e36 m3 decat questions

[///MPROVD 131]

hey sorry if this has been covered befor but i have a few questions about decating a 3.0l m3 exhaust

1. can oxygen sensor be placed anywhere between header and cat?

2. can sensor be mount at 90' to the pipe half way between header and cat. whats the effect?

3. can i simple cut cats out and replace with resonators.?

4. can muffer simply be replaced with resonators

5. has anybody done this or can link some info to me.

6.what sort of volume would doing this get so its not too loud. ?

any help would be awesome.

[jeffbebe 1560]

I've decatted an E39 528 and 540. The 528 had no post cat sensors so it was easy. I replaced the cats with straight pipes, removed the y-resonator and the stock muffler with a high flow muffler. Sounded good from the outside but without any resonators was horrible in the cabin.

Learnt my lesson with the 540 and replaced the cats with two bullet resonators and kept the big stock y-resonator (you won't get a better design for keeping cabin noise down). I also replaced the stock muffler with a nice high flow oval box. Sounds great, just loud enough without sounding common and very little unpleasant noise in the cabin. The 540 had post cat sensors but they were fine without spacers or ECU flash. It's possible that the DME will try and compensate but I've found fuel consumption is way down (it has had a tune though).

I reckon if you remove the muffler altogether but replace the cats with resonators and keep the stock y-resonator (if it has one) it will be loud but not unpleasant. Whether it will sound loud and good is another matter - I think a decent box at the back is a great help or it can send a bit rough.

[Eagle 1679]

I think the Y resonator is an E39 V8 thing, i havent seen them on any other bmw myself. My 540i has had its Y removed and running some sort of single round muffler at the rear, sounds good with being too loud and you can hardly hear it when cruising.

[jeffbebe 1560]

The y stops the drone dramatically. It's an exceptional design!!

[Eagle 1679]

Thats how my car was when i got it but nothing i dislike about it as it stands. I rather have the E38 setup over the E39.

[duvey 245]

Mine has been de-catted. Pre-cat O2 sensors only, so just replaced cats with resonators, still running the O2 sensors in the same place (in the resonator). No drone in the cabin at all. Doesn't make it a lot noisier, but maybe a bit more rasp at high rpms.

As far as i'm aware, the later models have the post O2 sensors, so it might be a bit more complex. Maybe spacers or a ECU flash would be in order. Would be worth working out whether you have only pre-cat sensors or not.

edit: spells

Edited January 24, 2014 by duvey

[///MPROVD 131]

Mine has been de-catted. Pre-cat O2 sensors only, so just replaced cats with resonators, still running the O2 sensors in the same place (in the resonator). No drone in the cabin at all. Doesn't make it a lot noisier, but maybe a bit more rasp at high rpms.

is your 3l or 3.2? It wouldnt be a problem if it had a 3.2l as the sensors are on headers. do you have photos of sensors etc

As far as i'm aware, the later models have the post O2 sensors, so it might be a bit more complex. Maybe spacers or a ECU flash would be in order. Would be worth working out whether you have only pre-cat sensors or not.

only 2 sensors before cats they face at about 45' to the pipe, if i was to weld threads for sensors at 90' just before the resonators?
my goal is mostly performance im not a fan of fart cannon's and dont wont police attention but i would not complain about a little grumble

i dont mind keeping rear muffler just want better flow i know the power will not be much but if i simply remove cats and replace with resonators the cost if i weld it will be pretty low

thanks for the reply guys

the photos show what mine look like, and the other is my question can i mount o2 sensors at 90' and not change efficiency

[hybrid 1044]

1. can oxygen sensor be placed anywhere between header and cat? - No too far away from the headers, while the o2 sensor has a heating element for preheat open loop start-up programme, it needs to be close enough for the hot gases to maintain that heat on closed loop operation.

2. can sensor be mount at 90' to the pipe half way between header and cat. whats the effect? -- about 70 to 80 degrees slopping towards the headers so the o2 sensor tip is in the stream, this reduces turbulence in the mid pipe.

3. can i simple cut cats out and replace with resonators.? - yes, for a sporty sound it should be of straight through design, perforated tubing and steel wool design, this will remove/reduce drone without reducing performance too much.

4. can muffer simply be replaced with resonators -- hmmm its a big ask, the back box should not be of straight through design or else it will be too loud.

5. has anybody done this or can link some info to me. -- here http://bimmersport.co.nz/topic/24294-operation-weapons-grade/page-15#entry329797 although in these photos mine is missing the centre muffler / straight pipe flaged bolt in/out solution I added later.

6.what sort of volume would doing this get so its not too loud. ? -- with a straight through perforated center muffler and a non straight through design at the back with a fairly tight pack .. quite good.. whats it going on ? m3 ? Mine is quite raspy because I dont join the pipes on purpose .. to keep torque through the rev range. Its a very hard thing to get right using an x pipe and tuning the length. Lots of dyno and fabrication time.
To answer your question, the sound /tone entirely depends on the back box design. They all sound different.

Finally, you will either need to get the car tuned or have the correct software loaded into the DME for cat removal. the car will run slightly more rich without the cats, emissions air pump removed. Mine is yet to have the software loaded (long story but will be remedied soon) and economy is effected because of it.

[///MPROVD 131]

Ohk so if I leave my muffler as is and cut cats out and fit 2x 2.25' resonators and move oxygen sensor to about 150mm from cat position at a 70 to 80' angle towards headers? The intial sensor positions are away from headers?

Thats the performance going to be like also is there away to avoid reprogramming?

[hybrid 1044]

Ohk so if I leave my muffler as is and cut cats out and fit 2x 2.25' resonators and move oxygen sensor to about 150mm from cat position at a 70 to 80' angle towards headers? The intial sensor positions are away from headers?

Thats the performance going to be like also is there away to avoid reprogramming?

It shouldnt be so the o2 plug pipe ends up scooping air, it should be so the base of the tip is further away from the headers than the top where the wires are .. does that make sense ? if not will draw a nice mspaint mashup.

The performance will be fine, very little effect however your fuel economy will be slightly worse in closed loop (o2 sensor reading) mode, slight black soot build up on the back bumper etc. New OEM de-catted software flash or a aftermarket dyno tune such as what hi velocity offers will correct it.

[///MPROVD 131]

ohk cheers for the info might as well save for abit and get gavin to fully tune the car and do it all at once

[hybrid 1044]

cool, up to you .. We can now change the software to de-catted.

[///MPROVD 131]

when you say we? do you work at hi-velocity? ive heard that the 3.0l s50 make good power from a tune whats the power increase like with a decat

[jeffbebe 1560]

The tune will do a lot more than the de-cat. I don't think Josh is suggesting he can remap the ECU but he can flash a S. African version of your engine s/w onto the ECU. Cats were not standard in SA so the s/w is setup to run without them.

Tuning would be separate.

Edited January 28, 2014 by jeffbebe

[hybrid 1044]

What Jooles said.

No I dont work for them. We being Brendon and myself.