E46 Cat Removal?

[MISS BM 712]

Probably a question asked before, but couldn't find a lot on it.

Wanting to do small improvements to increase the level of sound out the backend, not drone loud, but a nice, louder than now sound.

I don't want to loose my Supersprint muffler as it has a really nice pitch to it (despite its odd looking elephant trunk tip).

Just looking to increase that sound a few notches.

In my 728i, I eliminated the rear mufflers and replaced with Coby's. Sound was loud, but a drone and personally not very desirable.

So.. I have been in and discussed, and they recommend loosing the cats and replacing with small Coby resonators.

Will this modification trip my O2 sensors, Being they are engine side of the cats?

Plus the side affects of loosing the cats, increase smoke output on cold starts and fume smell?

And I am sure it is okay in Nz to eliminate, but best to confirm with those who know facts, not 'I think so'

Thanks in advance.

[B.M.W Ltd 950]

You don't need to remove the cats to getter a slightly better sound. Removing them will give you other issues. Talk to a reputable exhaust specialist.

[MISS BM 712]

You don't need to remove the cats to getter a slightly better sound. Removing them will give you other issues. Talk to a reputable exhaust specialist.

It was the last option given, first was to customise and remove my supersprint, but not keen to remove it, then tips are restricting so to replace them, but unsure on that either, so last resort was Cats.

This was after going in. Im just making the job difficult by wanting to keep my SS.

My question is what issues will I see by removing cats. Need to weigh up to cancel the job or go ahead with it.

[Michael. 2313]

I think cats should be kept, firstly you avoid the annoying smell without them and secondly they act like small resonators taking some harshness of the exhaust noise.

If they are in good condition power losses would be negligible.

Best case scenario IMO would be to put an M3 mid section in, and have it adapted to your rear muffler, I did this on my old 325i, it sounded awesome!

[gjm 3258]

My 320d has (had) two cats. One is in the downpipe, the second in the middle section of the exhaust.

Had? It now has just one cat - the one in the downpipe has been removed and replaced with a straight-through piece of pipe. It's a bit of a faff to do as there are heat shields and suchlike in the way, but it does mean that a cursory visual inspection won't notice there's anything different.

However, some exhaust systems have sensors and the like in them, reporting to ECUs. Removing a cat can cause the signals reaching the ECUs to cause them to function in ways you don't want, leading to more problems - nothing that can't be sorted, but it is something to be aware of.

I don't know what facilities there are here in NZ, but in the UK companies that sell aftermarket exhaust systems will often be able to provide a re-flash of an ECU, or a piggyback, so that the car continues to run properly. I'd expect something similar to be possible here... Isn't there someone in East Tamaki who does stuff like this? It's worth a look.

Removing the cat will change the note of the exhaust but it tends to be subtle. A good free-flowing cat won't affect performance, either. It's only when you have a 350,000km+ sooty and smokey diesel that has clogged the cat that removing it really brings benefits!

[MISS BM 712]

Thanks guys.

Was going to replace with resonators but will can the idea now.

Sent from my KM-UQM11A using Tapatalk

[zero 1162]

Thanks guys.

Was going to replace with resonators but will can the idea now.

Sent from my KM-UQM11A using Tapatalk

Hey Amber - how many kms on your car?

Mine had about 120k when I did the cat. I cut it out and it was completely blocked.

I simply removed the internals with a hammer and chisel and got it welded back in.

A little louder exhaust note, and a tiny bit more power (my main car is only a 4 cylinder).

I think you have a m54 engine?

If so you should have cats in the exhaust manifold, and a cat midway down the length of the car.

The cat halfway down the car can be gutted or replaced without effecting the sensors.

The ones in the exhaust manifold require some mucking around to get the sensors working properly.

I plan to do this on my second car which is a m54.

[MISS BM 712]

Hey Amber - how many kms on your car?

Mine had about 120k when I did the cat. I cut it out and it was completely blocked.

I simply removed the internals with a hammer and chisel and got it welded back in.

A little louder exhaust note, and a tiny bit more power (my main car is only a 4 cylinder).

I think you have a m54 engine?

If so you should have cats in the exhaust manifold, and a cat midway down the length of the car.

The cat halfway down the car can be gutted or replaced without effecting the sensors.

The ones in the exhaust manifold require some mucking around to get the sensors working properly.

I plan to do this on my second car which is a m54.

Somebody who sent me some useful information - thank you!

My car has done double that (hence my project thread to potentially swap around when the engine dies.. not soon by the way she purrs!)

Spoke to another member who had his done on newer bmw.. seemed all fine and well.

I have M52TU - prefacelift to the M54.

