Modern cars cats

[Mr Whippy]

My BMW 3.0i Z4 has these:

http://bmwfans.info/parts/catalog/E85/Roadster/Eur...

http://bmwfans.info/parts/catalog/E85/Roadster/Eur...

Basically it goes:

Manifolds with built in pre-cats, and lambda sensors pre/post pre-cats
Front silencer which just appears to be another cat per exhaust
Centre section which is a cross-over mixing box for both exhausts
Back box which is just a big box


My query is, what is the purpose of seperate cats and which are doing the most work...

Do I need those second small ones in the front section? Or are the manifold ones the less useful ones for MOT passability?


Just wondering where I can save weight/flow restriction/cost in the search for better efficiency and a nicer noise (will look at the back box mainly, but just wondering on these centre cats if they are really needed or not)

Many thanks


Dave

Edited by Mr Whippy on Friday 16th October 17:33

[ridds]

First ones are to achieve early cat light off to meet stringent emissions targets (euro IV and V)

Pretty sure you could get away with just the rears for the MoT.

tbh I'd ditch the lot and replace the rears with race cells.

[TheEnd]

The front cats will be there to heat up quicker, and come online earlier which helps win BMW a green badge, but as for the second set, the only reason i can think of is that the front ones are too small to be fully effective, and packaging means they are as big as can be, so a secondary set is needed to take up the slack where they have more space.

That way, as least 50% of the cat converting can get going within a minute or so, and the other 50% comes in at the usual time.

What you'd really need is one of those BMW crib sheets that get sent out as dealer info before the model is released explaining what all the features are for to the techs.

[Mr Whippy]

Yeah, I imagined the first ones on the manifold heat up quick and get the dawdling around town/short journey emissions under control better...
Then the secondary ones (smaller by the look of it), help with the extra flow at higher revs etc.

I think what I am tempted to do is try fabricate some really shoddy pipes to sit in the middle ones place and then go to a friendly MOT centre and just let them do an emissions test on it... see if I can get away without or not...


I'd love to dump the lot, but the fronts are literally part of the manifolds, so a fairly large job to do anything with I would imagine, and with the lambda sensors at each end I guess the engine likes to know they are there and working which might be a pain to work around too.

Those back ones however seem to not be monitored or possibly needed


Will obviously flow test them to see if they are really that bad vs a straight pipe... anyone know what flow to simulate for a 1.5 3 pot engine at 6000rpm? Will temp of the flowed air make much of a difference for calculating back-pressure? (ie, will sucking air through them with a manometer on the sucking end, with a decent industrial vacuum, be good enough and indicative?)

Cheers again

Dave

[Tony427]

If the manifolds/exhausts are anything like the V8's they will be utterly crap at maintaining flow.

I would presume that the manifolds are double walled ( and hence far smaller bore than you would think) to get temps up high as quickly as possible in the first set of cats and then only get to single walled once past the second set of cats.

Simple removal of the first set of cats without moving the second lambda sensors behind a cat would simply cause the engine to throw up a fault code, on goes the engine check light and maybe also limp home mode. The second set of lambda sensors are there to check the cats are working.

If it was me and I had the cash this is what I would do.

I'd get a set of single wall manifolds, with a proper bore, and have the first Lambda fitted into the manifold close to the head.

I would then have a set of down pipes made going into a sufficiently large cat acting as a balance box with the second lambda sensors fitted in the pipe after that cat. Your gasses do not have to be as clean as BMW's to get past the MOT.

Thus you replace a number of small cats with just one large decent flowing one, remove lots of impediments to gas flow, increase power and response, improve exhaust note, emulate the OE placement of Lambda sensors ( thus not triggering fault codes) and you will still pass the MOT on emissions.

I took out the manifold cats on a Mondeo V6, leaving just one of the three, and it still passed MOT and my completely de catted V8 Lexus/ Cobra is a whisker away from passing anyway without any cats.

Cheers,

Tony