This is a summary of the work performed in developing a Stage 1 tune for my S4. I will try to incorporate the reasons for the changes made, many of which were suggested by others.
The XDF (definition) and BIN (ECU binary) files used as the basis to create this tune are located in this directory. You can use these files to develop your own tune. There is also a copy of my final Stage 1 BIN in the directory.
Stage 1 in my case is in reference to using stock fuel injectors, a stock MAF housing (initially) with Hitachi sensor, and stock K03 turbochargers. A number of other components are not stock, but for the sake of making a tune I feel the car is close enough.
The first thing I did was raise the rev limiter, NMAX, to 7000 rpm and NMAXF (RPM DTC limit) to 300 rpm over NMAX:
The rev limiter was raised to get additional rpm’s out of the engine before the throttle cut kicks in, which is set with the NMAXF parameter.
Next I adjusted the Pedal Position for Desired AFR (LAMFA Axis). This is one of three ways to adjust fueling, the others are with BTS or KR.
When stock, the values are 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 and they are changed in the snapshot below to 50, 60, 70, 80, 90, 97.5
The desired AFR is altered in the last two columns (90, 97.499) from 3000 rpm and above, enriching the mixture from 14.7 to 12.5.
Next up was modifying LDRXN to alter the boost profile, raising it slightly from the stock profile.
Then I made some timing adjustments. On the left side are the original tables and on the right are the current values. For KFZW2 I lowered the values slightly to try and eliminate the ignition retard that was occurring around 2000-2500 rpm. For KFZW I raised some of the timing values to try and have a smoother transition around 3500-4000.
And lastly, a comparison of this tune versus the baseline stock file and a pull from a different APR chipped S4.
This linked page shows some data collected from the car while using this tune. Note that boost peaks around 13 psi and tapers down to around 10 psi. Fueling stays below 95% injector duty cycle, and there is no ignition timing being pulled. All in all it produces a decent bump in performance over stock while still remaining ‘mild’ compared to a typical Stage 1 tune.