Anyone know why MarkP used a double control on his MBC??

[Hal_TT::CBP]

[Mark P]

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

[LI-S4]

OK...I won't use that excuse anymore..but I am worried about you being able to quote those threads so quickly...

You must have a sick Bookmarks folder..the Search function is no where that good!!

Thanks Mark!!

[Mark P]

the search rocks[list][*]http://search.audiworld.com/index.ht...6&db=2006-01-0

[LI-S4]

[LI-S4]

I understand the "in parallel" concept much better.

Essentially, the ball valve gives you the best of both set-ups...a non-MBC set-up and an "in parallel" setup.

Do the MBC, ball valve and waste gate all see full boost? I'd assume yes... just curious what level of boost the ball valve has to sustain.

Sans the ball valve...is there any reason NOT to run the MBC in parallel as you have it..at a setting just above maximum boost level for safety sake? I am asking this in light of your email..you weren't pre the MBC fot the 3- setup. Did I interpret that correctly??

[Mark P]

I'm all for the MBC in parallel set just above the N75 to avoid any spikes.

I was just curious why you wanted to use one at the time as there's no real reason to lower the k03-chip-boost when running with K04s, and I wouldn't use an MBC-only setup to raise boost if you're still running S4 injectors - they're already maxed out with the k03 boost levels.

I think sometimes people want stuff just because others are using it and not fully know why the want it. I just wanted to make sure you were informed and had good reason

[LI-S4]

I understand you thoughts.. Aside from the X1 intake, which just gives me wood, I haven't done anything "just because". I have a fact finder personality and will research thing to death..so the MBC question I originally posed was what seemed to be working...

Now, are you saying k04 + k03 93 code = skip MBC all together? And, anything more such as stage 3 code insert MBC?

And that said, are there any negative effects of using the MBC "just for safety sake" in the first example..Ko4 + k03 software?? Like Jekyll?

[Mark P]

In general, I would say that people don't need an MBC in parallel unless they find out then need one. That is, I wouldn't put it on an initial shopping list, but rather only look for one if I felt I had some overshoot/spike condition.

That said, because an MBC in parallel is used as a safety type (boost limiting) setup (as opposed to MBC-only which could creates more boost), there's not a downside to having it. The only issues I can think of would be:
- installing it properly
- setting it properly

Assuming it's installed properly (connected to the right hoses, without pinching any of them or causing leaks), then setting it can be done with just a boost gauge. Logging can certainly help, but you can get it pretty well dialed in with just a boost gauge. This somewhat assumes you already know the desired boost curve of the chip you're using, however not completely necessary.

What I would do is find the RPM in a specific gear (3rd or 4th usually works) that causes the highest boost spike when you go from coasting to WOT. I've found this near 4000 RPMs in my current setup. So if I'm at 4000 in say 4th and go WOT, I'll create the highest boost spike, which will then likely cause and undershoot. So imagine (example) I normally see 30psi (this is where it's handy to know what the software is trying to achieve by looking at prior logs), but when I do this test, it gets up to 32psi, and then drops to 27psi right after, before heading back to 30psi and then continuing the normal curve. Now I can slowly change the setting on the MBC to get it so it only overshoots to say 31 psi and doesn't undershoot below 30psi following that. Sometimes it's easiest to make these changes by starting at low psi MBC setting (ie start at 10psi and make 1/4 turns or whatever seems a good increment on your MBC, slowly getting to that desired result). If you start at the high end and work down, it's harder to tell if you're doing anything (ie if it's set up at 40psi, and you're changing it 1psi at a time, you won't see any results for the first 9 test)

Hope that makes sense. Jekyll just pulled in my driveway, so I gotta run ;-)

[LI-S4]

So a few questions...

The N75 is what would normally detect the boost level (or the boost level is communicated to the N75 from some other sensor) and the N75 in turn communicates to the wastegate to bleed off boost?

Does the boost spike signal the N75 to bleed off boost resulting in the under shoot? And if you control the spike then in effect you prohibit (or discourage) the N75 from excessive bleeding which result in the undershoot?

You said " assumes you already know the desired boost curve of the chip". Since logs are subject to a gojillion variables inherent in each car that logs..even given the same chip manufacturer, where do you get the accurate baseline information for a particular chip? For instance, if I want to use a set of data logs for the APR 93 code to set an MBC, or to see what level of boost spike/undershoot, how do I know it is what the manufacturer intended as opposed to what someone elses car is doing with the code??