3.0i decat?

M

Mr Whippy

Veteran
 Harrogate
Just having a read/search about Z4 3.0 decats and not much info really.

Taking a look here:

http://bmwfans.info/parts/catalog/E85/Roadster/Europe/Z4_3.0i-M54/RHD/N/2004/june/browse/exhaust_system/lambda_probe_fixings/
http://bmwfans.info/parts/catalog/E85/Roadster/Europe/Z4_3.0i-M54/RHD/N/2004/june/browse/exhaust_system/front_silencer/

It seems that the 3.0i has pre-cats as part of the manifold (with pre and post lambda sensors) and low-capacity cats downstream of the sensors/pre-cats on the front silencer section.


I noticed the Milltek kit for this car is either the back/mid box for maybe £600, and then £1200 for the pre-cat/manifold back system. So basically you pay £600 more to replace those little cats in the front silencer section.


So, since the sensors are up-stream of those little cats, do they actually add much to the cleaning up of the exhaust emissions? Has anyone removed these on a system and had an emissions test?
Anyone done any flow testing against sports cats to determine how 'bad' they actually are?


Just pondering if they are needed or not. I have a friendly local MOT tester so I can go test a system I throw together, but if someone knows before I start tinkering it'd be good :)

Dave
 
The pre cat's won't affect it much. Would be amazed if removing them made the car fail a test. It is some euro legislation thing saying they have to have them.
 
Problem is the pre-cats are built into the manifolds, so not cheap to do much about :(

The mid-cats are easy to do something about however. I wonder which ones do the main work in the system.


I feel a test session is needed to work out what does what... when I have the time/money to try :)

Dave
 
Hi, have you removed the mid-cats on the underside of car?

If so, what did the car sound like? Did you get louder POPS/BANGS????

Just wondering as I'm wanting to remove them or either do a rear muffler/back box removal for straight pipes so I can get more POPS BANGS and CRACKLES out of my exhaust! :evil:
 
Hey there,

Yes I just had straight sections made up to replace the cats under the car. They're swappable and I still have the cats, but I've not had to swap them for MOT. Every year since I've had it done it's passed fine without them. Probably 4 years worth of tests now.

The noise is similar. A bit deeper at lower rpm, a bit more transparent and less muffled.

The overrun pops/bangs are much more noticeable. Generally they come through as obvious pops/bangs rather than really wooly sounding muffled ones. Occasionally or if you provoke it, it'll do a full on snap or pop that will echo in an urban environment.


Either way it's much better having them done I'd say.

I think I paid £200 and it's the best thing I did wrt getting a bit more of an interesting noise from the car, without it just getting silly loud.


Dave
 
The sensors are in the manifolds. Unless you place the secondary O2 sensors after a set of cats, swapping the manifolds will cause you to throw a CEL and fail inspection.

Are you sure you have cats in your midpipe? I just have a resonator on my 2003 3.0i. N52 cars do have another set of cats (so four total counting the set in the exhaust manifolds).
 
They have manifold cats with the sensors, and then a secondary pair just near the gearbox.

They are definitely catalytic converters as I've looked inside them.

They're just there as overkill I suppose, so it's extra clean for type approval... but they're not needed for clean enough running for the MOT test even.


Well worth dumping if you want a bit more of an interesting noise, but don't want to be paying £££ for a full exhaust system.

Dave
 
That's interesting. I'd dump the catted manifolds in favor of catless aftermarket headers, and extend the O2 sensors to the secondary cats. That should be good for 15+ horsepower.
 
Thing is, after-market headers and sensor loom extension are gonna cost £1000+ for 15bhp.

Also there is no guarantee the secondary cats will be good enough for MOT passes, which could add further cost to upgrade them.

For a quick £200 spend to improve sound and probably power a bit, de-secondary cat is a good move to make.


Eventually it's just easier to get a TS1 ESS supercharger kit for £3,500 or whatever it is these days and go straight to 300bhp!

Dave
 
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