To simply explain how an MBC works, you must first understand how a wastegate works. In an extremely simple explanation, your wastegate is what essentially limits the amount of exhaust gas that your turbo sees. Wastegates have a spring rated at x amount of psi. When this psi is reached, the wastegate valve opens up, diverting some exhaust gas around, and not through the turbine. So lets say you have a 2psi spring(requires 2psi to compress spring/open up valve). Once the wastegate "sees" 2 psi, the valve will open up, diverting the excess exhaust gas around the turbine. With the excess gases traveling around the turbine, you're limiting the amount of power the compressor sees, and an end result is a limit to boost pressure that the compressor can provide.
Now, for an MBC explanation. I'm no expert, but basically, an MBC is a controlled "boost leak" if you will. Installed inline with the pressure side of the wastegate, it bleeds off x amount of pressure. So, lets say for the sake of simplicity, you open up your MBC and it is bleeding off 2psi of boost. Doing so increases the amount of boost that will be required to compress that 2psi spring in the wastegate to open up the valve and divert the excess exhaust gases. Since you're bleeding off that 2psi, the wastegate stays closed, allowing exhaust gas to continue flowing directly through the turbine. Turbo speed increases, compressor pumps out more boost. Since you're bleeding off 2psi from the pressure side, you do the math, the spring will now open up at 4psi of boost. And end result of this is more power. I hope that was a good enough explanation to give you an idea of how an MBC works.
Tuning fuel and timing alone will significantly increase the output of your vehicle. Adding an MBC to this and tuning the 3 together will further increase the output of your vehicle. In my own personal opinion here is what I would consider a Good, Better, and Best setup on the evo:
Good = Tuning fuel and timing, letting the ecu still map (control) the boost level, full aftermarket exhaust (increased flow, less restricted= more exhaust that can flow out at a higher velocity) a significant increase in hp/performance
Better = all of the above, while adding on an mbc, seeing a good power increase with the added boost...when properly tuned.
Best = Vishnu PROcede + full exhaust. The PROcede is near limitless in the tasks it can perform and is very user friendly. Not only that, it doesn't rape your ecu like a reflash, instead intercepting the outputs and giving its own "2 cents" if you will.
I would recommend against doing just an MBC alone because you're not compensating for fuel/timing. Now I'm no tuning guru, so I could be completely wrong. However, I figure if you are running too much boost, and the ecu has the fuel scheduled at a setting which is rich, you won't be getting enough air for efficient combustion due to the increased boost pressure. This being said, it is possible that the ecu would compensate by pulling fuel, which would increase the amount of air in the mix which could essentially cause a knock situation. Doing so, the ecu would then pull timing. The whole situation I just described could result in poor performance and possible damage to your engine. Although, I'm not positive if this last explanation is correct, so don't quote me on it..unless of course I'm right about it! haha
Anyways, hopefully you grasped the basics of how an MBC works and such.
