De-cat and back box mod.

[Jollygiant]

So I've managed to get a free exhaust (partial swap for carbon) where the secondary cats have been cut open, removed and welded back up. Complete with guards still intact, so is this correct it will pass the MOT like this?

So I was wondering if anyone has done this AND the back box mod where you cut it open remove most of the guts and weld some pipes in and weld all it back up? If so what it horribly loud and droney on long runs?

Back box mod here https://z4-forum.com/forum/viewtopic.php?t=88783

And de-cat here https://z4-forum.com/forum/viewtopic.php?f=2&t=129420&hilit=cat


Forget to mention its 2.5 E85.

 

[Sidewaze Samm]

And de-cat here viewtopic.php?f=2&t=129420&hilit=cat

The car to which this link relates is a 2.5 and already had the back box modded. I would guess doing the cats on their own would still sound better and it will still improve exhaust gas flow. I'd try it for a while to see what you think before taking the plunge with the box. And the sound of that will depend on what is done to it and by who...

 

[Zedebee]

If the catalyst itself has been removed, it is possible you will fail an MOT on emissions, since the catalyst will no longer be doing its job of removing harmful CO and NOx from the exhaust gases.

 

[patriot66]

Hi Jollygiant. I have a Z4 2.5i (M54) and have had DLVs modified backbox and secondary de-cat for a few years now. I posted my views/experiences in the thread link below back in October '23, which may be of interest to you.
The car also passed another MOT in January this year without any issues, Si
https://z4-forum.com/forum/viewtopic.php?p=2035512&hilit=SECONDARY+DECAT#p2035512

 

[Ed Doe]

Personally I wouldn't bother - it'll be unbelievably loud.

Maybe I'm an old fart, but I did the backbox chop on my old 3.0si, I detested it and put the Stock back-box back in as soon as I possibly could! It droned like hell below 3k-rpm, and as I used it as a daily as well as for weekend fun, it was not something I wanted to live with. God only knows how much louder it'll be with a decat too - but I'd bet it'd be painful to you, and more than a mild irritant to anyone unfortunate enough to be in your vicinity with the car running.

 

[enuff_zed]

Personally I wouldn't bother - it'll be unbelievably loud.

Maybe I'm an old fart, but I did the backbox chop on my old 3.0si, I detested it and put the Stock back-box back in as soon as I possibly could! It droned like hell below 3k-rpm, and as I used it as a daily as well as for weekend fun, it was not something I wanted to live with. God only knows how much louder it'll be with a decat too - but I'd bet it'd be painful to you, and more than a mild irritant to anyone unfortunate enough to be in your vicinity with the car running.

I would agree that a backbox delete and replace with straight pipes is abominable, but I think the OP is considering the DLV-style of cutting open the back box, piping it through and replacing some of the sound insulation. That has always seemed like a much better compromise.

 

[Jollygiant]

Hi Jollygiant. I have a Z4 2.5i (M54) and have had DLVs modified backbox and secondary de-cat for a few years now. I posted my views/experiences in the thread link below back in October '23, which may be of interest to you.
The car also passed another MOT in January this year without any issues, Si
https://z4-forum.com/forum/viewtopic.php?p=2035512&hilit=SECONDARY+DECAT#p2035512

I think that is the only exhaust link I didn't look at on the search!

But yes this what I'm looking to do.

I want it louder than standard, but not back box delete loud as I still would like my wife to want come for drive in it and be able to have a conversation with her at motorway speeds (yes I actually still like her!)

I didn't pay for the exhaust so I can cut the back box open and play around it over the winter and fit it on my drive and if it costs me £50 to get it welded up thats bargain. But swapping the de-cat pipes over I'll need to pay for that as I cant be bothered with lying on the ground for a couple of hours trying to reach under there in a 6 inch gap.

But its good to know it'll pass an MOT with the de-cats in a that would be a pain to swap them over then back again just for the MOT.

 

[Rockhopper]

In theory it should fail as "(a) Emission control equipment fitted by the manufacturer: missing, obviously modified or obviously defective Major.

In reality it'll probably pass assuming the emissions are still okay.

 

[enuff_zed]

In theory it should fail as "(a) Emission control equipment fitted by the manufacturer: missing, obviously modified or obviously defective Major.

In reality it'll probably pass assuming the emissions are still okay.

I suppose as cars get older it is harder for testers to remember what should be under there anyway.

 

[Rockhopper]

True, there was talk of an on-line guide for them to refer to. I suspect they are only going to get stricter about things like this though.
For modified one signs of welding on the CAT might be enough to prompt further investigation.

 

[Jollygiant]

But if the de-catting is done properly it'll be under the guard or done from the top, so the tester can't see.
You can't fail a vehicle with a loud exhaust, just give it an advisory as loud, as this is what happens to my motorcycle every year.

 

[Rockhopper]

Maybe bikes are different?

Exhaust noise from the vehicle must not be unreasonably above the noise level you’d expect from a similar vehicle with a standard silencer in average condition.
(a) Exhaust noise levels in excess of those permitted - Major

 

[Jollygiant]

So I did the back box mod and it makes a nice difference.

Deeper than standard on tick over, but when you rev it's louder but not back box delete loud and sounds lovely when accelerating like a twat.

I got it swapped over just before driving it London and back (from Norfolk) to pick my son up for Christmas and I was chav at every roundabout and junction and I just giggled every time and even my son who is now a cool 24year old Londoner laughed a lot coming home, so it must be good!!

The drone is slightly louder than standard but not that much and just having the radio on will drown it out. With it being a 2.5 and only 5 gears you are sat at 3k for 70mph.

If it was my daily it is liveable with when driven normally its still not much louder. But its not my daily so the de-cat pipes will be going on in the new year and the guy that fitted it said he knows a tester that passes de-cats, but the rest has got to be spot on, so we'll see!

With the weather being grim here in the UK in December and not nice for lying on a cold, wet gravel drive trying to get under it I paid someone to fit it for me.

Total cost was £145.
£25 for stainless pipes
£5 for cutting discs.
£15 for exhausts sleves for fitting.
£50 for welding and another £50 for fitting.

I won't bother how I did it as its detailed pretty well here https://z4-forum.com/forum/viewtopic.php?t=88783

But I just cut this section out.

 

[ChrisGUK]

Did you put anything back in or just weld the 'lid' back on? How does it sound?

 

[Jollygiant]

So seeing as it passed its MOT I've now added these...


Look closely and you'll see two weld squares on the cat's, thats where they were cut open and the guts ripped out and weld back together. Cost £50 to get the fitted as I didn't fancy doing it on my gravel drive with the rusty studs that he had to liquify to get out.

Together with the modded back box it sounds damn fine now. The de-cats have made it a deeper tone that burbles nicely and pops slightly on over run, not a pop, bang map like the 'lads' have on their Golf's and A35's.
​​​​​​​
It seem to revs better, but I have left the standard airbox on at the mo as it's going to be dyno-ed on Thursday.