At this stage i'm fitting a MBC and thought it might be good to add some information which directly relates to tuning in general and the drawbacks of MBCs. It's not mentioned/summarised well in the searches I've done specifically relating to MX5s and it may also explain some of the issues had in terms of boost spiking/fluctuation when you are at the same stage as me (and why you needed to get rid of the stock boost control solenoid).
The best article I have found is:
http://blog.perrinperformance.com/boost ... explained/ and
http://blogperrinperformance.com/boost- ... ed-part-2/
In short, the boost solenoid does not operate in a simple fashion ie long time spans of open/closed. It is part of the wider control rationale programmed into the stock ECU and is likely quite comprehensive/complex. It operates in conjunction with other systems (fuel, ignition etc) to maintain driveability and boost levels.
The main drawback of an MBC is that it stuffs up some of the control loop functions - ie Tune details including fuel mapping, gear/speed/pedal position ('3D') based boost mapping, idle speed mapping....
As an example, your aftermarket ECU (and the stock ECU) will modify boost delivery based on ambient temperatures/humidity, throttle position etc to deliver a consistent and desirable amount of power and drivability. As mentioned in the article you don't necessarily want full boost (power spike) to arrive at mid throttle position in a sudden fashion - it'll behave like an RS500 escort and be 'unsettling' to drive.
Putting in an MBC is likely to cause issues such as this and have an inherent inability to cater to situational factors so you'll have different boost levels on different days/situations.
Those 'tune details' mentioned are where you sort the wheat from the chaff in terms of good and bad tuners. Note that Mazda engineers are unlikely to be stupid and have disproportionate resources to be able to fine tune these things which may go some way to explaining why aftermarket tuners struggle to replicate drivability etc.
Note; this is also why you have to take the solenoid out of the equation when you add the MBC. The MBC doesn't simply add a couple of PSI to the wastegate actuator pressure when you have the solenoid in place. It will in an initial short spike, but then the solenoid (etc) will try to compensate for the boost spike and bring it back into check (by the ECU increasing it's frequency signal).
I do wonder if you could curb the feed signal to the ECU by 3 PSI or whatever and match it to a 3 PSI MBC increase to the wastegate and increase boost with nominally factory driveability..... but I digress.
Thought i'd share a good article. Basically, the MBC/EBC is a bandaid solution for a spike in power and is probably not the best for drivability. It is cheap fun, but certainly not a substitute for an ECU upgrade/re-tune.
