de cat issue's on s4

s4marsh

s4marsh

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i'm just thinking of different exhuast options and have been thinkin about gutting my precats asap,but when talking to a audi mechanic/600bhp audi owner he said removing the main cats would have more benefit which is obvious so looking at this picture above can anyone tell me what exactly the main cats are doing in relation to the engine management or making lights appear on the dashboard as the main cats are beyond the lambda probes so should have no effect if i leave the pre cats in as it looks to me like they're doing the work of cleaning the gases and reporting info to the ecu??? am i wrong????i know emmisions next mot time are the issue but thats ages away and refitting them easily enough the way i plan to delete them. whats peoples thought on the subject???
 
if im right, it looks like there is a lambda after the big cat

so getting rid of either cat is going to upset the ecu
 
on the milltek system yes but the standard system which i have i forgot to say. both lambda's are before the main cat. i got a price yesterday to cut my system off just in front of the flexi joints fit 2 new ones and a dual 2.25" system no main cats that'll look exactly like the milltek dual system


the total cost would be £535 but that leave's me with strandard downpipes with my precats in to keep the ecu happy. now i know this is not the usual way people do it but if a cat back milltek system produces an extra 10-20bhp for example then surely the restrictive main cats should also give me something or are my precats left in gonna cause any issue???
 
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What year is your S4?

Even if you do throw error codes, a tuner can alter the ECU so that it doesn't register a fault. My S4 has never had any problems with the lambda readings since gutting the pre-cats.

Also not sure why it was recommended to leave the pre-cats, as these are presumably more restrictive than the main cats seeing as how they are closer to the turbos and therefore the hotter gas.
 
Ye as Blue says, the cats right infront of the turbo really slow the gas flow out of the turbo down.

The best option is to create what the milltek pipes do, but you dont neccesarily have to spend £££ on the milltek pipes. Do the "piggy mod" and gut your precats, then you can replace the main cats with 200cell freer flowing items, or get a set of RS4 downpipes, and do the piggy mod to those as they already have 200cell main cats
 
The dual lambdas depends on the age of the car.

All pre-facelift S4's have one lambda before the first cat.

Some later ones have the two.
 
Pre-'99 cars, including regular 30V V6 had just the 2 lambda sonds before the catalysts (OBDI). Later OBDII cars - S4 and regular 30V had the extra 2 after the catalysts.

The 1,8T versions also went through the same changes. Milltek build all their systems as OBDll compliant, and supply a blanking plug for OBDl models.
 
If you cant afford a retune of the ecu for any cels just add some anti foulers , they dont cost much at all. What year is the car?
 
my s4 is a facelift 99 car so i'm assuming it has the 2 lambda on each pipe, i was just running through cheaper options really i can have a full used rs4 system for £200 from a lad off here gut the precats and fit that as the main cats are 200cell right??? i was just thinking of simplicity, but if the precats cause more restriction than the main cats down the line it seems a MUST do job if i'm gonna bother doing anything with the exhaust i was just thinking of the benefit of no main cats instead and just a chop off and weld on the new system, i can get my map adjusted for nothing i've spoke to my tuner already he gave me a very good price to do my downpipes aswell but its another cost on top of what i cant afford already at the moment...lol the quote of £535 replacing the flexi joints onwards seems tempting for a dual milltek replica but removing precats adds the price to do that plus to high flow cats so it still ends up around £850 so no matter what i end up doing its expensive...it wasnt the tuner reccomended leaving the precats it was the audi specialist mech who mention it. but said to check with the tuner. i also know you guys have been there and done that so value your input.
 
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i've just discovered that the demontweeks website has the full monty turbo back milltek with 100cell cats its very expensive but more importantly they do FINANCE............should i?????? i cant afford it but what does that matter they're not exctly gone knock on my door wanting it back
 
If you go for the full RS4 system you will need to chop off the exhaust tips as they don't fit the S4 bumper.

If you get the RS4 DP's, cutting them and removing the pre-cats is a relatively easy job. You'd just need to get them rewelded.

An RS4 exhaust would definitely offer some performance improvement over the stock S4 exhaust, but you may be better off with something like an XS-Power catback and RS4 DP's for maximum performance improvement on a budget.
 
*Yes and no.

*Didn't hint that they would do. My thoughts are more toward sound/perfomance/fit/service/how much maximum power they're good for etc. A general review from somebody would be delightful, but I simply haven't heard of ANYBODY who's tried one yet.

I want to give KC a ring (they're only a few miles down the rod from me), but KC is a notoriously arogant son of a gun. S'pose I'll email him.
 
i could use the gutted rs4 downpipes and pipe work with maybe a milltek silencer @£285. i've not seen that qst system i'll have a look now
 
you dont want any more front antiroll bar, but an upgraded rear will help reduce the cars naturally understeery tendancies.
 
so these kits.. is the front one not really upgraded that much over the stock item? a reduction a understeer would be very nice it can also be achieved by running a slightly lower ride height on the front of the car only 5 mm is needed to make a big difference in weight transfer and balance. anyone with adjustable coilovers that can drive well should try it and report back. have you done a rear only roll bar yourself???? or anyone??? i dont like the story's of subframes cracking with h+r tho i'll tell you that for free
 
The fwd's only have a front bar so i've not tried it on ours, but its very popular over on the B6 forum, they all fit RS4 rear bars which are a nice upgrade over the stock B6 rear.

