This is the OEM Spacer Nissan uses to eliminate the iIdler pulley from the later (August 2006+) model VQ35DE Engines. This Spacer is placed in between the engine and the power steering mounting bracket in the space the idler pulley bracket would be normally.
By eliminating the idler pulley you remove almost 1lb of rotating mass from the engine and eliminate a wearing part that will eventually fail anyway. The idler pulley and bracket together weigh almost 3lbs. When using the spacer instead of the pulley you must also use a shorter belt found HERE.
Choose to add this as a FREE Option to the Z1 350Z / G35 VQ35 Lightweight Crank Pulley.