Exhaust upgrade

I have recently rebuilt the head on my Metro 1.0L and all has been well so far except I got a "catalyst threshold code" showing probably since the car was running on 2 cylinders for a long time. I am looking to upgrade the exhaust and maybe even do away with the cat, but the car has 2 oxygen sensors on it and I don't want to hurt performance or mileage.

I work at an Auto/Truck garage and have enough exhaust parts laying around to put 5" stacks on it, but seriously, I need some input.

The exhaust on this car is pretty rusty and already leaks some. I would like to modify the exhaust by going from the manifold 90 degree bend and just opening it up to 2 1/2 to 3 inch pipe all the way to the end and add a turbo muffler or glasspack.

What ideas do you guys have for my exhaust upgrade? Do I need a cat? Do I need O2 sensors with the OBDII system or will eliminating them screw up the car? Could I run my custom exhaust and simply weld in O2 sensor ports for the sake of the OBDII system?

Input please!

you can switch the exhaust to a bigger pipe for better breathing but you will need to run both O2 sensors on the car for it to run right. also if you eliminate the cat no exhaust shop will touch the car. if you do put in a new system run some sort or resonator like a glass pack or something in the middle of the car or it will sound like a angry lawn mower. they have a really weird exgaust noise being a 3 cyl and are obnoxious without resonator

The car will run fine minus the cat and the downstream o2 sensor, but you'll be illegal, have a check engine light, and you wont be smogable. You need to keep the upstream o2 sensor because it controls fuel trim in closed loop mode (operating temperature and not WOT).

As far as pipe diameter goes, bigger is not always better, especially if you plan on getting rid of the cat.

If you keep the cat (or replace with a high flow cat :bigthumb:) along with an aftermarket muffler/resonator setup, don't go bigger than an additional 3/4in.

On the other side of the spectrum, if you plan on getting rid of the cat, and having just an aftermarket muffler with no resonator, don't exceed 1/4in. additional pipe diameter.

The reason being is back-pressure and exhaust scavenging. Zero back-pressure (or no resistance in the exhaust stream) is never a good idea because you lose the 'slug' effect of the escaping exhaust gasses which is crucial to the systems ability to clear the combustion chamber of the spent air/fuel charge at higher RPMs.

:2cents:

-MechanicMatt