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most hotwire airflow meters use an output voltage from about 0-5v (0 being engine stopped and 5 being the highest airflow that sized airflow meter can maintain the hotwire temp at.) once this "wall" is reached it is time to get either a bigger airflow meter (pipe diameter) or change to a completely different meter (ie nissans use Q45 infinity meter) or go for map sensed load axis. either way you will need a retune on your ecu. it is possible to max out an airflow meter on a nissan without hitting the boost cut by using an aftermarket turbo with stock boost levels, the subaru ecu does not have af meter out of range cut, only a boost cut, when you disable the boost cut with an fcd the ecu will adjust fuelling to excessively rich to the point where the fuel system is not capable of maintaining. ie as boost is raised the stock ecu gets excessively rich to a point where the fuel system runs out of flow and results in a lean condition often ending in cracked ring lands or melted piston crowns or both. this is why the stock ecu shows little extra power when boost gets above 15-16psi on a stock turbo as it runs air fuel ratios in the low 10:1s to protect itself and looses power because of excessive rich mixtures. the subaru engineers have tuned the fueling within the confines of the factory turbocharger and factory boostcut with an excellent compromise between safety and power. if you go beyond the factory boost cut with an fcd or aftermarket boost control you can measure the life of your engine in seconds or minutes. anytime boost or airflow (via turbo replacement) exceeds the factory limits (boost cut) a full ecu retune should be done. no exceptions. So the obvious conclusion is that boost controllers dont kill engines, boost controller USERS kill engines.
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