ok, here's my latest boost control setup:
essentially a cockpit-adjustable ball and spring mbc (hallman evo rx) in parallel with a GM "3 port" solenoid (in interruption-based boost control) which is controlled by a UTEC.
the UTEC boost map ends on 500 in the 100% tps column, and ramps down to 150 in the 60% column.
this setup allows for a couple of key points:
a) cockpit adjustability of max boost via a knob.
this is one aspect that i can't stress enough. it is by far the best way to account for atmospheric changes in boost levels as well as driving style changes (highway vs. city, wife driving, valet, etc). two seconds in the morning and it's set for the rest of the day.
b) throttle dependant boost levels.
obviously, this is key to avoiding stoichiometric boost, and also allows a ramping up of manifold pressure so that boost is controlled at partial throttle, instead of coming on when it wants to (ie pure mbc). this improves drivability and prevents engine damage due to PTFB scenarios.
c) rpm dependant boost levels.
haven't tuned this in yet, but the capability is certainly there. i have a strong hunch that tuning in a bit of boost taper at the top end of the rpm range while using the oem turbo is a "good thing," since it will cause the wastegate to open, reducing EGBP without decreasing manifold pressure, since that's pretty much capped by the compressor anyway. less EGBP for no hit in MAP is without a doubt desirable from an efficiency standpoint.
d) rock solid boost response.
when i say rock solid, i mean ROCK SOLID. those who have never used a GOOD MBC will not really know what i'm talking about. what i am talking about is a boost level which will NEVER be exceeded, even for the briefest of spikes. there are simply no spikes whatsoever to speak of.
e) very quick boost increase.
when the throttle is floored, the UTEC's boost map is set to 500, essentially max boost. the GM solenoid closes, which prevents any pressure signal from going through it to reach the wastegate acutator. the ball and spring mbc is a very good MBC in that it provides a very tight seal, and essentially no air gets through that until the spring tension is exceeded by the pressure signal. bottom line: boost response just as fast as a good ball/spring MBC, which is as fast as it gets.
i'm going on day two with the setup and i could not be happier... pretty much every thing i could ask for is addressed. since the picture was taken (during the testing phase) i have shortened all boost/vac lines as much as possible, and secured the junctions with zip-ties. having short, non-obstructed lines is also critical to having rock-solid boost control.
any questions, ask away...
ps a tip of the hat to TMS for finding the very economical GM solenoid
, which enables us to use a direct interruption based boost control method. i had tried the method i just described once before, using the oem bleed based solenoid (along with its attendant restrictor), and it didn't work at all, so this was key to the success.