As far as I know there is no need to vary the exhuast lift/duration as is done with the intake, You regulate the intake so as to allow more or less fuel. In the old days hi revving chevy v8's used cams of long duration and hi lift with the exhaust have a shorter lift longer duration so to be able to rev without freaking out the cam geometry, todays electronically/computer controlled motors are able to vary the intake for power or economy, the exhuast is just there to take out the trash so there isn't the need to open it longer than to clear the cumbustion chamber... Hope this helps..
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