What is really better? Removing pre cat oder main cat?

[pablo82]

hello...i have read here al lot of removing pre cat or cat....
but my german english is not the best...

so what is really better to get more performance?

removing the pre cats or the main cats?

if i remove the pre cat i must only place the second o2 sensor behind the main cat...right?
if i remove the only the main cat i must nothing change on the o2 senors?

did i feel a really strong difference in power?


ps. great forum here

[two2]

anything that is close to the engine will restrict more power, so that is why the precats are removed.. best performance would be to remove both main and precats, but in short, it's the precats which will yield the most gains.

if you remove precats an O2 spacer (non-fouler) is placed on the main cats.
if you remove main cats, you still need a spacer (non-fouler) due to the increase air flow.

[pablo82]

thanks...so

if i remove the pre cat i can use for the second o2 sensor a spacer-ilsolator or place the second o2 sensor behind the main cat...

what i am interested too...what ist the difference between removing the precat and the main cat....its clear that the pre cat will bring more, but how more than removing the maincat?
because its more easy to replace the main cat than the precat and its faster to install it back if i need....

is it really a huge win of power at the s4 v8 or only a little bit...

[SDS]

I'm curious too about power gains.....if stock chp is 340, is there a close estimation for:

catback = xxx chp
catback + dp's without precats = xxx chp
catback + catless dp's = xxx chp

or for what I'm planning - catback (resonated) and catless dp's + JHM tune = xxx chp?

[dparm]

The pre-cat is only used for cold starts. The main catalyst doesn't work very well until it is warmed up, and coupled with a high-idle when the motor is cold...you pollute a lot then. The pre-cats scrub the air at that point. Once the car is warm, they're not doing much and act as a restriction.


Stop looking at horsepower gains as X. It's more about learning to build a package of parts that extracts the most power. And as an extension, it's improving the area under your horsepower/torque curves where the engine is geared and cammed to make power. Who gives a shit if you can gain 10 peak horsepower? Gain 10 horsepower from 4000-redline and then you'll be talking.

[SDS]

How do you "build a package of parts" for these cars? It's not like there's a vast array of exhausts, cams, cranks, pistons, intakes, etc.....as far as hardware goes we've got catbacks and dp's, and that's pretty much it. Yea you can spend a shitload on long-tubes and a supercharger but I think the majority of us will be going with the catback + dp's routine. I don't see how simple curiosity about what the estimated chp would be is so silly.....and I'd figure de-restriction from the manifold back, and a software redo (JHM tune) would boost the hp and tq output all across the rev range.

[dparm]

There's a lot of differences in the downpipes and exhausts for this car.

[pablo82]

for me its not interessting how much horsepower i get...it is interessting if i can say after that: yes my car goes now much better than before.
my car goes not really bad and it runs nice to 270km/h, but i miss a little bit the kick in my back...
i have a catback exhaust with a h-pipe....and the difference between a stock exhaust i can fell...and when i remove both middle silencer a put there only a straight pipe i can feel a gain extra of power too...
i think the four cats are to much for the s4...
removing the pre cat is no problem...but i don´t want to have problems with the electronic...i hope your guy are right when you say put the second o2 sensor behind the main cat and there will be no problem... because when i decide to kill the precat it shoul be safe it works...another questions is the emissiontest wich i think should be harder to pass in germany
our s4 b5 pass them without precats...but they have a lower emissonclass then the s4 v8

in germany drives a lot s4 but to find a used cat is not so easy and when you have to paid about 900$ for it...

i also wondering why the sock exhaust have no x or h-pipe and why the precat are so close to the engine...

[SpeedETC]

This should answer some of your questions

http://www.audizine.com/forum/showth...ONG-SUPER-LONG

[dparm]

Precats are close to the engine so they warm up quickly. Like I said, they keep emissions down when the main catalyst is not hot.

[jfunkey]

Ditch the precat. No more needs to be said. Its best all around

[FWP!]

Ditch the precat. No more needs to be said. Its best all around

How?

[dextrek]

Ditch the precat. No more needs to be said. Its best all around

I second this.

Precat is too close to the engine, causing the gas flow to choke up before it can scavange.
The aftermarket DP's with precat's will perform less than the OEM DP with gutted precat.

However, the best solution is no cat. By far the best mod for my car.

[Hansel]

I second this.

Precat is too close to the engine, causing the gas flow to choke up before it can scavange.
The aftermarket DP's with precat's will perform less than the OEM DP with gutted precat.

However, the best solution is no cat. By far the best mod for my car.

how loud is it now?

[beemercer]

Read this http://www.audizine.com/forum/showth...9C-The-Formula

ii. Next comes the downpipes; the primary goal is to eliminate the very restrictive pre-cat, additional gains can be had by smoothing bends, increasing pipe diameter, removing main cats.
-Pre-cats are of particular importance because of their location nearest the exhaust manifold. They are able to trap large amount of heat within the exhaust manifold and combustion chambers, causing the ECU to retard timing advance. Also, they restrict gas flow at the vital junction between the exhaust manifold and remaining exhaust piping.
-Elimination of the main-cat further reduces gas flow restriction, but does not provide as much of a benefit as the pre-cat elimination.
-Larger diameter piping; stock is 2.5” necked to 2.125", whereas aftermarket units utilize 2.25”-2.5” piping. The closer to 2.5” the better.
-Smoother bends/ more flow conducive design.

[dextrek]

how loud is it now?

It toned down a bit, but still pretty loud. I love it.

[Hansel]

the apr is JUST perfect. BTW what coil overs do you have on your car?

[pablo82]

is it better to cut the pre cat (remove only the middle piece...so i can reuse it if needed) and close it again or put instead of the pre cat a straight pipe? i mean a pipe produce better flow as a gutted precat

[jfunkey]

Don't put in a stright pipe. Save the time and hastle. You can still pass emmisions both visual and functional with just the cat punched out. No need to save the cat

[Terry.Reese]

Sorry late to the party on this, I am looking into removing the precats as well, would it be better to remove them, or just gut them out? I guess I didn't understand what I would then need to do with the O2 sensor, i heard I needed to move them back to not get a CEL.

[jakeoboy67]

Sorry late to the party on this, I am looking into removing the precats as well, would it be better to remove them, or just gut them out? I guess I didn't understand what I would then need to do with the O2 sensor, i heard I needed to move them back to not get a CEL.

empty cats may make an unpleasant resonance/drone/tone, and on a lotof cars, they just sound like shit.

luckily on the S4's it doesn't affect the sound in a bad way.

as for the CEL, just get a tune (the CEL basically says that your cats are inefficient and dont warm up fast enough)

[Terry.Reese]

are piggies a better option? or will that still require a tune? I mean, I am going to get her tuned at some point, just wanting to know if that is something I should just do all at one time.

[badger.]

Piggies sound just fine. However, I would not tune without having piggies as the tune will not be as effective in the aspect of garnering more power.

Note that if you do just piggies you need to address the rear O2 sensors somehow if you don't want to throw SAI codes. Some people have luck with relocating them, some people have luck with spacers, and some people (like me) have luck with neither and still throw SAI codes all the time. The tune has an option to ignore SAI codes.

[macattac]

I've looked at JHM's site but I can't seem to find extensions/foulers.....where can I source such?