Since there seem to be a lot of questions on how N75 vs MBC vs Both affect boost, I drew this up...
 

Below you see 3 boost curves. First we start with just a hypothetical chip. The next two show two different ways of adding an MBC to the system and how they affect this chip's boost curve.

The <b><font color="red">N75 Only</font></b> shows a hypothetical boost curve with a chip and your standard N75 valve in place.

The <b><font color="green">N75 &amp; MBC in Parallel</font></b> shows the MBC set at ~17psi, such that any boost above that will get bled off by the MBC, but the N75 will still control the boost curve below the MBC's set limit.
- Here the MBC functions to remove spikes.
- In this configuration, the MBC is only capable of reducing boost lower than what would occur without it.

The <b><font color="blue">MBC Only</font></b> shows the MBC set at ~17psi, such that the MBC has complete control over the boost.
- Here the MBC functions to control the boost.
- In this configuration, the MBC has full control, and can either raise or lower boost, and thus caution is suggested when doing this. One should have knowledge of fueling limitations, turbo capabilities, etc.


<img src="http://ctny.audiworld.com/mark/s4/misc_data/mbc_vs_n75_boost.gif">

Here's Matt's drawing of the <b><font color="green">N75 &amp; MBC in Parallel</font></b>:

<img src="http://ctny.audiworld.com/mark/s4/misc_data/mbc_parallel.jpg">

PS - It should be noted that with the MBC in place (Only. Parallel too?), the initial ramp of boost may occur quicker as well.

PPS - Having a MBC Only set-up, if you set the boost to a value greater than the turbo can physically achieve, the resulting curve will merely be the max boost (for the given RPM range) that the turbo can sustain. An example of this is that the S4's K03's can not sustain the 17psi at high RPMs as illustrated above. In this case, once the turbos can no longer sustain the 17psi, the boost curve would begin to taper and follow the maximum achievable boost.

That about sums it up... This might be a good place also...
 

for the theory behind the PEB Throttle Cut Defender... Since it's burried deep in the other threads...

so should we run MBC's?

Add a few plumbing diagrams for each setup and make it a tech
 

article. With some advice on watching A/F and EGT's via VAGCOM blocks.

Nice work.

Should we assume w/K04's and K03 software, we are going to have fueling issues up high on the MBC setup only ? Will a 5 bar FPR provide a reasonably respectable A/F ratio up high ?

DAP's setup is interesting because he runs them in parellel but by freak of nature his actual is a bit higher up high which is very nice as long as it's controlled.

In parallel with the N75, sure wouldn't hurt it seems....
 

Although it would be one more thing for your dealership to yell at you about.

Great info - a side note to others about the blue line - it can still taper down
 

for example with stock K03s, they can't sustain over 1 bar of boost all the way to redline. With the MBC set at 17psi, basically the wastegate completely shuts as you approach redline, and boost will taper down, as the turbos are pushing out the max CFM they can but it isn't enough to sustain the desired boost pressure at those RPMs. Makes a good case to run the MBC in parallel with the N75 in this case. With K04s at 17psi this isn't an issue though - all depends on where you set your MBC and what your turbos can do.

Very nice simple explanation! Why didn't I think of that!?

Exactly! Here is an example of a k03 car running MBC only and tapering....
 

<center><img src="http://pictureposter.audiworld.com/51367/actual_vs_requested-57_d.jpg"></center><p>

So which is better? In you opinion, of course ;)

Totally depends on your setup and what you want to accomplish.
 

If you want to eliminate spikes, but still want boost to taper like normal--&gt;MBC in parallel

If you want to run more boost than requested up high--&gt;MBC alone

If you are happy with your car's boost delivery(different with the different chips)--&gt;