http://forums.audiworld.com/s4/msgs/2340924.phtml

http://forums.audiworld.com/s4/msgs/2249278.phtml

Here is the setup that I have:

I also added a shut-off valve on the green side of the MBC.

If I close the valve, then I just have the left half of the diagram (the standard operation: N75-only). If I open the valve, I have the parallel operation.

In parallel, the N75 still controls the the boost level *below* the MBC setting. The MBC is used to set a maximum level to which boost can not exceed. The MBC in this config does not raise boost, it only limits it. I set this at about 1psi above the programmed boost curve peak. This way, if boost ever goes above that desired value (ie spike due to quick hardware response; quicker than N75 can adjust for), the MBC will bleed it off.

Here's a diagram that shows some of the various configs, specifically parallel for the sake of this post (w/MBC set at 17psi):


Finally, here's what I saw pre and post MBC usage for a drag-style run where boost is spiking on hard shifts:

Note the boost spike (above say 19-20psi) at the begining of each shift. This causes the N75 to pull back boost, and thus the large undershoot after the initial overshoot:

With the MBC in parallel set around 20psi, this got rid of the overshoot and the subsequent undershoot:




Oh, and to further illustrate, here's what you get if you lower the MBC down further (ie 13psi):


Hope that helped - ask away

Mark P


Dyno: 469wHP / 451wTQ
93 oct - best 60':1.734 | best 1/4ET:12.239 | best 1/4mph:116.58
100+oct - best 60':1.698 | best 1/4ET:11.762 | best 1/4mph:121.72



ECUx template version 11.16 (3/12/07)
AWD Dynos by State - Race Gas in New England - B5S4 100k+ mile post

Current:
'02 Audi S4 Avant, Imola Yellow, APR Stg3 - imolavant website - 180k miles
'01.5 VW Golf 4dr 1.8T, Satin Silver, GT28R, EPL-tuned - 14.8 | 101mph - 195k miles
Previous:
'00 Audi A4 1.8T, Silver - Quattrobonium website