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Discussion Starter · #1 ·
Putting potential check engine lights or limp mode problems aside and getting into the true technical aspects of T7 cars, what is the real reason it is not a good idea to run a MBC on a T7 car? I am not starting this thread in efforts to "find an excuse" for using a MBC so please avoid simple comments such as "why would you want to use such a primitive device with such a sophisticated system". I agree with this but I am starting this thread to better understand the way the system functions. Please use specific technical details.

Does a MBC affect the a/f ratio at WOT? Does the turbo work harder than necessary because the MTB is trying to keep air out, increasing turbo back pressure, increasing intake temps and decreasing VE? Why would an aftermarket ECU allow a higher boost setting with an MBC? Is it simply a greater tolerance for higher boost pressures and more willingness to supply extra fuel? How does the BPC determine appropriate boost levels and what is the risk involved with tricking it using a boost controller? I am hoping for this discussion to be more about Trionic 7 than just the boost controller.
 

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Even though I know a lot of people use MBC's fairly reliably on SOME cars, they really shouldn't be used on Saabs. The simplest explanation is that Saabs follow the "knock curve" VERY closely and are constantly adjusting the ignition/boost/fuel to deal with it. Putting an MBC on the car makes it unable to do its job well.

To quote Corky Bell:

"We are generally agreed that accountants and lawyers determine tolerable boost pressures, but suppose for a moment that the engineer with a graduate degree in thermodynamics was actually the man responsible. Then we are in trouble if we arbitrarily turn up the boost. Take your pick. Chances and logic suggest that we should take some heat out before we turn up the boost."

It's not worth it on a stock car. For a modified Saab there are lots of ways to make power that don't even involve turning up the boost, and upgraded ECU's aren't nearly as expensive as a new engine.

But if you just want to know the quick and dirty:

T7 boosts to a finite air-mass/combustion/RPM. It is measured by the Mass AirFlow sensor (though corrected during quick load changes by the MAP sensor) and because the throttle is under ECU control if you over shoot the expecte value it will throw a code at you, and try to bring it back down.

But with some race gas as a safety precaution I tried once for a few mimnutes. (Had one sitting around.) It can be made to raise the boost. However, I saw no additional accelleration. The boost was up, but the car just didn't go any faster. Presumably it was counteracting my actions some other way.

I was also careful not to do this more than once or twice, and like I said, was running on Unleaded race fuel just in case. No problems as a result, though I did find out where the fuel cut was. (1.5 bar or so, if you must know. )



Dubbya~
 

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Discussion Starter · #3 ·
After reading my adaptation post on the other thread again, I think I know what it is.

With an MBC, the throttle control and turbo control lose balance with each other. This would explain why you saw higher boost with no improvement in acceleration. The throttle was open to a certain angle expecting a certain boost but since the BPC was no longer managing boost, the MBC was causing the turbo to supply a higher level of boost that could not be accomodated by the current throttle angle. The turbo is basically trying to force charged air through the MTB which was not opened to the correct angle.

This would increase turbo back pressure and intake temps significantly as well as forcing the turbo to work harder than necessary.
 
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