I was only planning to spew out the mid ones, not the both of them. And just replace with small resonators.

Not a total Cat overhaul.. Haven't made a move yet, as lot's of ups and downs and this and that regarding them.

[twisted 45]

Your car would have post cat sensors? If so, it can still be done, but the o2's may need to be spaced out. Been looking into this for my m52b28 e39 schnitzel. Deff has post cat sensors

[MISS BM 712]

Your car would have post cat sensors? If so, it can still be done, but the o2's may need to be spaced out. Been looking into this for my m52b28 e39 schnitzel. Deff has post cat sensors

Already had it on the hoist at exhaust shop - no sensors post cat.

[jeffbebe 1560]

The cats will be munted anyway in a 10+ year old car. Remove them and you'll have no issues as you have no post cat sensors but definitely replace with something. I used a couple of small resonators and the sound is fine but a little bit droney at low rpms (but sounds great at high RPMs). I'd recommend replacing with some high flow cats so - as Mike says - you still get some of the benefit of cats in terms of emissions. If it's still too loud fit another resonator in the single pipe to the supersprint (or two small ones if it's a dual in exhaust).

[zero 1162]

Somebody who sent me some useful information - thank you!

My car has done double that (hence my project thread to potentially swap around when the engine dies.. not soon by the way she purrs!)

Spoke to another member who had his done on newer bmw.. seemed all fine and well.

I have M52TU - prefacelift to the M54.

I was only planning to spew out the mid ones, not the both of them. And just replace with small resonators.

Not a total Cat overhaul.. Haven't made a move yet, as lot's of ups and downs and this and that regarding them.

If you have as many Km's as that your cats will be blocked for sure.

Cheap fix is gut the cat and reinstall (sounds better than a straight pipe imo).

More expensive fix is replace cat with an aftermarket one which gives you more control over the sound.

Its up to you what your budget is.

Personally I think replacing or gutting your front cats would be beneficial because of age as they are bound to be restrictive.

[jeffbebe 1560]

I have also decatted an M54B28 with straight pipes and removed the Y resonator... Loud, droney and generally horrible. The cats do help with excessive noise, especially in the cabin so whatever you I agree that straight piping isn't the best option.

[KwS 2429]

My M3 has resonators replacing the cats. Sounds great, but at 100KPH it does have a drone (thanks to sitting at just under 3000RPM). If you do pull them out, do replace them with resonators as itll just keep the sound much more refined.

[MISS BM 712]

Did anyone actually read my orginal post?

I will be placing resonators in place of the Mid cats...

I did the straight pipe thing on the 728i. I think I lost brain cells from the excessive drone over 50kmp/hr. Never. Again.

I will be keeping my Supersprint Muffler in place - it's actually rather quiet now anyway. It won't do a drastic increase.

[KwS 2429]

I read the original post, im just clarifying that even with resonators you can get a drone. If the cats are OK and not blocked i dont see the point in removing them. The increase in noise wont be too substantial.

Also, my car does smell a bit from the exhaust, more so when cold (due to cold start enrichment). I dont mind it, just smells like burnt fuel. Cant notice it when the engine is warm unless backing up with windows down (and then its only a hint of exhaust). I wouldnt worry to much. Mine will be made worse by having no O2 sensors and overfuelling.

Edited May 4, 2014 by KwS

[zero 1162]

Did anyone actually read my orginal post?

I will be placing resonators in place of the Mid cats...

I did the straight pipe thing on the 728i. I think I lost brain cells from the excessive drone over 50kmp/hr. Never. Again.

I will be keeping my Supersprint Muffler in place - it's actually rather quiet now anyway. It won't do a drastic increase.

Sorry Amber.

Will you be doing anything with the front cats?

Its highly likely they will be blocked.

[MISS BM 712]

Sorry Amber.

Will you be doing anything with the front cats?

Its highly likely they will be blocked.

LOL! I'm not angry, and no sorry's needed. Just facepalming here being told something I already know

Will need to investigate front cats.. Not sure I'm confident enough to touch them :/

[MISS BM 712]

I read the original post, im just clarifying that even with resonators you can get a drone. If the cats are OK and not blocked i dont see the point in removing them. The increase in noise wont be too substantial.

Also, my car does smell a bit from the exhaust, more so when cold (due to cold start enrichment). I dont mind it, just smells like burnt fuel. Cant notice it when the engine is warm unless backing up with windows down (and then its only a hint of exhaust). I wouldnt worry to much. Mine will be made worse by having no O2 sensors and overfuelling.

Thanks for that, it's the smell that I was a bit unsure of.. A mate did a full cat removal on his VW and I think I almost died if I stood behind it while it started up..

[KwS 2429]

Its the smell of power!