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Well that's unusual, 3" exhaust normally gives better boost due to extra flow. What revs are you getting max boost? Under load it should be slightly higher but if the atmospheric pressure is low you will get low boost as the map (manifold air pressure) measured boost is above atmospheric. Do you live at high altitude? Even dry weather lowers boost hence humid conditions increases boost. Try going uphill under full throttle and check boost then. The ecu alters the vanes on the turbo to deliver what the ECU demands.