[bigdonnyd40]

Hi folks. I come in peace from the G30 forum (I have a 540i). I thought I'd post here as the F30 folks have been tinkering with the B58 platform the longest and most aggressively (which is awesome).

Like many of you, I love the sound of a more free flowing exhaust with subtle burbles and running a tuned B58.

As we all know, the factory catalytic converter being right behind the turbo is the biggest bottleneck. The problem is that I'm sensitive to the uncatted smell so I can't run catless and the good catted options are very expensive, but more importantly, they all add unneeded back pressure right behind the turbo and they deteriorate over time if you run even OEM-level burbles.

I recently had a catted downpipe (CG DS-1) and while the performance was there, the fitment was a little off and with a 200 CEL cat smelled basically like catless so I'm back to stock now (main issue was bad fitment).

My idea is the following: run a catless DP which will not deteriorate with burbles AND the best flow right behind the turbo, and to mitigate smell, run a downstream 400 or 200 CEL cat around the midpipe (possible one of these).

This cat relocation idea is pretty popular in the Audi community as they have cats right behind the turbos as well. I haven't seen anyone doing this on the B58 in my limited reserach. This should in theory mitigate smell IF the cats can get hot enough that far back from the turbo (I'm thinking right where the downpipe meets the mid-pipe). It will still be far more free flowing than stock and likely sound better, but it will mute the catless DP sound a bit (which is fine).

Would love thoughts on this or to know if anyone has tried it.

Thanks!

[toxik]

question about the Audi guys, is the cat relocation being done on their hot V engines?

[bigdonnyd40]

Originally Posted by toxik

question about the Audi guys, is the cat relocation being done on their hot V engines?

No. Typically you see it with the 3.0T motors. I used to have an S4 and they did this a bunch.

[gantona]

Hello,
This is a really good question btw...

From my humble perspective, cats behind turbo are placed here for a specific purposes : efficiency on lowering emissions
The closer to the turbo the cat is, the more hotter it will be, increasing his efficiency.

Other tweak to accelerate efficiency operational range is done by DME at each startup : cold start procedure.
But since it's also running when engine is hot, you can remove the "cold"

that's being said, this procedure create load at startup in order to quickly create heat, to warm up the cat faster and being efficient and emission friendly.

So, taking the cat away from the turbo, may sounds good for the turbo, but may drasticaly limit his efficiency in term of emission.
Also, I don't think you will gain performance, since the global flow remained limited by the cat.

my 2 cents, but I may be wrong !

[bigdonnyd40]

Originally Posted by gantona

Hello,
This is a really good question btw...

From my humble perspective, cats behind turbo are placed here for a specific purposes : efficiency on lowering emissions
The closer to the turbo the cat is, the more hotter it will be, increasing his efficiency.

Other tweak to accelerate efficiency operational range is done by DME at each startup : cold start procedure.
But since it's also running when engine is hot, you can remove the "cold"

that's being said, this procedure create load at startup in order to quickly create heat, to warm up the cat faster and being efficient and emission friendly.

So, taking the cat away from the turbo, may sounds good for the turbo, but may drasticaly limit his efficiency in term of emission.
Also, I don't think you will gain performance, since the global flow remained limited by the cat.

my 2 cents, but I may be wrong !

Yes, the global performance will be as good as a some of the higher end high flow cats which typically run 200, 300, and 400 CEL (CG DS-1, Fabspeed, and Active Autowerks respectively), however the placement of the cat being further down the exhaust will mitigate premature cat failure caused by a burbles tune BUT also accrue biggest benefit of a high flow cat over catless - less smell (in theory).

I guess the only risk is that exhaust temperatures at the midpipe section are not hot enough to properly active a cat (but I doubt that).

Anyone know?

[gantona]

Hum…
You will have to move the post cat lamba sensor, after the mid pipe cat.
I’m almost sure,
The efficiency will be worse than expected by ECU, resulting in a CEL for low efficiency cat.
Something you will turn off with your tuner

[n55david]

Originally Posted by gantona

Hum…
You will have to move the post cat lamba sensor, after the mid pipe cat.
I’m almost sure,
The efficiency will be worse than expected by ECU, resulting in a CEL for low efficiency cat.
Something you will turn off with your tuner

Yes, you have to move it and it will cause issues..

[DreamerJoe]

I have a 2017 340i and running a catless DP, dorch stage 2, and mhd. After the tune the smell really isnt bad at all, once going with an e85/91 tune its almost non existent. I smell 0 fumes when at a stop light, even if all windows are open, and not even bad standing behind the car during idle.. just cut the cat man, or get a high flow, but dont worry too much about the smell. Ive had other cars that were bad, but this one honestly isnt.

[bigdonnyd40]

Originally Posted by n55david

Yes, you have to move it and it will cause issues..

Why would I need to place the second o2 sensors behind the mid-pipe cat? The car is tuned so there will be no CEL regardless.

The mid-pipe cat is only there to reduce the smell.

[bigdonnyd40]

Originally Posted by DreamerJoe

I have a 2017 340i and running a catless DP, dorch stage 2, and mhd. After the tune the smell really isnt bad at all, once going with an e85/91 tune its almost non existent. I smell 0 fumes when at a stop light, even if all windows are open, and not even bad standing behind the car during idle.. just cut the cat man, or get a high flow, but dont worry too much about the smell. Ive had other cars that were bad, but this one honestly isnt.

Fair enough. Are you running E85 or E30?

[DreamerJoe]

Originally Posted by bigdonnyd40

Fair enough. Are you running E85 or E30?

The tune is the mhd e40 but I mix my gas 50/50 cause calculators are annoying. I didnt like the full e85 tune because you actually notice the fuel efficiency and cold cranks take like 2-4 seconds.