it be wise to place it on my cat but right before the filtering materials?

 

before cat yes.

and ditch the precat and go for a downpipe, if you can