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While in Audi today , a few fault codes showing small loss of back pressure...on inspection it appears that the flexible part of the exhaust( looks like its covered in wire wool webbing) just before the cat is fraid, and it appears maybe bottoming it out on a speed hump has created a small fracture in the pipe, as this is a flexible part im not sure if this crack can be wielded up or if it will need to be replaced?

i wonder if a slight drop in back pressure could result in power loss / overheating on the track, next week.?
Anyway going up on hoist at tyresome in darlinghurst on monday for a look (Audi dont wont anything to do with it). hopefully a wield or something will fix it or a custom piece to replace it could be built, hope i dont have to get it from milltek..... we all know how long that takes

Fitting such an exhaust system will always show a little lack of back pressure because it breathes so easily, but this is more apparant on forced induction engines, where torque can slightly decrease/

Easy weld it and be done with it. Plug up vag-com erase code and possibly do some data logging, check the a/f ratios and when ya get your DNA tuning performance ecu upgrade ask jordan for a more custom tune, since you have this exhaust.
The most important issue though is your a/f ratios, very, very,very easy to blow a cylcliner if a/f ratios are high due to the exhaust.
Cheers

RS4Molar, what's the optimal A/F ratio for our RS4? I read 12.8-13.2:1 will give best / safe power for n/a engines but some say 14.7:1 is ideal. What's your take?

Ozrs4 wrote:RS4Molar, what's the optimal A/F ratio for our RS4? I read 12.8-13.2:1 will give best / safe power for n/a engines but some say 14.7:1 is ideal. What's your take?

lambda of 14.7 is ideal, however slightly more fuelling is a safer option from the factory. Often there may be poor quality fuel, owners that don't regularly service and poor drivers. Lowering the lambda will help with these issues and prevent any lean outs. It will certainly decrease perfromance a touch, but reliability is high on the AUDI's checklist.

Fitting a miltek without custom map is questionable due to the increase in the A/F ratio, It would be very advisable to place a sniffer and put it on the rollers before PING - and there goes the motor. AUDI won't want to know about it.
Try pricing the RS4 FSI motor ?? WOW

Ozrs4 wrote:RS4Molar, what's the optimal A/F ratio for our RS4? I read 12.8-13.2:1 will give best / safe power for n/a engines but some say 14.7:1 is ideal. What's your take?

In theory the optimal A/F ratio for maximum torque is 13.2:1.

This is the air ratio that provides the most complete burn - any richer AFR and there will be unburnt fuel, as there aren't enough oxygen molecules.

For NA cars, they are normally run at 12.5-13:1 AFRs - leaner AFRs generate more heat which can cause detonation and excessive engine wear. Thus most road cars run a little bit richer for the sake of engine reliability and also to accommodate the various grades of pump gas.

For the RS4, you could probably run it at 13.2:1, but on pump gas you would likely get detonation due to the heat generated at those AFRs, hence timing would need to be reduced. Put in C16 and it would probably run fine at 13.2:1.

mixja wrote: In theory the optimal A/F ratio for maximum torque is 13.2:1.

This is the air ratio that provides the most complete burn - any richer AFR and there will be unburnt fuel, as there aren't enough oxygen molecules.

For NA cars, they are normally run at 12.5-13:1 AFRs - leaner AFRs generate more heat which can cause detonation and excessive engine wear. Thus most road cars run a little bit richer for the sake of engine reliability and also to accommodate the various grades of pump gas.

For the RS4, you could probably run it at 13.2:1, but on pump gas you would likely get detonation due to the heat generated at those AFRs, hence timing would need to be reduced. Put in C16 and it would probably run fine at 13.2:1.

Mixja, what's C16? thx.

RS4Molar wrote:

Ozrs4 wrote:RS4Molar, what's the optimal A/F ratio for our RS4? I read 12.8-13.2:1 will give best / safe power for n/a engines but some say 14.7:1 is ideal. What's your take?

Fitting a miltek without custom map is questionable due to the increase in the A/F ratio,
Try pricing the RS4 FSI motor ?? WOW

Does this apply to a cat back system as well? I've only just put one on as i was under the impression it would have no adverse effect. If this isnt the case i may well take it off.

pippyrips wrote:

RS4Molar wrote:

Ozrs4 wrote:RS4Molar, what's the optimal A/F ratio for our RS4? I read 12.8-13.2:1 will give best / safe power for n/a engines but some say 14.7:1 is ideal. What's your take?

Fitting a miltek without custom map is questionable due to the increase in the A/F ratio,
Try pricing the RS4 FSI motor ?? WOW

Does this apply to a cat back system as well? I've only just put one on as i was under the impression it would have no adverse effect. If this isnt the case i may well take it off.

I believe cat back only shouldn't be an issue.

Ozrs4 wrote:

pippyrips wrote:

RS4Molar wrote:
Fitting a miltek without custom map is questionable due to the increase in the A/F ratio,
Try pricing the RS4 FSI motor ?? WOW

Does this apply to a cat back system as well? I've only just put one on as i was under the impression it would have no adverse effect. If this isnt the case i may well take it off.

I believe cat back only shouldn't be an issue.

Good news

pippyrips wrote:

RS4Molar wrote:

Ozrs4 wrote:RS4Molar, what's the optimal A/F ratio for our RS4? I read 12.8-13.2:1 will give best / safe power for n/a engines but some say 14.7:1 is ideal. What's your take?

Fitting a miltek without custom map is questionable due to the increase in the A/F ratio,
Try pricing the RS4 FSI motor ?? WOW

Does this apply to a cat back system as well? I've only just put one on as i was under the impression it would have no adverse effect. If this isnt the case i may well take it off.

cat back is good,no probsl

Ozrs4 wrote:
Mixja, what's C16? thx.

C16 is a leaded race gas made by VP Racing rated at 117RON...

On my old Subaru STI Spec C I made 270KW ATW on 98RON and 303KW ATW on C16, which shows you the difference.

It also costs around 2-3 quid per liter...