i have a 92 9k t and i had a mbc with a ball spring in it and i got ot to boost to 15 constant just fine and spikes to about 20 i suggest a chip upgrade because i ran into problems with my mbc, i am speaking from a broke college kid standpoint so if your broke do it but if not go with the sqr upgrade cause youll boost to about 22 psi and about 17 constant, an mbc is just quick power and its really no that safe for your motor
It works well between solenoid and actuator (ran with mine like that for a while). However, to do this you will need to block the vent hole on the body of the MBC. JB weld, superglue or a dollop of Blu-tak should do the job.
I run 1.2 bar (17.4 psi) on my ex-LPT with a bleed valve MBC. The difference is that I have a 2.5 bar FPR. Not sure why, but it seems to work. However I would say that since you have an APC you can get the same performance by tweaking the pots while retaining knock protection and dispense with the MBC altogether. I always say this - its like a disclaimer. Mine works OK but I'd hate someone to melt a piston with an MBC by turning it up too far.
Is the MBC you've got a bleed valve or a variable pressure opening valve (AKA Dawes)?
If it's a bleed, then yes, simply putting it in line netween the solenoid and the actuator will work; you just need to set the right amount of bleed. You may also need to nobble the overboost protection switch. My T16s ran like this for a while (I now have a tweaked APC- more in a mo)
If it's a Dawes type device, then again it goes inline between the solenoid an actuator, but here I'd have to disagree with Julian about blocking up the hole. This is needed to allow the actuator to close again, as when you come off boost the valve shuts and there is no longer anywhere for the pressure in the actuator to go, so the wastegate would stay open. This sort of device shouldn't significantly affect your peak boost as long as it's not set too high.
However as Jezzadee suggests, I'd opt for the tweaked/red box APC route. It works really nicely on mine; I get a nice intial hit o boost to get me going before it drops back to a more sustainable level- remember the I/C has limited capacity and heat saturation is a real problem. You won't notice it so much at this time of year, but you really will in the summer.
As for how much boost, I'm not sure there's a "right" answer, because of the limted IC capacity I mentioned before. In winter it can be higher than in summer.