Timmay

No way! Don't change the color whatever you do. White is the best!

RI A6

Vijay, you're correct.Max power occurs between 13.23 and 12.5, where the power curve has a fairly flat spot in the hump. The ECU should adjust for a 14.7 ratio for all torque settings below WOT. Then at WOT, a fixed enrichment multiplier is applied. If an air fuel ratio of 12.5:1 represents a good adaptation, then the enrichment "kicker" that Audi has programmed is around 17-18% additional fuel. This would mean that adaptation was not yet complete for the Miltek exhaust (with a 13.23 ratio) and there is room for the engine to develop even more power, as adaptation drives WOT into a more enriched ratio, more cylinder wall cooling, and a higher spark advance. This would mean that the current plots are actually on the low side, and we should expect even better gains as the ECU fully adjusts.

RI A6

and having done a bit of reading, I actually think that the ECU was adapted correctly with the stock exhaust, since it is on the rich side of max power air/fuel ratio. And I think that with the Miltek exhaust the ECU has a while to go before adaptation is complete, as it is on the lean side max power air/fuel ratio range. Both the lean side and the rich side will put out around the same power, if detonation does not occur. I think that Audi want's to see A/F ratios around 12.5, to maximize cooling and decrease the possibility of detonation. This ratio will allow for a bit more spark advance and a slight bit more power.

My conclusion is that the comparison is valid, and that the Miltek exhaust will actually produce a little more HP when the ECU is fully adapted. Thank you for supplying the data and the additional plots for comparison.

Vijay

As I said, most of my tuning expereince has been with forced induction cars. First was my 1993 supercharged Honda Prelude, then my 2000 S4, then my supercharged Nissan 350Z roadster. A/F over 13 at WOT under boost is trouble.

Again, I just didn't know if A/F in 13's with 12.5 compression is safe. Interesting that you should mention about the stoichiometric A/F at condtions other than WOT, since everyone's dyno plots for power comparison are at WOT!!!!! As such we often don't think about other driving condition that occur 99% of the time.

With regards to optimal power, I have heard two schools of thought. One is that slightly lean will insure combustion of all the fuel. On the other hand, slightly rich does afford the cooling effect you mentioned.

The real secret here is in knowing what the "normal" A/F parameters are for the RS4's ECU at WOT. If for instance, it will accept an A/F of 13, it will not correct to below that. The question also becomes, will it necessarily enrichen the mixture, retard timing, or a combination of both when it senses the higher A/F ratio. If the sole response is to enrichen the mixture, you are right, there is more power to be had. My only concern was that part of the response would be to retard timing, and not just add more fuel. In this scenario, the 20hp power gain may not hold up over time. In either case, however, there is still a decent gain to be had. So it might be 12hp in the worst case scenario.

You sound like you know a fair bit about cars. You must admit, this result is almost too good to be true on a car like this. What possessed Audi, on a "no compromise" model to leave this much power untapped with such a simple mod. I am surprised to say the least.

Vijay

Craig Lieberman went over the design issues on the stock exhaust that restrict power. All valid points to me. It's a few threads down, so I thought I would just repost the link URL.<ul><li><a href="https://forums.audiworld.com/rs4b7/msgs/29093.phtml">https://forums.audiworld.com/rs4b7/msgs/29093.phtml</a</li></ul>