My friend said that he hollowed out his cats on his car and then put them back on. He said it makes his car louder and makes the exhaust less restrictive.

Does anyone know what steps I should take when doing this, or special tips I should know?

Will it effect my air to fuel ratio?
Will exhaust gas leak from the cats?
Should I run a straight pipe throught the cats to prevent an exhaust leak?
Will it make my car sound unnatural?

P.S.
I know it is illegal but I don't car

Well, I havent personaly seen or heard any 350Zs with a new exhaust, but I'm pretty sure they will make a different sound. I think the Nismo should stay a little loser to the same, but with more deeper sound. There shouldnt be any leaks or anything like that.

My friend said that he hollowed out his cats on his car and then put them back on. He said it makes his car louder and makes the exhaust less restrictive.

Yes it does because a catalytic converter is basically a ceramic brick in your exhaust that gets really hot and burns most of the dirty gases in your exhaust, making it cleaner. Removing this brick clears up the exhaust passage and makes it flow easier.

Does anyone know what steps I should take when doing this, or special tips I should know?

Well frankly I would say don't. If you really want to get more power out of what your profile says is a 94 Thunderbird then I suggest you look into an actual aftermarket exhaust system, which will provide much more in the way of power gains and legality.

Will it effect my air to fuel ratio?

Not likely, if you went from a stock system to a full 3 1/2" pipe from the headers back then it may lean out a bit on the top end but even then I would doubt it.

Will exhaust gas leak from the cats?

Again, not likely because the cats are sealed tubes of metal, as long as you didn't puncture them when gutting then you wouldn't have a problem.

Should I run a straight pipe throught the cats to prevent an exhaust leak?

See above.

Will it make my car sound unnatural?

If by unnatural you mean different from stock then yes. If your worried that it would make your car sound like a two stroke being run in a coffee can like most Civics sound, then no, I can't see it doing that.

P.S.
I know it is illegal but I don't care

Unfortunately the government does.

Spellcheck?

Oh, and go with the aftermarket straight pipe has a potential gain of about 20hp "no chance of a so called leak not that it really matters" and if you get caught or busted buy the cops or DMV with an illegal straight pipe you can always go back to the original and you won’t need to buy a new one from the factory, since you didn’t “gut” the original, big money saver
(minus the hefty fine you will receive from the government since it’s a federal law that you have a catalytic converter) but other than that I say go for it.

Do not "gut" the cats. There are several reasons not to do so. Especially if the engine is still normally aspirated. They are as follows:

The major one being that the current generation of catalyst elements are not very restrictive at all. In fact, they actually work like a tuned collector on normally aspirated engine. As the exhaust gases pass through them, they act like a check valve for exhaust flow. As the exhaust valve closes, momentum causes the gases to continue to travel through the exhaust system. This is plenty of energy to pass through the cats. This causes a vacuum to build up between the exhaust valves and the catalytic converter. The energy required to pull the gases back through the element can't build fast enough to work before the next exhaust valve opens. When that next exhaust valve opens, it is opening to a vacuum environment which helps to PULL the gases out of the cylinder due to what is called "scavenging effect". This translates into more power as the piston now does not have to do all the work to evacuate the cylinder of spent combustion gases.

Second, if you were to remove them, you would definitely want to use a cat removal pipe. The chamber left after you would "gut" the cat is actually worse at losing energy than a catalytic element (understand I mean element designs after around 1990). This is because as the gases enter the empty chamber, exhaust velocity slows down. As it then regroups to enter back into the exhaust piping, it does not have the energy it did before, therefore it "stacks", causing backpressure. This backpressure now works against the flow of spent gases out of the cylinder when the exhaust valve opens, causing the piston to do all the work. Now, energy is being wasted for gas removal that could otherwise be used to turn the crankshaft.

My recommendation would be to leave the cats alone and concentrate of every other part of the airflow system. After all else is done, come back to them, and if not using a high flow catalytic converter, then use a cat-delete pipe.

Shawn @ Street Sports

It wasn't a Z that I hollowed out...it was a Hyunda Excel. The change in sound was not noticable to the non-driver (very mild change in sound). I don't recall any noticable increase or decrease in power, it was many years ago. But if you still want to do it, just remove the cat and take a screw driver (or chisel) to the stuff inside. It looks kind like a honey comb. I don't know if the stuff has a dollar value, but I think someone told me it had platinum in it. Anyway, if you do it...just remember your car probably won't pass emissions testing anymore.