at part throttle the car runs leaner to get better gas mileage. the motor can easily handle the lean condition below probably 4500rpms, as long as the boost doesnt go over like 6psi.
the problem is if u install an aftermarket WGA, and you arnt being careful with the throttle, u can boost well past 6psi at part throttle, and the ecu is still aiming for a target AFR of around 14.7:1. this is fine at lower boost, and lower rpms, but as u rev higher, and boost more, the combustion temps start to raise higher, and that can damage your motor. at full throttle your afr at max boost should be around 11.5:1 - 12:1, which is a little richer and safer for the motor.
the reason the car does this is because at part throttle, the ECU is looking at the upstream o2 sensor, and trying to acheive 14.7:1 afr. it doesnt care rather you are running 6psi or 20psi, because it thinks you are driving easy, and it is trying to give you good gas mileage. when you hammer the gas pedal, it stops reading the upstream o2, and just dumps all the fuel the injectors will allow it to, and then uses the data it sees from the map sensor to calculate how much boost you are running, and tells the ECU how much timing to pull based on this number. this is why people use map clamps and piggy backs to help lean the car out a little more at redline and raise timing back up. i hope this helps.
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