Is a decat on a Z4M an option?

[jontZ4M]

Hi guys

I bought my Z4M roadster a couple of months ago and have been busy sorting out various bits with it. My next job is to tackle the exhaust. Currently the headers are on the car, but the rest is removed.

The exhaust is completely OE. It works but has two problems: 1) it's got a few blows 2) it's too quiet in general.

On examination the blows seem to be from the welds just before the cats. I see black sooting there.

My natural instinct at this point would be to replace this section with aftermarket decat pipes. This would fix the blows and from my experience with 90's Jap cars, increase sound without going over the top.

I come from Skylines and Silvias, and the internet is crawling with £100 decats for them. You just fit them and they never cause a problem(obviously reversion required for MOTs). But with the Z4M I can find almost no decat solutions for it. Do we know why this is? Is it because there are sensors in the cats and this causes problems with the electronics, making it impossible?

What are you guy's experiences with this please?

Thanks

 

[buzyg]

I kept mine OEM and had inserts fitted to fix the joint problem that occurs at that point of the OEM Exhaust. Always sounded OK to me. But never a patch on the 5 pot burble of my previous Fiat Coupe.

 

[jontZ4M]

I kept mine OEM and had inserts fitted to fix the joint problem that occurs at that point of the OEM Exhaust. Always sounded OK to me. But never a patch on the 5 pot burble of my previous Fiat Coupe.

Did these inserts cover the welded section like a sleeve? Or were they cut into the pipe?

 

[Vanne]

lots of decat options for z4m, just not for 100 pounds..

https://www.hackengineering.co.uk/product/supersprint-decat-pipes-z4-m/

 

[buzyg]

I kept mine OEM and had inserts fitted to fix the joint problem that occurs at that point of the OEM Exhaust. Always sounded OK to me. But never a patch on the 5 pot burble of my previous Fiat Coupe.

Did these inserts cover the welded section like a sleeve? Or were they cut into the pipe?

They inserter sleeves inside. So that the inserts matched the original pipe width and didn't restrict flow. The pipes were eased out to a slightly larger diameter to accommodate them.

 

[hiltonfa]

Had mine removed and straight piped very similar to the Hack Engineering pipes above, the rasp was terrible. Had to then have new silencers made up to deal with the rasp, the car is loud and does have some drone at certain RPM. Overall I'm very happy with the end result, my butt dyno tells me the car feels stronger I live in South Africa so don't have to worry about MOT.

 

[Argenta]

Can't see the point really, unless you're after poisonous gasses? Which of course could be fun as a joke. If you have a weird sense of humour.

 

[jontZ4M]

Thanks for opinions guys

So options so far seem to be:
1 - re-weld the leaking welds myself (probably won't hold for long). Do we know if the OE steel is 304 grade SS?
2 - get a pro metal worker to add the sleeve sections as you guys have described.
3 - get a new decat pipe for £900 and deal with changing back to OE for MOT.
4 - replacing with OE is not an option, I checked with BMW and no longer available. And anyway it would be well over £2000.

hmmm

 

[Ed Doe]

lots of decat options for z4m, just not for 100 pounds..

https://www.hackengineering.co.uk/product/supersprint-decat-pipes-z4-m/

That price is absolute f*cking robbery for a pair of pipes!!!

 

[TomK]

https://z4-forum.com/forum/viewtopic.php?t=138219

See Martyn's solution about half way down page 1

 

[maupineda]

Dont do decat. Car will sound horrible and smell like crap. You gain 2-5 hp, not worth it. Just repair the welds and move on. If you want more noise remove the resonator first. Or get a set of louder mufflers.

My car with SS x-pipe and remus was loud as f*k

 

[Mikey_Boy]

^^^ This.
Decat is an awful option. I bought a new OEM catalyst last year after a brick came loose after 14 years… It flows pretty well. Speak to Cheltenham BMW as they do them on an exchange basis - comes in a lot cheaper if I recall.

For the right noise, go for Supersprint or Remus or another established manufacturer. Loud exhausts are great until they get on your pecks, then they’re just annoying…

 

[jontZ4M]

See Martyn's solution about half way down page 1

Very clever, so he used E46 cheaper pipes and modified them. Probably above my skill level.

Dont do decat. Car will sound horrible and smell like crap. You gain 2-5 hp, not worth it. Just repair the welds and move on. If you want more noise remove the resonator first. Or get a set of louder mufflers.

My car with SS x-pipe and remus was loud as f*k

This car has a resonator? Where is it please? Is it that bulge where the two pipes meet? I thought that was just a joiner section.


So far repairing the Cat section seems best solution, then modifying it downstream for extra sound.

 

[TomK]

This car has a resonator? Where is it please? Is it that bulge where the two pipes meet? I thought that was just a joiner section.

Yes, where the 'X' is. I put a supersprint x-pipe on mine (already had their fairly loud 'race' back boxes) and the volume change was pretty extreme, I think my car went from measuring ~92dB @5000rpm to ~98dB static. Had to go back to stock for track day compliance, but I was happy, it was far too loud with the SS X-pipe. I have stock cats btw.

 

[Vanne]

lots of decat options for z4m, just not for 100 pounds..

https://www.hackengineering.co.uk/product/supersprint-decat-pipes-z4-m/

That price is absolute f*cking robbery for a pair of pipes!!!

yep, i didnt say it was a good deal, just that there are options out there..

 

[maupineda]

lots of decat options for z4m, just not for 100 pounds..

https://www.hackengineering.co.uk/product/supersprint-decat-pipes-z4-m/

That price is absolute f*cking robbery for a pair of pipes!!!

Then this car is not for you. Parts can be very expensive. This is not the usual cheap and cheerful Japaneese car. Wrong selection for low cost motoring

 

[jontZ4M]

Yes, where the 'X' is. I put a supersprint x-pipe on mine (already had their fairly loud 'race' back boxes) and the volume change was pretty extreme, I think my car went from measuring ~92dB @5000rpm to ~98dB static. Had to go back to stock for track day compliance, but I was happy, it was far too loud with the SS X-pipe. I have stock cats btw.

Ah that makes a lot of sense.

OK so the plan now is the following:

1) repair the cat section (not sure how yet) and refit.

2) modify/replace the X-section and rear mufflers to get the volume up. Either a new cat-back Supersprint set up, which will cost around £2000. Or modifying the OE cat-back system to remove the middle resonators and replace the mufflers with free flow units.

Thanks all.

 

[Ed Doe]

lots of decat options for z4m, just not for 100 pounds..

https://www.hackengineering.co.uk/product/supersprint-decat-pipes-z4-m/

That price is absolute f*cking robbery for a pair of pipes!!!

Then this car is not for you. Parts can be very expensive. This is not the usual cheap and cheerful Japaneese car. Wrong selection for low cost motoring

Lol behave mate, I don't care what car it is, a bit of bent 304 stainless pipe is not worth 900quid unless you're a certifiable clown.

I'm not getting into a dick waving contest over who's spent what on their cars but I'm fairly sure I've had the bigger end of the engine bills from an s54, I'm perfectly aware of and prepared for the cost of ownership.

 

[jontZ4M]

I'm not getting into a dick waving contest over who's spent what on their cars but I'm fairly sure I've had the bigger end of the engine bills from an s54, I'm perfectly aware of and prepared for the cost of ownership.

Not me sadly, my wife will cut my balls off if I spend too much on car that never gets used. I'll find a way of making it affordable.

 

[TomK]

Then this car is not for you. Parts can be very expensive. This is not the usual cheap and cheerful Japaneese car. Wrong selection for low cost motoring

Says the man with H&R coilovers