If you look at the majority of dyno sheets that show torque & manifold pressure, the torque curve will to a certain extent mirror the changes of the MAP. As in, boost drops, torque will drop. The reason why boosts drops will be a couple of reasons, but generally poor waste gate operation / boost solenoid or control operation or simply the spring inside the waste gate is too soft for the controller to handle.

AVCS pays a large part also in boost levels & how air flows through the engine & then that flow influencing the pressure.

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Cheers,

Simon.