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At some point the engine will be ingesting so much air that it will max out the reading on the Mass Air Flow (MAF) meter. At that point the ECU will not be able to know if any more air is coming into the engine, and it would begin to run lean. This is no good! Of course there is a fix for this -- actually, there are a couple fixes.
The first way we could address the upper MAF limit would be to simply enlarge the pipe that the MAF sits in. This would lower the voltage for a given air flow rate, getting us back under our maximum voltage. We would lose some resolution, but that's a small price to pay for still being able to accurate meter air at much higher-than-stock boost levels.
There's an alternative way to deal with the air flow limit, though, and that's to scrap the whole mass air metering scheme all together, in favor of a speed-density system. There are several reasons you might want to do this. First, your maximum airflow is limited only by the maximum pressure you can record, and pressure sensors are inexpensive and easy to get. Secondly, the metering system will no longer be sensitive to small leaks in the intake system, or other variations caused by things like backfeeding from the turbo, or the blow-off valve venting to atmosphere. Finally, although the MAF is a minimal restriction, the plumbing that it sits in can be a restriction to intake flow. Removing the MAF allows you to run a very short, very large intake pipe. Some builders just position a large air filter directly on the turbo inlet and call it a day.
Speed-density metering requires the installation of an aftermarket manifold absolute pressure (MAP) sensor with a higher limit than the stock sensor, and an intake temperature sensor. Along with the sensors, you'll need a custom-tuned ECU map to handle the new metering system. If you're not sure what all this means or where to start to make it happen, give our tuning experts a call.