My S15's exhaust system consists of a CES dump/front pipe and MagnaFlow muffler, which is awesome by the way. Great performance and great sound. The only real issue I had was that the front pipe sat too low which meant it scraped a lot. It was also a defect because being the lowest part of the car, it only had 85mm ground clearance.
I did some research and found a few front pipes that have a lower profile so they sit above the chassis rail. Companies either use an oval cross section pipe or dual 2" pipes side by side. Below is a front pipe made by MoonFace.
I eventually decided to keep it local and took the car back to CES Racing for a custom oval cross section front pipe, also had a secondary O2 sensor bung welded in for the AEM wide band.
Ordered some Defi gauges for boost, oil temperature and pressure. I also wanted something to display and log air fuel ratios, so ordered the AEM wideband kit for that.
Got to work right away and start removing the interior pieces to test fit the gauges. Was a pain in the ass to solder and route all the wires neatly, so many fuses and crap.
Next part involved installing the wide band O2 sensor into the front pipe, was a pretty straight forward process. The harness then had to be routed back into the cabin to connect up to the AEM gauge.