What a plenum spacer is? What is better, a plenum spacer, or a complete aftermarket plenum?

Several companies make a "spacer" which rests between your lower plenum and upper plenum, to increase plenum volume and reduce restriction from within the plenum. For example, Motordyne engineering's 5/16" spacer, or AAM's angled spacer. Increasing plenum volume typically has the effect of increasing horsepower. However, if you increase the volume too much, you will start to lose throttle response. Plenum design is a crucial aspect of engine design -- companies that make spacers are simply trying to get the most function out of the plenum.

Some people swear by aftermarket plenums (i.e. crawford, kinetix), but for all intents and purposes, they accomplish the same exact thing as adding a spacer. So most people would tell you to choose whichever one is cheapest, which is usually the spacer.

You will definitely feel this one. The stock plenum slopes down towards the front of the engine. Nissan designed this in order to clear the stock strut bar and hood on the 350Z (function after form). This sloping design restricts the airflow to the front cylinders, but an aftermarket plenum can help the front cylinders breathe. Aftermarket plenum spacers either increase the overall height or remove the downward slope in the design, allowing more volume and greater airflow to all cylinders. Higher airflow is needed to achieve more power.

Note: Some plenum spacers will cause you to loose the stock strut bar.

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