MBC & APC [Archive] - SaabCentral Forums


Marko A
28-04-02, 02:16 AM
There has been a lot of discussion about MBC. My experience is that there´s not much use for a such with APC still functioning.
Trouble is that it messes up APC´s way to control boost as fast as it should, and also the boost will be raised only below 4000 rpm, after that APC will be able to take the lead again and starts to lower boost down to pre-set value (F). You can get similar results just by turning P higher. Causing boost overshoots.
So if you still like to use MBC, I suggest you to have fuelcut switch working, just for the safety.


28-04-02, 03:04 AM
I found this as well when using an MBC with an APC still running in active mode, above about 4500rpm the boost started swinging wildly about its desired target.

An MBC can be used if the APC has been modified with the passive mod. As there is no active boost control the wild swinging doesn't occur. The MBC then allows faster spoolup with out the huge peaks that a high P pot setting produces.

28-04-02, 03:06 AM
what do you mean by the "passive mod"

28-04-02, 03:40 AM
This is the one that used to be listed on Tweakers site, in which only the P pot is used to control boost. The APC is effectively used as a bleed valve which can react to knock. Unfortunately, to obtain a decent amount of boost at the top end large peaks are experienced at lower rpms. I think Alanb's car peaks at about 1.75bar but holds about 1.1-1.2ish.

28-04-02, 05:29 AM
Alex is right. Passive mod gives me too much boost at, say, 3000rpm which is good for 3rd gear wheelspins but not much else.