First, does it defeat the purpose of putting on an aftermarket header if you leave the stock cat on? And if so, does anyone know if the car has to be a certain "age" before you can put on a hi-flo cat? I had heard that a car has to be at least 5 years old before you can change the cat on it. Thanks.

 

I put my cat-back on with stock headers. Had my Cobalt for about only 4 months.

It wouldnt matter putting aftermarket headers on, with stock exhaust still on.

No worries!

 

the only thing i'm worried about is having the extra exhaust flow from the aftermarket headers and then have it run into the restrictive stock cat

 

The total restriction is a sum of the header, the downpipe, the cat, the rest of the exhaust pipe and the muffler. If you drop the restriction of the header, the entire system will drop restiance. If you want to get the most out of it, do the entire exhaust, but an improved header will only benifit you.

Yes it does have to be of a certain age, assembled.

 

your stock cat flows just as good as any hi-flow cat out there. It's all the same really. The only way to really drop the restriction of your cat is to take it out or hollow it out.

 

no cat = jail time

 

yeah i heard that if you take the cat off and the cat isnt 5 years old that the cat cops will come arrest you and put you in jail for violating emmisions , the cobalt has an electronic sensor in the ecu to be able to tell if the cat has been taken off or hollowed out

 

thats only the half of it. that sensor is hooked to a built in cell phone in your car that will call the cops and alter them that you have removed your factory cat. then they throw your ass in the smaller, no trial no nothing. and the 4th amendment doesn't apply to you any more, bring on the cruel and unusual punishment...

the laws will vary state to state, but in colorado, to be absolutely technical ANY exhaust modification is illegal. this includes replacing a rusted out factory muffler with a replacement from midas. obviously its not enforced (replacing a defective one might get around that law, i'm not entirely sure) what is enforced is that you have a cat on the car, and even then its only checked when you go to have an emissions test done, which 06 (maybe 05) and newer vehicles are exempt from. a cop can check if he or she feels like it, but first off they won't, and if they did thats probably the least of your concerns.

as for an age to replace the factory one with a high flow one, its most likely not true, depending on your state. even if it is illegal and you do go to a high flow cat, all you have to say is the factory one was defective, if anyone would even notice. which they won't.

 

What are you, a fear monger? All bull...

Here is what you heard about the "5 year" thing. Depending on your state, you car has to have it's emissions tested starting after the car is 5 years old. Before that, you don't have to have a test done.

Aftermarket cats are perfectly legal as long they in fact do their jobs. If you pass emissions with it, your fine. If you don't, you just get a better one. They don't fine you, at least not here in Illinois.

As for your other question about restriction. Each part does help (header, downpipe, cat, exhaust). And yes, it's a "weakest link" sort of thing. Any restriction is a restriction. You will gain improvement from a single part, but you won't get the best results until the whole system matches up.