Gutting your Catalytic Converter

[Evil S14]

i have a 97 with a Apex-i N1 dual, i just wanna know if i gut my cat (drill all the stuff out and make it hollow) if itll create a backpressure problem or if ill feel a big gain in HP, will it cause some problems with some sensors?  I dont have to worry about emissions here in Indiana so i can do whatever

[wa-fu]

Hey, I have a 97 with a N1 Dual also.  Are you talking about gutting the thing that the N1 bolts to?  Because on mine it was already hollow.  Was yours?  If you're talking about the "primary cat" that is before the O2 sensor then never mind.

[hurleyboi514]

go to the links section, then to 240sx.  theres a place called PDM racing, you can get a replacement pipe there.  i think its like 75, but it will do alot more good than a hollow cat.  the point of guttin the cat is to improve flow, so why not take it up one more step and make it actally flow better and have no restriction?

[prInce]

Won't guttin cat kill backpressure the NA 240 engines need?

[wa-fu]

PDM website says the dump pipe won't work on s14's.

[prInce]

yeah, cuz the S14s have two oxygen sensors (one before and one after cat), unlike the S13s which have only one! <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'>
but eliminating cat will just trigger the ECU to display the "check engine" light, nothin major... right?

[Roly]

On an OBD2 and OBD3 car, if you remove the cat before the rear O2 sensor, it will trigger the "Check Engine" light. It's not really necessary to replace the cat unless you've decided to go F/I. But, if you still want it gone, then simply replace the cat with a Dynomax V8 cat, which is a hi-flow cat. And if I'm correct, isn't the second O2 sensor on the cat? &nbsp;If that's the case, have the muffler shop place a bung on the exhaust after the new cat so that it can be screwed in and still be functional without throwing a code.

[9591]

The PDM dump pipe won't work on S14s because it physically won't fit. Remember that the cats are in different places in S13s and S14s. Plus there's all that O2 sensor stuff already mentioned.

[tnord]

1. cats only produce 1-2lbs of backpressure, and a decent exhaust sysetem probably makes around 6lbs of backpressure. cats aren't that big of a problem
2. hollowing out cat will hurt performance. the extra large empty area it creates causes exhaust gas to become stagnent. there are two factors that are important when talking about airflow, gas backpressure and velocity. obviously, stagnent air is bad.
3. if you feel the need to modifiy your cat, just get one from random technology. they flow well, maybe giving a 2hp gain at best, and don't hurt environment (call me a hippie if you like, but if it's not a race car, it should have a cat). but to answer your question, replacing the cat with a section of pipe is the solution that will give biggest power gains.
4. backpressure is not needed to produce torque. this is an old myth.

[prInce]

really!?? I'd like to quote what you said:
<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/whatsthat.gif" border="0" valign="absmiddle" alt=':whatthe:'> "backpressure is not needed to produce torque. this is an old myth"
I defintiely agree with you that if the car's not a race car, it should have a cat and random tech is a great solution.
But, I've definitely been a victim of this myth. Isn't it true that our NA cars do need backpressure and eliminating the cat kills it? It's only the turbo charged that can benefit from cat removal... No? <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/confused.gif" border="0" valign="absmiddle" alt='???'>

Putting a testpipe does make the car backfire a lot though, or is that a myth too? <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'>

Please do explain to me how this is actually a myth, because I used to think this too but a whole group of people changed my mind.

[misnomer]

Backpressure is bad. This is not a myth. Think if it this way, the engine is trying to blow air through a really long straw. Backpressure makes it tougher to blow through that straw, takes more energy to do so. On the exhaust stroke, the engine has to push the air back out, greater backpressure puts more force on the pistons opposite the direction they are trying to move.

What is good is high gas velocity for a n/a vehicle. Think of it like a syphon, fast moving air farther back in the exhaust will help pull the air from the cylinders (to an extent). Pulling air out of the combustion chamber instead of the cylinder having to push it out is where you improve torque. On most exhausts, this is most effective at the lower end, 'cause when you get your engine spinning so fast, it's too much air being pushed through. This is one of the main reasons our cars stock die at the top end. This is simply an example, because of inherent backpressure in all exhausts, higher gas velocity proves to lower backpressure, not counteract it and suck the exhaust out of the engine <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'>

For this, you want smooth pipe with few bends to create the least turbulence. The narrower the pipe, the faster the air has to move. Ideal from what I hear is 2.25-2.5 inches in diameter on the KA (correct me if your smarter, please). Go too narrow and you increase backpressure, go to wide and you lower gas velocity. Essentially, you can help tune your powerband this way to an extent. Going a bit narrower will improve low end torque, but be too narrow for as much exhaust to flow through at the top end. Widening a bit will improve your top end, at the cost of losing that range of higher gas velocity down low. Look at some reviews on exhausts, you will fairly regularly find that the wider exhaust weaken the lower end while opening up the top end. This is the main reason gutting a cat doesn't help much, as a pipe that goes from two inches wide to a squarish four inch pipe back to two causes turbulence, and slows down the exhaust.

