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With the MBC set just below fuel cut off (which seems to be a rather fuzzy line at times based on the temperature and humidity) I have never experienced the dreaded "honking" sound others have described, maybe something else is worn out. Many have purchased a forged BPV (?) to eliminate this sound. What gives?
-The honking occurs when increasing the throttle from closed to open and the manifold pressure is near 0. The turbo is starting to blow, but the BPV has been open and the flow is bypassing and pressure is not being built up. At this point the throttle position is not very open and there is not much boost potential anyways. So the valve is starting to close I expect and flow is getting cut off. The air flow causes an occilation of the valve. But I am not convinced that I fuly understand why this is happening. A piston type valve just does not do it.
Also, what is any benefit to change out BOV as some have done? Now that my 9-3 is gone, I get to play with my old 94SE again.
-If it makes noises that you don't like, a piston type BPV just make things less of a PIA. In theory, a better BPV or a BOV will allow the turbo to loose less speed during shifts. This has an advantage for someone you is trying to get max acceleration across shifts. For sane street driving I don't know if it makes a difference. But if someone installed a better valve and is running a stock BPC valve, their turbo boost lag will be rather crappy anyways, and getting more performance with a new valve is sort of like rearranging the deck chairs in the Titanic.
Also, thought you had one of these (MBC) on your car, get tired of it? , or come up with something better like an ECU or some late night invention from your kitchen you have not let us in on yet.
-Have had a MBC since early 2000. The first reported here for the NG's. I had a Hallman then tried a HP TurboXS unit. I can't see that one has merit over the other, and from what I have read about the Dawes units, I don't expect that there is a difference there either. No ECU mode here. I did get a 25mm blowoff valve with a hose adapter, converted to a pneumo-adjustable pop-off valve by attaching a pressure regulator to the units hose barb. So it seems to help with venting boost pressure overshoots, but I don't know how it will work in cold weather when the rate if rise of boost can be very fast. I may put a softer spring in it and then increase the pressure regulator setting. This would increase the valves sensitivity or 'gain'. So it is still an experiment, not a solution, and not really cheap. I do have the MBC pressurized from the TB, whick maintains boost well at high rpm's but is worse case for pressure overshoots caused by the time day issues of this connection. I do feel that the Viggen IC has helped reduce that tendancy a bit.
So the pop-off valve is experimental and will provide some insight into things, but it is costly and required fabrication of a hose adapter manifold. I am getting ready to introduce a mild MAP sensor resistor hack what will make the ECU think that it is at a high altitude. It will get pissed at first but will soon re-adapt to things via the O2 sensor signals. I only need to shift the MAP sensor by a relatively small amount to get head room for pressure overshoots. If I was then to increase the MBC setting to just under that, then the same problems would come back, and that is not a technical problems, but human factors. Now others have done this mod with stock and modified ECUs. This requires quite a large change in the MAP signal and the fuel pressure can become insufficient to allow the ECU to adapt. That is large MAP signal change. But with the MBC, all we need is a minor shift, which will move the fuel cut pressure by a modest amount. Withthe large MAP signal changes one has to shift the signal to get the boost target pressure moved, which also puts the fuel cut pressure way up. So I feel that a modest MAP signal hack will be a minor issue for the ECU to deal with. So there are two projects.
The pressure overshoots occur because of the time delay efect, but also because the actuator cannot open the wasteage fast enough. This is a rate of rise, or slew rate issue and the control mechanism needs more gain, or needs to overwise be faster. Air pressure can be supplied and there are flow time constants. I drilled out a restriction orifice in the HP TurboXS to address that. But the actuator spring is very stiff and a lot of energy is required to open the wastegate. If the spring was softer, it would open faster. The actuator adjustment could be backed off, trick do do over and over again experimentally, or a pressure regulator can be fed from the compressor hose barb and the output pressure applied to the wastegate to provide an advance pressure loading to make the valve ready to move. But as soon as the MBC starts to flow, the pressure from the MBC will exceed the regulator st point and the regulator flow will stop. The regulator would have to be of the type that is not a down reducing type that will vent overpressure. So once the MBC gets going, the pressure regulator then will contibute no energy to the displacement of the actuators spring, so this will not do anything to solve the energy time constant. So a softer mechanical actuator adjustment or a spring to off set the actuator could help. So as you can see this gets rather complex, but some of the issues can be explored and evaluated as mind experiments.
Note that with the ball spring MBC, the MBC is the basic regulator and that the actuator will have minor regulation role, it not like stock. With the MBC the actuator settings can be changed to some extent with no adverse results. This is not true with a bleeder valve MBC or the bleeder like BPC valve.
posted by 65.68.10...
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