Several S4s using MBCs (in place of the N75) and bypassing N249s are still seeing tapering boost.
 

Can anyone provide thoughts on why this is happening? In theory, the MBC should hold the boost level at whatever it has been set to (and most Stage 3 cars are capable of holding boost at these levels).

-Dave Pramanik<ul><li><a href="https://forums.audiworld.com/s4/msgs/1653861.phtml">Long thread</a></li></ul>

has anyone bench tested the MBC in question?
 

i.e. hook it up to a pressure supply and verify its working as it should.

I *think* there might be multiple MBCs in question, but a bench test wouldn't be a bad idea.
 

Still it seems to work fine with respect to the initial set pressure... just tapers off slightly after that point.

-Dave Pramanik

Most likely it has to do with the fact that the turbo hits max spool
 

and then the boost drops. This is going to happen no matter what you set the boost till, since the turbo can no longer produce a constant level of pressure once it is running at full spool.

Thats the thing... Stage 3 S4s with the SAME turbos (K04s) can sustain 1.4 bar with no problems.

Well... 1st boost pressure and turbo capability aren't 1:1
 

if you think about a couple of things:
1. The turbo is rated to flow, not boost, a certain amount of air. For example the GT28r is a 32lb/min or 242g/sec turbo.
2. Boost pressure is the potential of velocity
3. As the RPMs increase the velocity of the airflow will be forced to accelerate.

So... if you take the above into consideration with a free flowing exhaust, such as one that doesnt have cats, your back pressure is minimized, thus you will FLOW more air up top.

If you are flowing more air there will be less of a static pressure build up in the intake manifold.

now that being said... the old "engine load" factor seems to come back into play
 

I'll talk to you more offline about this...
but I figured out my "tapering boost" issue, is a non-issue.

but what are they running downstream?

?
 

Not sure what you are asking, but Mark's car holds ~1.4+ bar to redline.

-Dave Pramanik

That's what I thought too, but others (including 1 Stage 3 car running manual boost control)
 

are seeing the same thing. Just thought it was odd.

-Dave Pramanik