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Re: Base Boost setup needed with MBC
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Posted by Dean (more from Dean) on Fri, 2 Aug 2002 09:34:31 Share Post by Email
In Reply to: Base Boost setup needed with MBC, tom_k, Thu, 1 Aug 2002 09:06:13
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There is no need to adjust the base boost with the MBC and it is not done. However, I have long speculated that a lower base boost setting would allow the MBC to open the wastegate faster once the MBC starts to flow. So I expect that the valves slew rate would increase and that one could reduce the pressure overshoot that one gets with a MBC. So lower base boost might have some advantages, but it might take a a lot of fiddling to verify. I suggest that you use a file to mark the threads and adjuster nut to be able to return the original adjustement easily if needed.

If you pressurize the MBC from the intake manifold, then you will get less boost droop at higher rpm's. But that will increase the time lag and will increase the pressure overshoots. The best compomise might be to tap pressure just after the IC, as to keep the distances shorter. I have a tap at the throttle body and have a pop-off valve there as well to vent overpressure. The pop-off valve is a small 25mm blowoff valve and a pressure regualor hooked to its hose barb allows for easy adjustment with the regulators knob.

The issues with pressure overshoots may come back and bite you with the start of cold weather. The rate of rise of boost is much stronger and the dynamics change.

With the C900's, I gather that boost dropping at higher pressures has long been considered good practice or survival skills, or perhaps an artifact of the APC system. So you might want to keep some of the boost droop.

95SET, 143000 miles, MBC and intake mods, exhaust etc.

MBC since early 2000 (first ng900 MBC posting)

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