Back pressure
 

is it necessary? i have a 98 2.2 camry the codes i'm reading suggest my catalytic converter is bad so i'm wondering if cutting it out would be good for my fuel efficiency i have NO care about performance or speed in this car just comfort and fuel efficiency i have the capability to gut it out but does my engine need backpressure? should i get a "high flow" cat,no cat, gutted cat, new exhaust piping all together? i was thinking of replacing my muffler because the current one is HEAVY i think i'm gonna put the one off my old wrecked 89 camry on. also before i bought it it ran on 1-3 cylinders so there is ALOT of soot in my exhaust is it worth trying to get out? and what if i took my current exhaust manifold off and polished it? stacy david of gears suggested it lol
also the bend for my o2 sensor gets really small should that be dealt with or not? my exhaust has been welded on and patch so many times it changes from stock pipe to slightly bigger pipe 4 times it has one convertor one resonator and one muffler i'll post pictures tommorow.

No, backpressure is not necessary and it doesn't help. If you cut all that out of your exhaust though your car will be insanely loud and will piss everyone off.

what about just the cat and keep the muffler and all? i hate loud cars with an extreme passion.... to be honest i hate them entirely too much

Ignoring what the codes tell you for a minute, does the vehicle run poorly and is your fuel economy bad or gotten worse recently?

Your cat may be bad and gutting it may be fine, but it may not be your problem, be careful of jumping at shadows, look deeper to diagnose the real problem, it may actually just be a sensor problem.

Have you gone to vehicle specific discussion groups to see if yours is a common problem and how others have resolved it, everyone here has different vehicles so the info is more general and not vehicle specific.

Back pressure is a commonly misused term when people speak of exhaust systems.... Velocity and flow are the important things in an exhaust.

The purpose of an exhaust system is to simply provide a way of exit for exhaust gasses. In order to get the maximum performance you want an exhaust system free of as many restrictions as possible. Obviously having a larger pipe diameter can increase flow capability...However you can go too far! An engine can only put out so much air. If the pipe is too large you loose velocity of the exhaust gasses. When that happens the exhaust cools and condenses. The engine literally has to push the exhaust out of the pipe. This pushing creates a differential which people refer to as "Back pressure".

Ideally you want a smooth pipe of the proper size that flows the perfect amount of air at the engine's most efficient operating RPM. This keeps the velocity of the exhaust gasses high which helps efficiency.

But wait... there's more! Just putting a pipe on there is going to make 5-0 unhappy... so you have to account for a muffler which controls noise and a resonator which controls drone. Placement of the muffler and resonator is very important. If you are in the USA you will also need a catalytic converter to make the EPA happy...

The valve timing will be set up with a certain amount of back pressure in mind ie with a stock exhaust.

Removing all the mufflers and silencers with the intent of reducing back pressure might negatively affect low speed running and driveability - and fuel economy.

Replacing the catalytic converter with a straight pipe shouldn't make much difference.

It's a pretty simple to check for exhaust restriction. Remove an upstream O2 sensor and screw a 5/8" spark plug adapter from a compression tester into the bung. Connect a vacuum gauge that also reads positive pressure to the adapter. Fire up the engine and observe the gauge. Less than 4" of positive pressure = a serviceable exhaust.

If it ain't broke, why fix it?

race cars a loud but they use wide open throttle most of the time

Cutting out the catalytic converter will not make the dash light go out and like MTXA said, you can measure your back pressure, but the larger chunks of exhaust pipe you have could be the root of your problem, causing the exhaust to slow in the pipe and screwing with the function of the catalytic converter and o2 sensor.
Try fixing the exhaust piping first with something as close to stock as you can, reset the check engine light and see if it keeps coming back as being an issue.

If you gut it, it will throw a 420 cat code. Fix it right. Back pressure on this vehicle is not as critical. Gutting it you wont notice much difference in sound or performance. Make no mistake, on performance applications back pressure IS very important. Rather than running open headers some drag racers switch out tuned extended header collectors to change torque curve and make more power at different rpm ranges. On street cars it's easy to install a larger exhaust than you need and you can actually decrease horsepower.