I have a 93 240sx with a completly stock exhaust system. I have a feeling that its pretty clogged up and was thinking of just ripping out the catalitic converter and replacing the muffler. Has anyone removed their cat converter and just replaced it with a piece of pipe? is there any issue with o2 sensors or any other complications? did you notice an increase in power? Basicly, do you recommend this? Thanks.
well i couldn’t tell you if it makes much power just replacing the cat alone as i replaced my entire exhaust sytem which did increase power
i will tell you that i had no check engine lights when i did have my cat removed on my 91
you can buy a de-cat pipe from lots of different dealers. Most of them have a bung for your o2 sensor. Generally there pretty cheap like 100bonesish.
Just take your cat off the car. Then take a piece of pipe and a hammer… Vwala, you now have a test pipe. :E
Start with the muffler.
Leave the cat on. While it is not illegal in Alberta to remove the cat it is bad for our air. There are high flow cats available.
smashy smashy , gut that cat and take out all the guts … if you have a stock exhaust then it would be best to do this as most aftermarket “test” pipes are 2.5 or 3 inch and would be useless to purchase unless you were upgrading some more of your exhaust system at the same time
Gutting the cat is not a good idea.
Read this… http://www.sportcompactcarweb.com/tech/9912scc_intake_exhaust_systems/index.html
Kojima knows his shit.
"Whatever you do, do not remove or gut out the catalytic converter on your street car. The monolithic, straight through design of modern 3-way catalytic converters is usually quite free flowing on most modern sport compact cars, producing at the most, only a pound or two of extra backpressure. A gutted cat can actually hurt power as the empty box can cause flow stagnation, which effectively shortens the length of the moving gas column in the exhaust pipe. The empty box can also reduce important flow velocity. This can be felt as a loss in bottom end power.
Because of these factors, some of our project cars have actually gained power with the addition of a cat. As the number of vehicles on our highways grows every year, we must all do our part to manage pollution. If every last bit of power must be extracted, as in real, off-the-street sanctioned racing, then the cat can be removed and replaced with a length of pipe, not simply gutted.
When changing a factory cat for a larger high flowing one, consider that Random Technology and Pace Setter make replacement cats with 3-inch ore even larger inlets and outlets."
But i love sitting is traffic with the window down smelling premium gas. And the occasional fireball never hurt anyone.
And the modern cat is that effective. have a look at a 5 year old cat.
Or any cat on any factory s13 or 14 . . .not so modern anymore eh . . .
I am willing to loose a little mid range torque for the gains I see on the top end. Its not like any of my cars are torque monsters anyway.
dont remove your cat, odds are youll lose power. if you think its gone bad, get a new cat. or better yet get a full new exhaust system.
Apexi has those high flow Cat’s, They work much better. I had an option mag with dyno runs with every Cat including a gutted one and if I remember the gutted had more hugh end HP but the apexi has it up high and down low! There expensive though 
Talk to paul at the skyline shop. Thats where i got mine!
http://www.theskylineshop.com/
pdm also sells magnaflow high flow cats with 2.5" flanges already welded
While taking out a new cat may end up lowering the amount of power you make, if your car is over 10 years old its likley that your cat is at least partially melted anyway… you will gain power… I will post up a picture when I get home of what ends up happening to a cat after it starts to degrade.
Just for clarification, my earlier post wasn’t about new versus old but rather leaving the cat box (he ehehehe) in, regardless of age, and gutting it.
My point was if you have the big empty cat box (again…h ehehe eheh) still in place it won’t be as effective as either a new cat or a decat pipe. Just wanted to clarify.
So, save us all from this suspense. What did you decide to do?
So, save us all from this suspense. What did you decide to do?
i think i’m going to replace it with a straight pipe. What do we need clean air for?
Exactly… If you are lucky enuff to drive a car thats turbocharged your cat is only hurting you. The turbo itself will give you way more back pressure then you will ever need.
Ok here are the pictures I took for you guys.
As you can see the top side is basically fine, all little blocks just like it should be. But the bottom side has been melted and is now about 50% blocked. This came out of what is likley the worst designed Cat system in the world. One that honda still uses to this day. The Cat is actually built into the exhaust manifold and hangs vertically. I belive this was done so the melted bits didnt’ pile up on the bottom side of the exhaust but all it ended up doing is making the cat almost $900 to replace because you have to swap the whole header to.
What are you talking about “all the backpressure you NEED”!? This whole discussion is about exhaust flow rate and not backpressure…read the article I posted. I am definitely not an expert but I trust Mike Kojima’s opinion.