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GM's New Family of VVT Gen IV Cam Cores
GM introduced a second style VVT cam actuator (aka phaser) in 2007 and ran two different cam actuators through 2007 and newer.
The new style became the exclusive unit in most 2009 and newer engines.
Original (old style) GM Part #12585994: 2007‐2008 (uses cam core 156‐family)
Current (new style) GM Part #12606358: 2007-2009 and newer engines (use cam core 189‐family).
The difference in the camshaft core is the pin to centerline position. The difference in the pin between 156 and 189 works out to 13 degrees cam retard (if a 156 is used where a 189 is required).
Tech Bulletin courtesy of Comp Cams