Suspension theory says that if you make the front of the car stiffer it will move the balance towards understeer, and if you make the rear stiffer it will move it towards oversteer.

This applies to both spring rates and damping, as well as the ARB's.

So fitting an upgraded front and rear will in effect cancel each other out in terms of balance changes, and all you'll have done is reduced the independance of the suspension.

I would fit a slightly uprated rear bar, perhaps 20mm at the thickest and leave the front alone, which should produce a nice balance between understeer and oversteer.
 
i'm gonna fit the 19mm rs4 factory one then with upgraded bushes i think and see how that goes when funds allow, cheers mate, right sort of back on topic... by the looks of things exhaust wise i'm gonna do a deal on the full rs4 system gut the precats and then get a milltek/other silencer for it as this is by far the most economical pound/performance gain. i would like to fit the new dp's myself but my confidence far outweighs my skills and as from what i've seen underneath the car and in posts on here its a right awkward job if i wasnt so impatient it would be ok but i just hate waiting once i have a plan developing in my head just hope it doesnt set fire to all the saw dust thats in there....lol
 
i checked my roll bars today and i have a 29mm on the front but only a 15mm on the back and the only S4/RS4 one's i can find listed for the rear is 15 or 16mm so i'll try the 16mm with powerflex bushes and change the bushes on the front too so should be a nice subtle handling change
 
i wouldnt waste your time going from 15 to 16mm

Find an aftermarket bar thats somewhere between 17 and 20mm, and go with that.

The 17mm Quattro Gmbh upgrade for the RS4 doesnt have a part number, the catalog just lists a fax number:

stabilizer / * audi-exclusiv-teile * * bestellung: * * nur ueber quattro gmbh * * fax: (+49) (0)7132 318216 *
heavy-duty 17MM /

I'm sure one of the aftermarket kits with the strengthening brackets for the subframe will be fine, but dont go too thick.
 
whiteline seems to do an 18mm bar, and H&R do a 21mm bar

Might also be worth investing in the strengthening brackets from MRC or the subframe can crack.
 
audi upgrades
Has anyone had any experience with the xs downpipes?
http://cgi.ebay.co.uk/AUDI-S4-B5-2-...s_SM?hash=item35a343190f&_trksid=p3286.c0.m14
I noticed they dont have flexi joints on like the standard downpipes, wouldnt this put more strain on the turbo's. hmm...
I was considering fitting these then getting a custom exhaust from these pipes back, Would this be a good idea?
I know i'd have a problem with emissions but im sure i can fine a freindly garage :D
 
have a look on pistonheads classifieds, parts section there is a few stainless, rs4, s4, systems going cheep on there is that helps you out mate

worth a look, good luck
 
i was so temted to try the xs power downpipes i kept looking at them every few days for months thinking should i shouldnt i..the flexi joints are to relieve the stress of expansion and contraction with the heat fluctuations and also isolate the movement of the main exhaust from the turbo/downpipe.it cant do any good not having them in and will cause trouble at some point. a pic was posted on here last week of a 6 month old xs system that looked worse than my 9 yr old system already so that was enough to put me off. i'm fitting a full rs4 system with gutted pre-cats in a week or 2 and maybe put a milltek or similar back box on too
 
Im getting a custom exhaust built on friday at longlife in leeds, I'm going for turbo-back system with no cats. But with flanges so i can put the cats back in when needed. Then im getting the car mapped a week later at MRC, Would my car be running too lean after the exhaust has been done and damage my engine one the way to MRC?
 
my rs4 system will be here this week ding dang dooo

fire up the grinder................
 
i gutted my rs4 precats last week and ordered some new lambda bosses from merlin motorsport on monday to relocate the rear 02 sensors i waited all week watching the letterbox but they never arrived untill saturday when i was out.......the best bit is i dont even need the sodding things i think... it seems that only the U.S s4's and euro rs4's had the four sensors, i'm a right stupid c**t. i'm gonna weld the precat section back up 2moro and wait for the cat back system coming at the weekend and attempt to fit them, i also need to find some new tips for the rs4 back box, i know this cut looks wonky but my arm was tired after doing the other with a blunt hacksaw, i have my new gaskets, new dp flange nuts new heatshield bolts + new jubilee clips, what could possibly go wrong.....
picture.php
 
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audi upgrades
all the write-ups of the precat gutting i've read have shown 4 sensors so i just presumed as usual...silly boy.i've just been and picked my gaskets up from audi £4 each which is better than i thought they would be, i passed a black R8 on the way and had a little race with an S3 which had a predictable outcome so all in all a good trip out
 
i've just welded my rs4 dp's, there was 3 out of 4 lambda's still in the pipes so i needed to remove these, i couldnt believe how tight they were so i used a bit of heat and 2 of them came out but left the threads in the bosses. is this quite a common problem? and am i gonna face the same problem when i come to remove my downpipes?
 
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