to 7K due to the loss of exhaust energy at higher RPM (above 6K or so) with small turbos. All small turboed Zs will have a predictable drop in (high) boost as the car gets into the end of it's powerband. You'll find bigger turboed cars can hold boost 'til redline with their increased ability to spin the turbos (due to their ability to produce more exhaust energy). Hope that makes sense, Mark