I omitted the cats when I installed my LT headers and ended up putting them back on. I agree with truckin' that there was a perceptible loss of low end torque.
I also agree with Hemiswapped in that backpressure is bad. In a nutshell, it's engine inefficiency.
I went all thru this big-time with Marty when he had KRC Performance when I did my headers. He suggested the local coating shop that I had do my ceramic coating and never had a problem with me picking his brain, even if I was doing the work and he wasn't seeing a dime out of it.
He had dynos on his laptop of Hemis b4 and after headers/omit cats/ omit Y and better flow muffler. Peak torque is increased, no question, BUT the torque curve moves up in the rpm band CONSIDERABLY. So, in a nutshell, you DO get torque gains with headers and a better flow, BUT it's actually worse down low in the RPM band.
The ideal fix for this is to do engine mods that enhance the low end torque (usually at the cost of high end HP - think diesel here, they are low end torque monsters, but usually don't have near the HP as a gasser - un-modded, of course).
I could have had the best of both worlds, next to non-existent backpressure AND gobs of low end torque IF I went with a cam designed better for low end than the stock 5.7 cam and had a tune to support it. Due to funding, I simply opted to put the cats in which created just enough backpressure to feel I lost no low end torque. My butt dyno feels about the same amount of low end as stock, but mid-high rpm gains are substantial. I actually had him order a cam (210) with a slight (-4*) modification to enhance my low end torque, but after it sat in the garage in a box for nearly 3 years, I sold it.
Bottom line - having no backpressure is great for power, more efficient, better on fuel, etc. BUT you really need other mods to support it and get the full benefit from it.
As for the O2 sensors, the purpose of the forward sensor is to measure the amount of unburned oxygen in the exhaust system left after combustion. Based on this reading, the sensor genrates a voltage which is read by the fuel managent program within the PCM. Based on this reading, against a pre-programmed "range" for residual O2 in the exhaust, the PCM will either release more or less fuel for the next combustion cycle.
The rearward O2 will take the same reading and sends the corresponding voltage to the PCM which is read against the reading of the front O2. There is a "range" of acceptable ratios pre-programmed into the emissions program within the PCM which contain a higher residual 02 count, meaning the cats are removing contaminants from the exhaust gasas and it has more oxygen than the front measured. If the ratio is outside of these pre-programmed parameters, a message is sent via a CEL.
The PCM makes NO fuel trim adjustments based off of readings from the rear O2 sensor. It is in essence, a warning system alarming you of a problem with a cat or a leak in the system...
