Ok guys... there are a couple major gaps in the info here.
First of all, the stock boost control solenoid raises boost by pulsing and bleeding boost before it gets to the wastegate actuator. So the solenoid needs to pulse in order to make boost, not the other way around like someone here stated. This is in essence a failsafe system so that if the solenoid goes bad (and they have been known to), you don't get uncontrollable boost that grenades the motor but rather just end up with wastegate spring pressure (~7psi).
This brings us to the issue of response. The stock solenoid can only bleed so much boost, so Subaru uses a series of restrictors to help it out and to try to smooth out the operation. These work ok for stock boost levels, but once you start running higher boost the restrictor size becomes inapropriate. This can certainly be tuned around, although some cars require a replacement restrictor to reach desired boost levels. Using the appropriate restrictor and propper tuning, you can get pretty much any boost curve you want... that's not really the issue here.
The real issue is when you you're just cruising along and decide to hammer on the gas. The ECU sees the TPS increase, then looks up the WDC for that RPM/TPS, ramps the solenoid to that duty cycle, looks at the boost level reached, then makes incremental adjustments to the duty cycle to try to hit the target boost for that RPM/TPS. This is how any closed loop system works. The issue is how quickly the ECU can actually ramp up the duty cycles while maintaining stable boost without spiking and how quickly it can make the small changes to reach target boost. Depending on setup, external conditions, and target boost levels, the system can hit boost pretty quickly and accurately or can take its sweet time about it and introduce a good deal of lag on quick throttle transitions. Remember that the ECU is having to respond to many different inputs to zero in on its target boost. Combine this with the inherent limitations of a bleeder type solenoid setup and you get the idea of why this system is not ideal for all out response.
One solution here is to replace the stock 2-port solenoid with a 3-port solenoid that can be set up in interrupt mode rather than as a bleeder. In this case you don't need to use any restrictors and instead of bleeding boost before it reaches the actuator, the solenoid actually interrupts the signal before the actuator sees any boost at all. In essence this type of setup acts much more like a ball and spring MBC and allows significantly quicker response than the stock bleeder solenoid. Response is, however, still limited by the fact that it is a closed loop system and the ECU must still ramp boost up at a controlled rate in order to reduce spiking, while also having to make small adjustments constantly to get closer to target boost. Granted, the response of this system is much more precise and much better overall than the stock setup, but there are still limitations. Even considering this, I recomend this setup to about 95% of my customers because its a great overall solution that comprimises a bit of response for a system that's fully controlled by the ECU and requires no user intervention.
This brings us to the remaining 5% of my customers who would benefit from the best possible response and aren't afraid of learning about something a bit different. For such people, like Cody and myself, I recomend using a good quality MBC such as the Hallman. I've tried many MBC's in the past and this unit has proven to be the best by far. Some older, low quality MBC's only used a ball and spring and resulted in significant spiking. There were also some units that only worked as a bleeder and resulted in inconsistant boost levels with varying conditions and less that ideal boost response. The newer units use a combination of the two systems. While the ball and spring is the main method of boost control, they also have a small bleeder hole to stabalize the boost curve. These units provide unmatched boost response with very consistant boost levels. While boost levels will in fact vary slightly based on temperature, these variances are very appropriate and very similar to what you would see while using the stock boost control system. These variances actually occure moreso from the efficiency of the IC rather than varying performance of the actual MBC. Remember that the MBC is referencing boost before the IC, so while it may be maintaining totally consistant boost levels pre-IC, if the TMIC is heatsoaked manifold pressure will appropriately be lower.
Now lets talk about boost tapering. No MBC I've ever seen runs X boost all the way to redline... atleast not on a stock turbo. The ball and spring is not a binary on/off system. It reacts much in the same way as your wastegate actuator spring. When it sees some boost, it opens a bit, when it sees more boost, it opens more allowing the actuator to see more boost. This results in the actuator seeing a fairly constant pressure variance under boost which results in an almost constant wastegate opening angle. As RPM's increase, the turbo runs less efficiently and requires a higher wastegate opening angle to run the same boost levels. Since the wastegate remains at a fairly constant angle, boost tapers off naturally with RPMs. This taper can be manipulated by the size of the bleeder, and of course will vary quite a bit with turbo size. In general, however, this taper is very appropriate and results in a boost curve that I would end up running using a solenoid for boost control.
The great thing about an MBC is that its an extremely simple mechanical pressure regulating system that does a much better job in most cases than the relatively complex closed loop solenoid system. The MBC also acts much more precisely and naturally than a solenoid which pulses air in small quick bursts. I've always been a fan of good MBC's for this reason.
Now lets talk about partial throttle high boost situations. While this is a real issue with a stock ECU, most tuners choose to reprogram the OL/CL switchover so that they can also run higher boost levels at low throttle angles. This makes the car feel torquier and more fun to drive. Cobb does this in their base maps and every tuner I know does the same thing in varying degrees. This means that the ECU is ready for the higher boost levels at part throttle conditions and runs the appropriately programmed open loop fueling levels to accomodate this. This was always the big issue with MBC's on Subarus and where many of the misconceptions originated. Before I could afford full blown tuning software, I actually designed a simple controller that would watch boost levels and force the ECU into open loop once a pre-set threshold was crossed. This allowed 02/03 WRX's to run MBC's on the stock ECU with no danger and smooth operation. I ran this setup on my own car for 60k miles and sold about 60 units without a single reported issue.
Phew... ok I covered a lot here and I hope I didn't miss anything. If I did miss something or didn't explain something well, please let me know and I'll try to explain more clearly.
Thanks
-- Ed
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