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Has anyone... [Deleted secondary cat stock]

17K views 55 replies 27 participants last post by  Gustav 
#1 ·
I know that people have taken out the secondary cat when they put on their Eisenmann exhaust.

My question is, has anyone taken off the cat on stock exhaust? If so, how does it sound?


thnx!
 
#2 ·
I have 2nd cat delete and replaced sect II or resonators with x-pipe along with Tubi mufflers. Here's a sound bite of mine if you haven't seen it already. Hope this is what you were looking for.

BTW: I absolutely love it. It sounds so much better in person. No trouble in the cabin.

http://www.youtube.com/watch?v=amTRbHkXcHc
 
#7 · (Edited)
Keang24 is right. When I bought from eisenmann they said it should pass because of the primary still in place. Then, I take it to my muffler guy and he says it mostlikely will not pass smog here in Cali. I kept sect. 1 just for that reason. When the time comes I will try for a pass unless someone in cali says they failed with secondary cats deleted.

The sound is awesome and well worth it. It also seems like it has added strength the car.
 
#10 · (Edited)
sorry again, got mess-up with FogPuppy comment.

Yes. the 2nd cat deleted will give a little raspy groaning from the midsection. I have 2nd cat/resonator deleted with stock muffler for about a week while waiting for a new exhaust, so I definitely noticeable with 2nd deleted. Not much louder, just more raspy
 
#12 · (Edited)
To add to the decription of sound and raspiness, since I recently deleted secondary cat, I already had x-pipe, what I noticed was the raspiness, increase in sound starting at 3000 rpm's. This is much sooner than before. Also, a lot louder start up warm up phase.

I also believe there feels like greater power. I am finding the wheels are spinning a lot more than ever before.
 
#14 ·
#15 ·
Just remembered who has that set up. Mike, AKA Wolverine. Don't think there's a sound clip. http://www.m5board.com/vbulletin/showthread.php?t=102285
Haven't removed the secondary cats, just the resonator. I'd take the cats out in a heartbeat if there were real hp gains, and no noise problems. I've heard there is some 'raspiness' or resonance at certain rpms if you remove them though, and no real hp gains.
 
#16 ·
I'm not sure I understand the threads original question? Are you asking about removing the secondary cats, ie. section 1, and put back the rest of the stock exhaust?
If that's the question I've not heard of or seen anyone do that. Do not know if it has any effect on the sound or power. Probably not.
 
#18 · (Edited)
I dyno'd each section separately. You can see what the section 1 (secondary cat removal) did here:

http://www.m5board.com/vbulletin/showthread.php?t=96799

It increased the torque from down low to about 3500rpm, then there is a slight loss for about 1000 rpm, then a gain to near redline.

Noise wise, you definitely noticed the increase in sound. It also took a bit of the smoothness away, giving the exhaust tone during the rev up a more raspy bark.
 
#19 ·
I'm installing the Eisenhaus system soon; section 1 and 2 included. When deleting the secondary cats, what's necessary to avoid the "service engine soon" fault? I see lots of choices for software upgrades to disregard such emissions faults, but they tend to be more than just a bypass software and rather a performance software designed for headers +.

I just want to avoid the SES fault only since I'm not replacing the headers right now.

Thanks in advance,

Nick
 
#22 ·
Sorry..... i have questions !!!

Can someone advise which section is secondary cat? Any pictures? Is there any sensors attached to secondary cat? After removal of secondary cat i assume the pipe would be shorter, how to resolve this issue?

Thanks! I wish to remove it but thought it is going to be a big job, so i didn't start doing so.
 
#24 ·
I think it was T Bone that posted a video here a long time ago with M6 stock everything except secondary cats deleted. It was first posted in a thread about Kreissieg exhaust then later in a separate thread. The M6 was white and the clip was done at night with very little background noise. Try a search, as I'm too lazy.
 
#27 ·
So when someone adds an X-pipe, what are they replacing, section 1 or 2? I've done this and still don't know what I did. haha
 
#30 ·
Thanks Kenny.

So if section 1 is secondary cats, where are the primary cats?

Someone had posted a picture of the complete underside of the M5 and labeled the different sections. Can someone locate that picture and post?
 
#32 ·
Thanks Kenny.

So if section 1 is secondary cats, where are the primary cats?

Someone had posted a picture of the complete underside of the M5 and labeled the different sections. Can someone locate that picture and post?
It integrated into the headers.
 

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#33 ·
ok... i'm a little confused... i spoke to one of the evosport reps yesterday and was told that deletion of 2ndary cats will most likely result in power loss due to lack of back pressure (since he said the back pressure helps scavenge exhaust still in the headers?). active autowerke posted a dyno showing some gain with their exhaust system which deletes 2ndaries.
noblem400, did you really get any gain (or loss) with your 2ndary cat-less dixis system?
evosport said that people delete 2ndary cats more for sound (since it would only result in power loss). instead evosport recommended high flow 2ndary cats to maintain/possibly gain power.
chime in if you've got some better perspectives
 
#34 ·
It's not quite so simple as a pure gain or pure loss. Here's my graph of average runs (3 runs per configuration, toss top and bottom):



The red line is the baseline, which is with the Dixis section 2 & 3.

The blue line is with the Eisenmann section 1 (secondary cat delete) added.

The purple line is with BMC filters added on top of that.

As you can see, from the addition of the Eisenmann, there was a very slight gain up 3500rpm, then a slight loss for 1300 rpm, then a gain up to near redline.

Overall, it's definitely a gain, no question about it. The total area under the power curve is larger after the secondary cat delete.

I think it's important to remember basic exhaust theory: unless there are some serious design flaws or the engine is unable to adjust fuel delivery, backpressure always reduces total power. This is a point that's often confused. All major exhaust manufacturers agree on this point. Basic info:

http://www.magnaflow.com/05news/magazine/05sportc.asp

BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Here's another way to put it:

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.
At low RPMs, you need exhaust VELOCITY. At high RPMs, you need exhaust VOLUME.

The best way to do this is to size your exhaust piping properly, and to use the lowest possible backpressure canisters. It's the velocity that helps scavaging: higher the velocity, greater the vacuum.

Backpressure is a shortcut to velocity, but not a great one, as it hampers power through most of the powerband.

Proper exhaust piping sizing + proper muffler design is the best way to maximize total power.
 
#37 ·
Very informative post, with excellent explanatory pics. Thanks guys!
 
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