VerrÜckt

For the past few months, I've been running a Piggie MBC only for boost control and have had the N75 disconnected. This works fairly well and only causes hiccups under a few conditions. This setup will be adequate for most people. The advantage is that the boost response is basically digital. This provides the absolute quickest spoolup and wastegate behavior. The wastegates will stay shut until the desired boost level is met and "should" plateau from there.

The downside is that part throttle (0-20%) response can become too sensitive...especially at higher RPM's. The problem is that at high engine speeds, there is enough exhaust gas energy on tap to spool the turbos rather quickly. This means you can get high levels of boost at relatively low throttle inputs. This can lead to a "surging" behavior which I was never able to completely tune out.

Once the car arrived in Europe, my driving style changed. There are many situations where I will be cruising along at speeds of 120-150 mph. At these speeds, it is very difficult to maintain a steady velocity as the boost level fluctuates very quickly with the slightest change of throttle input. As I stated before, there is enough exhaust gas energy to spool the turbos even at very little throttle. This behavior became unacceptable to me.

So today, I hooked the N75 back up in parallel with the MBC. So far, I've left the bleedhole open. It works well, but I may experiment with the bleedhole down the road. Now my part throttle behavior has greatly improved. The jumpiness and surging is gone. Right now I have it set to spike to about 22 psi and taper to about 20 psi. This is a good daily driving setup.

Contrary to what some others have suggested, this is still a very "safe" setup. With the N75 back in place, the ECU has control over the boost again. Even without the N75 in, the ECU can still easily put the car into Limp Mode if needed. The TCD only limits the ECU from reading boosts over 12 or so psi. Because limp mode is about 6 psi, the ECU can read boost in this region and accurately regulate it. I ran some tests WITHOUT the N75, and I could put the car into limp mode without issue. The ECU will release boost through the N249 valve or cut throttle (even with TCD, the ECU can cut throttle for limp mode).

Now with the N75 back in place (and an MBC to prevent overboosting), I am very confident with the "safety" level of this setup.

sj6sp

Are you using K04 specific programming to keep boost up to 20 psi at higher rpms?

-M3=Lunch-

cuz i experiance that part throttle thing as well. It gets annoying, and if you drive a stock s4, then drive my car, you wonder what that is. I am used to it now so it doesnt bother me that much, but would like to get rid of it. Could i set it up where i could spike at 17psi still have the n75 hooked up? let me know, you seem to know your ****. thanks

VerrÜckt

the throttle cut defender will allow the N75 to increase the duty cycle above what it would with K03's. Both Andy (Quad D) and I have no problems getting high boost out of the K03 software.

VerrÜckt

This diagram should be self explanatory. It's about a 10 minute job. I left the MBC how it was set before (about 22 psi) and it needed no tweaking. There is a little bit of software induced taper, but it's not bad at all. You should have no problems getting 17 psi out of it if you want. (Thanks to Matt and Dave for the pictures.)

<img src="http://www.nogaroblue.com/S4/MBCdiagram.jpg">

<img src="http://www.nogaroblue.com/S4/N75mbcparallel.jpg">

PSmalls::DoggieStyle

Good stuff, Chris. That is exactly how I'm set up right now and it'll stay that way

VerrÜckt

I don't know if you remember (you were there), but we tried the N75 and MBC in parallel with my Stage III- setup. After some quick testing by Mike's parent's house, I was getting crazy taper down to like 1 bar so we quickly ditched the N75. I didn't think about trying it again until after Andy got his working. I have no idea why this happened.

Mike2kS4

I'm also throwing in a precision bleed valve to help eliminate the taper. I didn't have any notable boost spikes and such with no N75 and the MBC and bleed valve in series (used for anti-spike). Nevertheless, I'd like to reintegrate the N75 for your other previously-mentioned reasons.

VerrÜckt

FWIW, I'm not getting much more (1psi at most) taper than without the N75. I figure a little taper won't hurt me as I'm spending so much time at high RPM/high boost situations.

What do you think about plugging the bleedhole Mike. Have you tried it?

Mike2kS4

Got a bit more taper and a 1.5# less peak boost with the hole closed. Seems that the bleed affects the actuation pressure of the WG's. The precision bleed valve is going to increase this effect, hopefully inducing less taper and a more stable boost curve.

Oh, It's nice to see what a setup very similar to mine does on the 'bahn. Got any more videos?