Well I am running a stock setup right now, and I want to install a turboback exhaust and a uppipe. I am attending UTI, and the teachers there all tell me not to remove my cats. When you remove catalytic converters, the car will start to run rich. The ECU no longer sees a difference in O2 sensor signals between the first O2 sensor and the second O2 sensor. So the ECU (being safe as it is) retards the timing by 5 degrees, thus running rich. Then you get lots of carbon buildup on the pistons. Then later on down the road you get detonation. How can I avoid running rich? How can i still keep my O2 sensors sending the right signals to the ECU?

It is my understanding that installing a catless uppipe will not throw a CEL as long as you plug in a resistor for the EGT sensor.

If you install a downpipe that eliminates the two cats, I would think that you would get a CEL because there is no longer any difference between the two O2 sensors signals. Now if I were to install a downpipe with a cat in it, like the Perrin catted downpipe, I dont think I would get a CEl.

Now I wouldnt really care about running catless (well maybe id get one high flow cat) if it wouldnt retard my timing by 5 degrees and cause me to run rich. Would reflashing ECU and getting it dyno tuned eliminate that problem? I dont like piggy back systems because sooner or later the ECU learns around the thing and you are back to stock. So any good suggestions?

Here is a diagram of the intake and exhaust system on the WRX, courtesy of Mattr762 without his permission. diagram link

 

If you remove the second cat, you must install a voltage clamp on the second O2 sensor. Look for the Mil Eliminator. I am using one and it works fine. The computer will not retard timing 5 degrees and add fuel because you removed the cats. However, if you don't put a clamp on the second O2, it will eventually throw a code. What the ecu does at that point, I don't know, but I doubt it does much. I would make sure not to allow it to happen in the first place. I don't believe the factory computer can determine that you have altered the cats if you use a clamp. The second O2 is not used for fuel trim anyway.