Cat. Questions

[GT-X]

This may or may not have come up already.Has anyone X-ed their catilictic converter (spelling sucks... and to think I teach) anyway on my truck I was running a 2 1/2in. pipe to a flowmaster with not cat. and it ran great. I'm kind of new to the whole high compression 4 cly. thing. What would happen if I took out the inside to my cat. then stuck it back on??? I know it would open up air flow but is it going to mess with O2 sensor etc. and do I need a certain amount of back presser etc.. eventually I want to put on a aftermaket cat. back exhaust..let me know...

[Stang2Vibe]

I would also recommend a cat-back system that was designed for the Vibe. Hollowing the CAT will probably mess with the sensors and cause the vehicle to run poorly. One easy way to tell if the sensors will be affected is to check their location. I haven't looked for them on my Vibe yet, so I don't know where they are exactly for myself yet. Get under the car and look at the exhaust routing near the CAT. Make sure that you are looking at the CAT and not the resonator (the CAT will have a heat shield on it--looks like a metal mesh grille). If the sensors are in the exhaust pipe on the engine side of the CAT, you would probably be OK with hollowing it out. If they are after the CAT on the muffler side, then hollowing it will cause the sensors to read improperly and the engine won't run right. Most newer cars have the sensors on the muffler side of the CAT and this old trick can't be done without reprogramming the computer. Also, if you have emissions inspection required in your area, you might not pass it. A new CAT for most newer cars costs around $300-$400. So you don't want to purposely destroy it just to have to put a new one back on.

[MadBill]

I forgot to notice when I was rolling around under my base Vibe checking things out, but I believe all vehicles have an O2 sensor downstream of the converter these days. This is because the law requires a vehicle diagnostic for catayst function, and the ECM usually does this by monitoring the second sensor's output. The air fuel ratio on most engines is not controlled exactly to the chemically correct (stoichiometric) ratio at all times as you might expect, but "dithers" back and forth about ten times a second a small amount either side. This improves the converter's efficiency. It also means that the output voltage of the O2 sensor upstream of the converter cycles similarily. The downstream sensor's voltage however does not, due to a phenomenon called "oxygen storage". When the mixture swings lean, excess oxygen is stored in the pores of the cataylst; when it swings rich, this oxygen reacts with the excess fuel. The result is a steady voltage from the second sensor as long as the catalyst is functioning normally. If it is contaminated (or missing) the output varies just like the upstream sensor and the ECM sets a code. [There are inexpensive devices available to mimic the "normal" sensor output if one is determined to run without the convertor (or convertors, as the Vibe has two.)]

[d_m_kolb]

On a v6 or larger engine you'll probably notice a difference in performance by guttying your cat like you asked. But on a 4 cylinder the back pressure that the cat cuases in the exhaust system actually help improve the low RPM torque of the engine. If you remove this back pressure at low RPM you'll end up with a decrease in your performance. You might pick some up on the higher RPM but not enough to off set the low.alsdo if there is a O2 sensor after the cat and you gut it your rear o@ sensor will through a code causing the check engine light to come on and stay on.Plus cats are good in general for the enviroment. If you want try a high flow unit with a good cat back exhaust but remember a small 4 cylinder engine normally likes a little backpressure so if you open the exhaust up to much you'll be shooting yourself in the foot.I personally wouldn't go any larger than 2 1/4 on the Vibe. Maybe only 2" all the way out the back. This size should be large enough to let the engine breath well at high RPM and also keep the velosity of the exhaust high so it helps pull in the air fuel mixture on the intake stroke.Of course if your running forced induction larger is better but that's another topic.

[NovaResource]

Removing (or disabling) a working converter is illegal.