If you want to see what backpressure does, plug up one of your exhaust tips and see how well the car runs <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'>

[prInce]

niiiiiiice, i like the explanation... very well put <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'> this is what i wanted to hear. I agree with every single word you said. The folks at the nissaninfiniticlub definitely made me believe that backpressure was good for our engines... this is probably just being said because not havin a cat is bad for the environment. but a fact shouldn't be twisted like that <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/eh.gif" border="0" valign="absmiddle" alt=':eh:'>

ok, a friend of mine replaced his cat with a custom test pipe that he made by welding two 2.5 pipes together from autozone. his car backfires a lot at WOT, or when slowing down while in gear. now, i'm pretty sure that's not good.. how can that be fixed?

[tnord]

i think this is the reason that people think some backpressure is necessary.

at low speeds (low end) the combustion cycle is longer, thus creating less exhaust gas. a 2.5 or bigger pipe is larger than necessary for this amount of gas flow, thus the low end power/torque loss. but at high speeds/revs (high end) the engine creates far more gas, and the full 2.5'' is needed. and really, top end performance is what everybody (except for ace &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'> ) wants isn't it?

i believed that whole backpressure=torque thing for a while as well, but i then read an article in SCC about this not being true. call me a fool, but i trust what SCC says.

[misnomer]

Fool.

:-)

[HippoSleek]

Fool - don't trust those guys... a little knowledge is a dangerous thing! &nbsp;The bling-factor raises it's head over there too much!

misnomer &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>
-great explanation!

As for ditching a cat b/c you don't have emissions controls... I've heard that one before... wait... California, circa 1975 ("Who needs this emissions crap!"). &nbsp;Now look at them. &nbsp;Don't let CARB happen to your state! &nbsp; <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/lookaround.gif" border="0" valign="absmiddle" alt=':look:'> &nbsp;Keeping it clean keeps the regulators off all our backs.

[DuffMan]

Backpressure is definitely bad. But sometimes things that cause backpressure also improve scavanging, especially at low RPMs. This makes it looks somewhat like backpressure is needed to make torque.

What would actually make the most torque on an engine would be a straight pipe exhaust that increased in diameter as RPMs increased.

But back to the original question, no gutting your cat will not do much to improve your performance.

Besides you don't want to be like those rednecks that list gutting their cat as a "mod" to their trucks.

[prInce]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (tnord @ April 11 2002,01:44)</td></tr><tr><td id="QUOTE">...but i then read an article in SCC about this not being true. call me a fool, but i trust what SCC says.</td></tr></table><span id='postcolor'>
psst... you got a link to that SCC article online by any chance? i'd like to read that <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/cool.gif" border="0" valign="absmiddle" alt=':cool:'>

[DuffMan]

I Do! (http://www.magnaflow.com/05magazine/05sportc.htm)

[tnord]

hey duff, i would say you're absolutely right about the ideal situation being a pipe that increases in diameter with rpms, i think that this is the theory behind the variable muffler or whatever it's called that some makes are putting on their cars (i think the new maxima's have it), where there's a valve that opens up to allow better flow at high rpms.

in theory, couldn't you have a system that split at the header, with a small pipe on one side and a larger on on the other, with a valve at the intersection to control gas flow?

anybody else got any ideas on how this variable pipe diameter thing could actually be achieved?

[prInce]

so back to the original topic... <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/sigh.gif" border="0" valign="absmiddle" alt=':rolleyes:'> gutting the cat DECREASES overall perfomance when compared to the OEM cat, or does it just not produce as much power as the test pipe but does still perform better than havin the OE?

[tnord]

gutting cat should decrease overall performance

[prInce]

hey I found this website that has a test pipe for '95 and up 240sx! &nbsp;Call em up. &nbsp;I think it comes with the option of havin a space for the oxygen sensor so it won't show the check engine light.
btw, even for our '89-94 240sx, the test pipe is only $30! &nbsp;dang, pdm-racing is makin lotsa money sellin it for $75...
oh yeah, before i forget, here's the website:
www.testpipe.com

have fun!

[prInce]

hey i called these guys and ordered the test pipe for my '90 S13 and it was $30 + shippin'...
for ppl who have S14s with oxygen sensors on their cat convs & the answer to Evil S14's question:
i asked em if they made the test pipe and also provide a place in there for the oxygen sensor to prevent the check engine light to be lit and they said they DO!!! kewl?
all you have to do is that when you call them, give them the measurement for the cat of an S14. &nbsp;They said no one called em for a test pipe for S14 yet
<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/satisfied.gif" border="0" valign="absmiddle" alt=':satisfied:'>

[Cheater240SX]

I've seen a toyota (avalon and lexus sport ute, what is it?) "dual mode" muffler cross-sectioned at work. &nbsp;There is esentially just a big flapper valve that opens up a chamber to bypass much of the muffler, providing a less restrictive path for exhaust flow. &nbsp;Seems to work well at high rpms and lets OEM still pass regulatory sound requirements.

[BlackFox]

So from what I'm hearing is that as long as you plug the 02 sensor into the exhaust flow your computer will figure it all out? I though that with OBD2, 95 240sx se, your ecu needed to take a reading after your exhaust reacts with the cat, is this correct? My friends Type R has a stand alone computer so he can get pass the obd2, I'm planing to get a JWT ecu so can they compensate for not having a cat? What about the pipe diameter and matching a high flow cat to existing stuff, i'm going to get a Apexi N1 Dual, Thanks

[BlackFox]

Anybody?